Single-Isocenter Multitarget Stereotactic Radiosurgery Is Safe and Effective in the Treatment of Multiple Brain Metastases

HyperSHArc: Single-Isocenter Stereotactic Radiosurgery of Multiple Brain Metastases Using Proton, Helium, and Carbon Ion Arc Therapy

Purpose

This work presents a proof-of-concept study of HyperSHArc, spot-scanning hadron arc (SHArc) therapy for single-isocenter stereotactic radiosurgery of multiple brain metastases (MBMs). HyperSHArc plans using proton, helium, and carbon ions were compared with state-of-the-art volumetric modulated photon arc therapy.

Methods and Materials

Treatment design and optimization procedures were devised using commercial and in-house treatment planning systems. Planning and delivery methods considered dedicated energy, spot, and multiarc selection strategies. Proton, helium, and carbon HyperSHArc plans were generated for patients with MBM exhibiting 3 to 11 intracranial lesions with gross tumor volumes (GTVs) between 0.03 and 19.8 cc, at prescribed doses between 19 and 21Gy in a single-fraction. Planning target volumes (PTVs) considered a 1-mm isotropic margin around the GTV, and robust optimization with 2.5%/1 mm criteria for range and position uncertainty was applied. Photon hyper-arc volumetric modulated arc therapy (HA-VMAT) plans were optimized for the PTVs using the HyperArc® single-isocenter stereotactic radiosurgery platform (Varian, Palo Alto, CA, USA).

Results

HyperSHArc plans were comparable between particle species, achieving highly conformal target doses and satisfying clinical coverage criteria. Particle arc plans reduced V2Gy and V4Gy in the healthy brain compared with HA-VMAT, while intermediate doses (V8Gy-V16Gy) were similar or reduced depending on the number of lesions. Particularly for the case with 11 targets, a considerable reduction in V12Gy was observed that could be relevant for reducing the risk of treatment-induced radionecrosis. HyperSHArc using carbon ions boosted dose-averaged linear energy transfer inside the target relevant to overcoming radioresistance factors (>100 keV/μm).

Conclusions

We present the first particle arc therapy strategies for MBM. Results demonstrate that with HyperSHArc, dose conformity comparable or superior to HA-VMAT is achievable while reducing the low-dose bath and increasing mean dose-averaged linear energy transfer in the GTV. Our findings suggest that HyperSHArc using light and heavy ions could be an effective and efficient means of treating MBM. Further development of HyperSHArc optimization and delivery is justified.

AAPM-RSS Medical Physics Practice Guideline 9.b: SRS-SBRT

The purpose of this Medical Physics Practice Guideline (MPPG) is to describe the minimum level of medical physics support deemed prudent for the practice of linear-accelerator, photon-based (linac) stereotactic radiosurgery (SRS), and stereotactic body radiation therapy (SBRT) services. This report is an update of MPPG 9.a1 published in 2017. As SRS and SBRT services are rapidly adopted into the community-practice setting, this guideline has been developed to build on the work presented in MPPG 9.a and provide current appropriate minimum practice guidelines for such services.

Impact of enhanced leaf model on dose calculation accuracy in single-isocenter multitarget stereotactic radiosurgery treatments

PURPOSE

Single-isocenter multitarget (SIMT) radiosurgery has become increasingly popular as advancement in planning and delivery systems have made this approach clinically viable. With targets varying in size and distance from isocenter, SIMT plans are highly complex with dynamic multileaf collimator (MLC) motion. Our department recently commissioned Eclipse Treatment Planning System v18.0, which included a novel enhanced leaf model (ELM) for photon dose calculation. ELM represents the biggest update in MLC modeling on a commercial treatment planning system over the past decade, yielding improvements in leaf modeling and ray tracing. Considering its dependence on dynamic MLC movements, we set out to assess the potential clinical impact of ELM on SIMT.

METHODS

Dynamic zebra crosswalk (DZC) plans were delivered on a Varian Edge to investigate ELM. DZCs consisted of sweeping MLC gaps (ranging 1-15 mm) across a 3 cm width at isocenter, 4 cm, 8 cm, and 12 cm along the x-axis. Phantom dose calculations were performed using AAA v15.6 and v18.0 (with ELM) for 6 MV flattening filter free DZC plans and compared to measurements using stereotactic radiosurgery MapCHECK (Sun Nuclear Corporation) and Gafchromic EBT4 films (Ashland). To assess potential impact on SIMT, ten patients were retrospectively planned with RapidArc and HyperArc. Both versions of AAA were used for dose calculation.

RESULTS

DZC measurements showed improved agreement with ELM; differences between measured and calculated doses were reduced by as much as 19% for the smallest sweeping gaps at off-axis distances. Differences in central profile dose for DZCs increased with reduced gap size and increased off-axis position. SIMT plans showed up to 4.0% increase in planning target volume (PTV) maximum dose when switching from AAA v15.6 to v18.0.

CONCLUSION

Dose calculations with ELM mirrored diode and film measurements for highly modulated SIMT plans. ELM represents a major improvement in MLC modeling that more accurately reflects current treatment delivery practice.

The Impact of Margin Expansions on Local Control and Radionecrosis Following Stereotactic Radiosurgery for Brain Metastases: A Systematic Review and Meta-Analysis

PURPOSE

The implications of margin expansions on local control(LC) and radionecrosis(RN) for treating brain metastases with stereotactic radiosurgery(SRS) remain unclear. We performed a systematic review and meta-analysis to compare LC and RN between patients with brain metastases treated with stereotactic radiosurgery(SRS) planned with no margin vs. a margin.

METHODOLOGY

We utilized PICOS/PRISMA/MOOSE selection inclusion criteria for studies of patients with brain metastases treated with SRS with no margin or a margin. Primary outcomes were 1-year LC and radiographic and symptomatic RN incidences. Weighted random effects meta-analyses were performed to compare effect sizes.

RESULTS

Across 17 studies, we identified 5,015 lesions treated with SRS (1,360 lesions with no margin and 3,684 with a margin). The median total margin was 1.5 mm (range: 1-3). Single fraction SRS was most common with a median prescription dose of 21 Gy (range: 15-24 Gy). The estimated 1-year LC rate was similar with a margin (88.4% (95% CI: 83.7-92.4%) vs. without (83.0% (95% CI: 69.3-93.2%; p = 0.28)). Estimated incidences of radiographic RN following SRS with no margin vs. a margin were similar at 9.2% (95% CI: 0.2-29.6%) and 7.0% (95% CI: 4.1-10.7%; p=0.56), respectively. Estimated incidences of symptomatic RN following SRS with no margin vs. with a margin were 8.6% (95% CI: 5.2-12.7%) and 4.1% (95% CI: 0.9-9.3%), respectively (p=0.24).

CONCLUSIONS

Significant differences were not noted in either LC or RN for patients treated with margin expansions vs. without. Prospective evaluations are warranted to further assess this question while controlling for other relevant treatment planning and metastasis considerations.

Comparison of single- and multi-isocenter planning with Dynamic WaveArc for multiple brain metastases

Dynamic WaveArc (DWA) is a technique used for continuous, non-coplanar volumetric-modulated arc therapy on the Vero4DRT platform. This study aimed to evaluate the application of single-isocenter DWA (SI-DWA) for treating multiple brain metastases by comparing dose distribution and irradiation time with multi-isocenter DWA (MI-DWA) through retrospective treatment planning. Treatment plans were developed for SI-DWA and MI-DWA in 14 cases with 3-5 brain metastases. Parameters assessed included target dose indices, such as conformity index (CI) of the planning target volume (PTV), volumes of normal brain excluding gross tumor volumes (GTVs) receiving a single dose equivalent of 14 Gy (V14), V30%, V20%, V10%, volumes of normal brain, including GTVs receiving a single dose equivalent of 12 Gy (V12), D2% for other organs at risk, and beam-on time. SI-DWA showed inferior CI, V14, and V12 values for lesions with PTV volumes <1 cc, whereas it performed equivalently to MI-DWA for lesions with PTV volumes ≥1 cc. SI-DWA resulted in higher volumes of normal brain receiving low doses compared to MI-DWA. SI-DWA exhibited significantly shorter beam-on times than MI-DWA. In conclusion, SI-DWA is an effective method for treating multiple brain metastases with PTV volumes ≥1 cc, offering an index of radiation-induced brain necrosis comparable with MI-DWA while allowing for shorter irradiation times.

Dosimetric commissioning of a high-resolution CMOS 2D detector array for patient-specific QA of single-isocenter multi-target VMAT stereotactic radiosurgery

Stereotactic radiosurgery (SRS) using the single-isocenter-multiple-target (SIMT) technique by volumetric modulated arc therapy is increasingly popular for treating multiple brain metastases. However, the complex nature of SIMT SRS necessitates rigorous patient-specific quality assurance (PSQA). This study presents a multi-institutional dosimetric commissioning of a high-resolution complementary metal oxide semiconductor (CMOS) 2D detector array, the myQA SRS device for SIMT SRS PSQA. Basic dosimetric properties such as dose-rate, field-size, energy and angular dependencies were characterized for the CMOS detectors. Additionally, gamma index analyses were performed between the measured dose and the films for nine simulated and clinical plans. The results showed that the CMOS detector was dose-rate, field-size, energy and beam-angle dependent. Specific to SIMT SRS, angular dependence on gantry rotations was invariant to couch rotations but was sensitive to off-isocenter distances. With appropriate dose calibration and angular corrections, myQA SRS showed a high dosimetric correlation with films. The average gamma index pass rates were 99.9 ± 0.03% and 99.2 ± 1.1% at 3%/2 mm/10%thr(global) and 1 mm/1%/10%thr(local) criteria, respectively. The average dose difference between myQA SRS and films was 0.4 ± 1.3%. In conclusion, the CMOS 2D detector array has demonstrated its potential as a reliable tool for PSQA for SIMT SRS. The excellent dosimetric agreement with the films was consistent in multiple institutions, further validating the dosimetric accuracy and reproducibility. It provides a timely alternative to film dosimetry for commissioning and quality assurance.

Analysis of linear accelerator-based fractionated stereotactic radiotherapy in brain metastases: efficacy, safety, and dose tolerances

Objective

To assess the efficacy and safety of linear accelerator-based fractionated stereotactic radiotherapy (LINAC-FSRT) in patients with brain metastases (BM).

Methods

We retrospectively analyzed 214 patients treated with LINAC-FSRT, categorized based on biologically effective dose (BED10, α/β = 10) into two groups (≤55 Gy, >55 Gy). Stratified analyses were conducted based on targeted therapy to compare survival outcomes. To examine brain tissue dose-tolerance volume, patients were divided into two groups: the standard Hypofractionated Treatment Effects in the Clinic (HyTEC) protocol group and an adjusted HyTEC protocol group where dose-volume restrictions exclude the planning target volume (PTV).

Results

Results as of December 2023 showed median intracranial progression-free survival (iPFS) at 12.4 months, with median overall survival (OS) not reached and a one-year local control (LC) rate of 68.7%. Mild to moderate toxicity affected 17.3% of patients, while severe toxicity occurred in 2.8%. Multivariate Cox analysis indicated that uncontrolled extracranial disease significantly reduced iPFS (HR = 2.692, 95%CI:1.880-3.853, P < 0.001) and OS (HR = 3.063, 95%CI:1.987-4.722, P < 0.001). BED10 >55 Gy (HR = 0.656, 95%CI:0.431-0.998, P = 0.049) improved OS, showing statistical significance (P = 0.037) without affecting iPFS or CNS toxicity (P = 0.127, P = 0.091). Stratified analysis highlighted nearly significant OS improvements with high-dose FSRT and targeted therapy (P = 0.054), while concurrent therapy markedly enhanced iPFS (P = 0.027). No significant differences were observed in intracranial local failure (ILF-which represents progression in previously treated areas during follow-up), one-year LC rates, iPFS, or OS between dose-volume groups. Adjusting HyTEC volume restrictions did not significantly increase CNS adverse reactions (P = 0.889).

Conclusions

LINAC-FSRT is safe and effective in BM. BED10>55 Gy notably enhances OS post-LINAC-FSRT and may benefit LC. High BED10 FSRT with targeted therapy likely boosts synergy, and concurrent targeted therapy significantly improves iPFS. Diminishing dose volume constraints at different fractions based on the HyTEC guidelines is feasible.

Advances in the Management of Lung Cancer Brain Metastases

Lung cancer, both non-small cell and small cell, harbors a high propensity for spreading to the central nervous system. Radiation therapy remains the backbone of the management of brain metastases. Recent advances in stereotactic radiosurgery have expanded its indications and ongoing studies seek to elucidate optimal fractionation and coordination with systemic therapies, especially targeted inhibitors with intracranial efficacy. Efforts in whole-brain radiotherapy aim to preserve neurocognition and to investigate the need for prophylactic cranial irradiation. As novel combinatorial strategies are tested and prognostic/predictive biomarkers are identified and tested, the management of brain metastases in lung cancer will become increasingly personalized to optimally balance intracranial efficacy with preserving neurocognitive function and patient values.

Initial Experience of Implementing a Pre-treatment Dry Run for HyperArc Stereotactic Radiosurgery Treatments With Optical Surface Imaging for Intra-fraction Motion Monitoring

Linac-based stereotactic radiosurgery (SRS) with planning target volume (PTV) margins <1 mm has become increasingly common in recent years. Optical surface imaging for surface-guided radiation therapy (SGRT) is often used for intra-fraction motion monitoring during these treatments to facilitate the use of a smaller PTV margin by providing real-time quantitative patient positioning information. However, rotating the couch introduces errors to SGRT-reported translations and rotations that can be problematic for SRS treatments with non-coplanar arcs and very small PTV margins. This work presents a novel approach for decreasing the magnitude of these errors by performing a pre-treatment dry run and capturing reference surfaces with the SGRT system at each couch angle included in the treatment plan. Time from cone beam computed tomography (CBCT) to treatment initiation and total treatment session time were reviewed for 30 single-fraction brain SRS cases treated using this technique to determine the effect of including the dry run on treatment session times. Out of the 30 cases treated between April 2023 and January 2024, 23 treatments required only a single CBCT prior to treatment, with no additional mid-treatment imaging required to verify patient positioning after motion. The median time between CBCT and treatment initiation was 7.98 minutes (interquartile range (IQR) = 7.28 to 8.93 minutes). The median time from CBCT to treatment completion was 15.43 minutes (IQR = 13.67 to 21.97 minutes). In the six patients that required one additional CBCT, the treatment session times ranged from 24.32 to 32.83 minutes. There was one patient who required three mid-treatment CBCTs, and the treatment session time was 67.87 minutes. Incorporating the pre-treatment dry run with the acquisition of reference surfaces at each treatment angle decreased errors in SGRT-reported translations and rotations associated with couch rotation without significantly increasing treatment session times.

Dosimetric and Clinical Prognostic Factors in Single-Isocenter Linac-Based Stereotactic Radiotherapy for Brain Metastases

Background/Objectives: To report on predictive factors in Linac-based SRT for single and multiple BM. Methods: Consecutive patients receiving either one or three fractions of single-isocenter coplanar VMAT SRT were retrospectively included. The GTV-PTV margin was 1-2 mm. The delivered target dose was estimated by recalculating the original plans on roto-translated CT according to errors recorded by post-treatment CBCT. The Kaplan-Meier method estimated local progression-free survival (LPFS), intracranial progression-free survival (IPFS), and overall survival (OS). Log-rank and Wilcoxon-Mann-Whitney tests evaluated inter-group differences, whereas Cox regression analysis assessed prognostic factors. Results: Fifty females and fifty males, with a median age of 69 years, received 107 SRTs. A total of 213 BM (range, 1-10 per treatment) with a median volume of 0.22 cc were irradiated with a median minimum BED of 59.5 Gy. The median delivered GTV D95 reduction was -0.3%. The median follow-up was 11 months. Nineteen LP events and a 1-year LC rate of 90.1% were observed. The GTV coverage did not correlate with LC, while the GTV volume was a risk factor for LP, with the 1-year rate dropping to 73% for volumes ≥ 0.88 cc. The median LPFS, IPFS, and OS were 6, 5, and 7 months, respectively. Multivariate analysis showed that patients with melanoma histology and those receiving a second or subsequent systemic therapy line had the worst outcomes, whereas patients with adenocarcinoma histology and mutations showed better results. Conclusions: The accuracy and efficacy of the Linac-based SRT approach for BM were confirmed, but the dose distribution alone failed to predict the treatment response, suggesting that other factors must be considered to maximize SRT outcomes.

Challenges with hippocampal MR spectroscopy as a surrogate for pre-radiotherapy assessment of neurocognitive impairment in patients with brain metastasis

AIM

Patients with multiple brain metastases (BM) benefit from hippocampal-avoiding whole brain radiotherapy (HA-WBRT), the challenging and less available form of WBRT. This study explores potential of pre-radiotherapy (pre-RT) hippocampal magnetic resonance spectroscopy (MRS) measuring hippocampal neuronal density as an imaging surrogate and predictive tool for assessing neurocognitive functions (NCF).

METHODS

43 BM patients underwent pre-RT hippocampal MRS. N-acetyl aspartate (NAA) concentration, a marker for neuronal density (weighted by creatine (Cr) and choline (Cho) concentrations), and neurocognitive function (NCF) tests (HVLT and BVMT) performed by certified psychologists were evaluated. Clinical variables and NAA concentrations were correlated with pre-RT NCFs.

RESULTS

HVLT and BVMT subtests showed pre-RT deterioration except for BVMT recognition. Significantly better NCFs were observed in women in HVLT subsets. Significantly higher NAA/Cr + Cho was measured in women (median 0.63 vs. 0.55; P=0.048) in the left hippocampus (no difference in the right hippocampus). In men, a positive correlation (0.51, P=0.018) between total brain volume and HVLT-TR, between left hippocampal NAA/Cr + Cho and HVLT-R (0.45, P=0.063), and between right hippocampal NAA/Cr + Cho and BVMT-recognition (0.49, P=0.054) was observed. In women, a borderline significant negative correlation was observed between left hippocampal NAA/Cr + Cho and BVMT-TR (-0.43, P=0.076) and between right NAA/Cr + Cho and HVLT-DR (-0.42, P=0.051).

CONCLUSION

Borderline statistically significant correlations were observed with speculative interpretation underlying the challenges of hippocampal MRS as a surrogate for neurocognitive impairment. Further studies need to be done to ascertain the opportunities for imaging predictors of benefit from memory sparing radiotherapy.

Single-Isocenter, Multiple-Target Abdominal Cone-Beam Computed Tomography (CBCT)-Guided Online Adaptive Stereotactic Body Radiotherapy (SBRT)

Stereotactic body radiotherapy (SBRT) is increasingly being prescribed for treating patients with multiple metastases, especially in the setting of oligometastatic disease. Treating multiple targets presents unique challenges in radiotherapy planning and delivery, including practical considerations relating to treatment time, resource allocation, and treatment planning complexity. Treating targets in a common isocenter reduces the time required for treatment and simplifies planning, but historically, it has often not been feasible due to inter- and intra-fractional variation in relative target positions. With online adaptation, individual targets can be re-contoured on each treatment fraction to obviate inter-fractional variation, and with appropriate margin selection intra-fractional motion can be managed. In this case report, we describe single-isocenter, multiple-target treatment via online adaptation of a 93-year-old man with a history of metastatic hepatocellular carcinoma. He initially presented with a 9.1 cm liver mass, suspicious lung lesions, and an enlarged porta hepatis lymph node, which were biopsy proven to be hepatocellular carcinoma. Following 18 months of systemic immunotherapy, he demonstrated a favorable response, including a reduction in primary liver mass to 5.1 cm and resolution of pulmonary lesions; however, recent serial imaging demonstrated oligoprogression of two peripancreatic lymph node conglomerates that were biopsy proven to be poorly differentiated carcinoma. The patient was offered adaptive SBRT to a dose of 35-40 Gy in five fractions as a consolidative approach for treating both the primary liver mass and oligoprogressive lymph nodes. He tolerated treatment without any grade 2 or higher acute toxicity and had stable disease on three-month post-treatment imaging. By leveraging online adaptation, especially for the daily re-definition of target volumes, we were able to treat three targets in the abdomen accurately in a common isocenter. Treating in this manner vastly shortened and simplified the patient's radiation course. Quantitative evaluation of re-contoured targets and post-treatment imaging highlighted the value of online adaption with careful margin specification and alignment instructions.

Insights into the dosimetric and geometric characteristics of stereotactic radiosurgery for multiple brain metastases: A systematic review

BACKGROUND

GammaKnife (GK) and CyberKnife (CK) have been the mainstay stereotactic radiosurgery (SRS) solution for multiple brain metastases (MBM) for several years. Recent technological advancement has seen an increase in single-isocentre C-arm linac-based SRS. This systematic review focuses on dosimetric and geometric insights into contemporary MBM SRS and thereby establish if linac-based SRS has matured to match the mainstay SRS delivery systems.

METHODS

The PubMed, Web of Science and Scopus databases were interrogated which yielded 891 relevant articles that narrowed to 20 articles after removing duplicates and applying the inclusion and exclusion criteria. Primary studies which reported the use of SRS for treatment of MBM SRS and reported the technical aspects including dosimetry were included. The review was limited to English language publications from January 2015 to August 2023. Only full-length papers were included in the final analysis. Opinion papers, commentary pieces, letters to the editor, abstracts, conference proceedings and editorials were excluded. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. The reporting of conformity indices (CI) and gradient indices, V12Gy, monitor units and the impact of translational and rotational shifts were extracted and analysed.

RESULTS

The single-isocentre technique for MBM dominated recent SRS studies and the most studied delivery platforms were Varian. The C-arm linac-based SRS plan quality and normal brain tissue sparing was comparable to GK and CK and in some cases better. The most used nominal beam energy was 6FFF, and optimised couch and collimator angles could reduce mean normal brain dose by 11.3%. Reduction in volume of the healthy brain receiving a certain dose was dependent on the number and size of the metastases and the relative geometric location. GK and CK required 4.5-8.4 times treatment time compared with linac-based SRS. Rotational shifts caused larger changes in CI in C-arm linac-based single-isocentre SRS.

CONCLUSION

C-arm linac-based SRS produced comparable MBM plan quality and the delivery is notably shorter compared to GK and CK SRS.

Safety and Feasibility of Stereotactic Radiosurgery for Patients with 15 or more Brain Metastases

Background

Current standard of care treatment for patients with ≥15 brain metastases (BM) is whole brain radiation therapy (WBRT), despite poor neurocognitive outcomes. We analyzed our institutional experience of treating these patients with stereotactic radiosurgery (SRS), with the aim of evaluating safety, cognitive outcomes, and survival metrics.

Methods

Patients who received SRS for ≥15 BMs in 1 to 5 fractions from 2014 to 2022 were included. Cognitive outcomes were objectively evaluated using serial Patient-Reported Outcome Measurement Information System (PROMIS) scores. The Kaplan-Meier method was used for survival analysis and log-rank test for intergroup comparisons.

Results

Overall, 118 patients underwent 124 courses of LINAC-based SRS. The median number of lesions treated per course was 20 (range, 15-94). Most patients received fractionated SRS to a dose of 24 Gy in 3 fractions (81.5%). At the time of SRS, 19.4% patients had received prior WBRT, and 24.2% had received prior SRS. The rate of any grade radiation necrosis (RN) and grade ≥3 RN were 15.3% and 3.2%, respectively. When evaluating longitudinal PROMIS score trends, 25 of 31 patients had a stable/improved PROMIS score. Patients who did not receive prior brain RT had a longer median survival (7.4 months vs 4.6 months, P = .034). The 12m local control was 97.6%, and the cumulative incidence of distant intracranial failure, with death as a competing event, was 46% (95% CI, 36%, 55%). One year freedom from neurologic death, leptomeningeal disease, and salvage WBRT were 89%, 94.6%, and 84%, respectively.

Conclusion

We present here one of the largest studies evaluating SRS for patients with ≥15 BMs. SRS was safe, had favorable cognitive outcomes, and had comparable survival outcomes to contemporary studies evaluating WBRT in this population. Treatment-naïve patients had a median survival of >6 months, long enough to benefit from cognitive sparing with SRS. Our study supports randomized studies comparing SRS and hippocampal avoidance WBRT approaches for these patients.

Development of a 3D printed phantom for commissioning and quality assurance of multiple brain targets stereotactic radiosurgery

Single plan techniques for multiple brain targets (MBT) stereotactic radiosurgery (SRS) are now routine. Patient specific quality assurance (QA) for MBT poses challenges due to the limited capabilities of existing QA tools which necessitates several plan redeliveries. This study sought to develop an SRS QA phantom that enables flexible MBT patient specific QA in a single delivery, along with complex SRS commissioning. PLA marble and PLA StoneFil materials were selected based on the literature and previous research conducted in our department. The HU numbers were investigated to determine the appropriate percentage infill for skull and soft-tissue equivalence. A Prusa MK3S printer in conjunction with the above-mentioned filaments were used to print the SRS QA phantom. Quality control (QC) was performed on the printed skull, film inserts and plugs for point dose measurements. EBT3 film and point dose measurements were performed using a CC04 ionisation chamber. QC demonstrated that the SRS QA phantom transverse, coronal and sagittal film planes were orthogonal within 0.5°. HU numbers for the skull, film inserts and plugs were 858 ± 20 and 35 ± 12 respectively. Point and EBT3 film dose measurements were within 2.5% and 3%/2 mm 95% gamma pass rate, respectively except one Gross Tumour Volume (GTV) that had a slightly lower gamma pass rate. Dose distributions to five GTVs were measured with EBT3 film in a single plan delivery on CyberKnife. In conclusion, an SRS QA phantom was designed, and 3D printed and its use for performing complex MBT patient specific QA in a single delivery was demonstrated.

Assessing the sensitivity and suitability of a range of detectors for SIMT PSQA

PURPOSE

Single-isocenter multi-target intracranial stereotactic radiotherapy (SIMT) is an effective treatment for brain metastases with complex treatment plans and delivery optimization necessitating rigorous quality assurance. This work aims to assess five methods for quality assurance of SIMT treatment plans in terms of their suitability and sensitivity to delivery errors.

METHODS

Sun Nuclear ArcCHECK and SRS MapCHECK, GafChromic EBT Radiochromic Film, machine log files, and Varian Portal Dosimetry were all used to measure 15 variations of a single SIMT plan. Variations of the original plan were created with Python. They comprised various degrees of systematic MLC offsets per leaf up to 2 mm, random per-leaf variations with differing minimum and maximum magnitudes, simulated collimator, and dose miscalibrations (MU scaling). The erroneous plans were re-imported into Eclipse and plan-quality degradation was assessed by comparing each plan variation to the original clinical plan in terms of the percentage of clinical goals passing relative to the original plan. Each erroneous plan could be then ranked by the plan-quality degradation percentage following recalculation in the TPS so that the effects of each variation could be correlated with γ pass rates and detector suitability.

RESULTS & CONCLUSIONS

It was found that 2%/1 mm is a good starting point for the ArcCHECK, Portal Dosimetry, and the SRS MapCHECK methods, respectively, and provides clinically relevant error detection sensitivity. Looser dose criteria of 5%/1 mm or 5%/1.5 mm are suitable for film dosimetry and log-file-based methods. The statistical methods explored can be expanded to other areas of patient-specific QA and detector assessment.

Novel radiotherapeutic strategies in the management of brain metastases: Challenging the dogma

The role of radiation therapy in the management of brain metastasis is evolving. Advancements in machine learning techniques have improved our ability to both detect brain metastasis and our ability to contour substructures of the brain as critical organs at risk. Advanced imaging with PET tracers and magnetic resonance imaging-based artificial intelligence models can now predict tumor control and differentiate tumor progression from radiation necrosis. These advancements will help to optimize dose and fractionation for each patient's lesion based on tumor size, histology, systemic therapy, medical comorbidities/patient genetics, and tumor molecular features. This review will discuss the current state of brain directed radiation for brain metastasis. We will also discuss future directions to improve the precision of stereotactic radiosurgery and optimize whole brain radiation techniques to improve local tumor control and prevent cognitive decline without forming necrosis.

Stereotactic Radiosurgery in the Management of Brain Metastases: A Case-Based Radiosurgery Society Practice Guideline

Purpose

Brain metastases are common among adult patients with solid malignancies and are increasingly being treated with stereotactic radiosurgery (SRS). As more patients with brain metastases are becoming eligible for SRS, there is a need for practical review of patient selection and treatment considerations.

Methods and Materials

Two patient cases were identified to use as the foundation for a discussion of a wide and representative range of management principles: (A) SRS alone for 5 to 15 lesions and (B) a large single metastasis to be treated with pre- or postoperative SRS. Patient selection, fractionation, prescription dose, treatment technique, and dose constraints are discussed. Literature relevant to these cases is summarized to provide a framework for treatment of similar patients.

Results

Treatment of brain metastases with SRS requires many considerations including optimal patient selection, fractionation selection, and plan optimization.

Conclusions

Case-based practice guidelines developed by the Radiosurgery Society provide a practical guide to the common scenarios noted above affecting patients with metastatic brain tumors.

Multi-institutional investigation into the robustness of intra-cranial multi-target stereotactic radiosurgery plans to delivery errors

BACKGROUND

The use of modulated techniques for intra-cranial stereotactic radiosurgery (SRS) results in highly modulated fields with small apertures, which may be susceptible to uncertainties in the delivery device.

PURPOSE

This study aimed to quantify the impact of simulated delivery errors on treatment plan dosimetry and how this is affected by treatment planning system (TPS), plan geometry, delivery technique, and plan complexity. A beam modelling error was also included as context to the dose uncertainties due to treatment delivery errors.

METHODS

Delivery errors were assessed for multiple-target brain SRS plans obtained through the Trans-Tasman Radiation Oncology Group (TROG) international treatment planning challenge (2018). The challenge dataset consisted of five intra-cranial targets, each with a prescription of 20 Gy. Of the final dataset of 54 plans, 51 were created using the volumetric modulated arc therapy (VMAT) technique and three used intensity modulated arc therapy (IMRT). Thirty-five plans were from the Varian Eclipse TPS, 17 from Elekta Monaco TPS, and one plan each from RayStation and Philips Pinnacle TPS. The errors introduced included: monitor unit calibration errors, multi-leaf collimator (MLC) bank offset, single MLC leaf offset, couch rotations, and collimator rotations. Dosimetric leaf gap (DLG) error was also included as a beam modelling error. Dose to targets was assessed via dose covering 98% of planning target volume (PTV) (D98%), dose covering 2% of PTV (D2%), and dose covering 99% of gross tumor volume (GTV) (D99%). Dose to organs at risk (OARs) was assessed using the volume of normal brain receiving 12 Gy (V12Gy), mean dose to normal brain, and maximum dose covering 0.03cc brainstem (D0.03cc). Plan complexity was also assessed via edge metric, modulation complexity score (MCS), mean MLC gap, mean MLC speed, and plan modulation (PM).

RESULTS

PTV D98% showed high robustness on average to most errors with the exception of a bank shift of 1.0 mm and large rotational errors ≥1.0° for either the couch or collimator. However, in some cases, errors close to or within generally accepted machine tolerances resulted in clinically relevant impacts. The greatest impact upon normal brain V12Gy, mean dose to normal brain, and D0.03cc brainstem was found for DLG error in alignment with other recent studies. All delivery errors had on average a minimal impact across these parameters. Comparing plans from the Monaco TPS and the Eclipse TPS, showed a lesser increase to V12Gy, mean dose to normal brain, and D0.03cc brainstem for Monaco plans (p < 0.01) when DLG error was simulated. Monaco plans also correlated to lower plan complexity. Using Spearman's correlation coefficient (r) a strong negative correlation (r ≤ -0.8) was found between the mean MLC gap and dose to OARs for DLG errors.

CONCLUSIONS

Reducing MLC complexity and using larger mean MLC gaps is recommended to improve plan robustness and reduce sensitivity to delivery and modelling errors. For cases in which the calculated dose distribution or dose indices are close to the clinically acceptable limits, this is especially important.

Dose-Volume Tolerance of the Brain and Predictors of Radiation Necrosis After 3-Fraction Radiosurgery for Brain Metastases: A Large Single-Institutional Analysis

PURPOSE

Stereotactic radiosurgery (SRS) is the current standard of care in patients with brain metastases and controlled extracranial disease. Radiation necrosis (RN) is the dose-limiting side effect of SRS, but the dose constraints especially for fractionated SRS remain poorly defined. We assessed the risk of RN after 3-fraction SRS with a goal to identify specific dose-volume constraints associated with grade 3 or higher RN (G3RN).

METHODS AND MATERIALS

A single-institutional retrospective review of patients treated with 3-fraction SRS was performed. The primary endpoint was G3RN, which was defined as severe symptoms with evidence of necrosis on magnetic resonance imaging with perfusion and/or biopsy confirmation. Tissue volume around each target lesion was contoured, and volumetric doses per lesion were recorded. Logistic regression models were used to estimate the relationship between RN and each volumetric dose, and normal tissue complication probability modeling was performed using a modified Lyman-Kutcher-Burman model.

RESULTS

From 2015 to 2021, 434 patients underwent 539 courses of linear accelerator-based SRS; 2518 lesions were treated. Median SRS dose was 24 Gy. Median follow-up after SRS was 7.9 months, and the median overall survival was 9 months. A total of 93 patients (17.2%) and 123 lesions (4.9%) developed any RN. Forty-two patients (7.8%) and 57 lesions (2.3%) developed G3RN. On logistic regression, V20 and V23 were best predictors of any grade RN and G3RN, respectively, with cutoff values of 4 cc, 10 cc, and 20 cc associated with <5%, <7.5%, and <10% risk of any RN, respectively, and V23 < 15 cc associated with <5% risk of G3RN. With constrained optimization of the normal tissue complication probability Lyman-Kutcher-Burman model for G3RN, we obtained a TD50 (uniform dose resulting in a 50% complication risk) of 31.4 Gy (95% CI, 27.8-35.1 Gy).

CONCLUSIONS

In patients receiving 3-fraction SRS, G3RN was seen in 7.8% of patients, and 2.3% of the lesions were treated. V20 and V23 were the most robust dosimetric parameters associated with RN. Further studies evaluating the outcomes and RN in patients treated with fractionated SRS compared with single-fraction SRS are warranted.

Stereotactic Radiosurgery for Women Older than 65 with Breast Cancer Brain Metastases

BACKGROUND

Breast cancer is the second most common cause of brain metastases (BM). Despite increasing incidence of BM in older women, there are limited data on the optimal management of BM in this age group. In this study, we assessed the survival outcomes and treatment patterns of older breast cancer patients ≥65 years old with BM compared to younger patients at our institution.

METHODS

An IRB-approved single-institutional retrospective review of biopsy-proven breast cancer patients with BM treated with 1- to 5-fraction stereotactic radiation therapy (SRS) from 2015 to 2020 was performed. Primary endpoint was intracranial progression-free survival (PFS) defined as the time interval between the end of SRS to the date of the first CNS progression. Secondary endpoints were overall survival (OS) from the end of SRS and radiation treatment patterns. Kaplan-Meier estimates and Cox proportional hazard regression method were used for survival analyses.

RESULTS

A total of 112 metastatic breast cancer patients with BMs were included of which 24 were ≥65 years old and 88 were <65 years old. Median age at RT was 72 years (range 65-84) compared to 52 years (31-64) in younger patients. There were significantly higher number of older women with ER/PR positive disease (75% vs. 49%, p = 0.036), while younger patients were more frequently triple negative (32% vs. 12%, p = 0.074) and HER2 positive (42% vs. 29%, p = 0.3). Treatment-related adverse events were similar in both groups. Overall, 14.3% patients had any grade radiation necrosis (RN) (older vs. young: 8.3% vs. 16%, p = 0.5) while 5.4% had grade 3 or higher RN (0% vs. 6.8%, p = 0.7). Median OS after RT was poorer in older patients compared to younger patients (9.5 months vs. 14.5 months, p = 0.037), while intracranial PFS from RT was similar between the two groups (9.7 months vs. 7.1 months, p = 0.580). On univariate analysis, significant predictors of OS were age ≥65 years old (hazard risk, HR = 1.70, p = 0.048), KPS ≤ 80 (HR = 2.24, p < 0.001), HER2 positive disease (HR = 0.46, p < 0.001), isolated CNS metastatic disease (HR = 0.29, p < 0.001), number of brain metastases treated with RT (HR = 1.06, p = 0.028), and fractionated SRS (HR = 0.53, p = 0.013). On multivariable analysis, KPS ≤ 80, HER2 negativity and higher number of brain metastases predicted for poorer survival, while age was not a significant factor for OS after adjusting for other variables. Patients who received systemic therapy after SRS had a significantly improved OS on univariate and multivariable analysis (HR = 0.32, p < 0.001). Number of brain metastases treated was the only factor predictive of worse PFS (HR = 1.06, p = 0.041), which implies a 6% additive risk of progression for every additional metastasis treated.

CONCLUSIONS

Although older women had poorer OS than younger women, OS was similar after adjusting for KPS, extracranial progression, and systemic therapy; and there was no difference in rates of intracranial PFS, neurological deaths, and LMD in the different age groups. This study suggests that age alone may not play an independent role in treatment-selection and that outcomes for breast cancer patients with BMs and personalized decision-making including other clinical factors should be considered. Future studies are warranted to assess neurocognitive outcomes and other radiation treatment toxicities in older patients.

Comparison of the accuracy of Monte Carlo and Ray Tracing dose calculation algorithms for multiple target brain treatments on CyberKnife

Single plan multiple brain targets (MBT) stereotactic radiosurgery dose difference between Monte Carlo (MC) and Ray Tracing (RT) algorithms has not been studied. A retrospective study and dose measurements were performed to access factors influencing dose differences. Fifty-three RT treatment plans with a total of 209 brain metastases were extracted from Precision Treatment Planning System (TPS). These plans were generated using fixed cones and were delivered using the CyberKnife M6 system. The same treatment plans were recalculated using MC algorithm and keeping the beam parameters unchanged. MC calculated plan parameters were extracted and dose differences were normalised to MC calculated dose. Correlations were investigated. RT and MC calculated off-centre-ratio (OCR) and tissue-phantom-ratio (TPRs) were exported from the TPS and compared with measured. Plans with 5 gross tumour volumes (GTVs) were created on a phantom and dose measured using a CC04 ionisation chamber and microdiamond detector for comparison with calculated doses. Calculated and measured TPR agreed within ± 1% beyond depth of maximum dose. The OCR showed differences up to 4.3% in the penumbra and out-of-field (OOF) regions. Largest RT and MC calculated GTV mean dose difference was - 5.7%. An increase in the number of GTVs and reduction in the geometric separation of metastases were associated with increased differences between RT and MC calculated doses. In conclusion, calculated dose disagreement in MBT depends on the number of GTVs per plan, number of GTVs within a certain separation distance and plan complexity. MC dose calculation is recommended for complex CyberKnife SRS of MBT.

Single-Isocenter Linac-Based Radiosurgery for Brain Metastases with Coplanar Arcs: A Dosimetric and Clinical Analysis

The efficacy of linac-based SRS/fSRS treatments using the single-isocenter coplanar FFF-VMAT technique for both single and multiple BM was investigated. Seventy patients (129 BM) treated with 15-21 Gy in 1 (n = 59) or 27 Gy in 3 (n = 11) fractions were analyzed. For each fraction, plans involving the intra-fractional errors measured by post-treatment CBCT were recalculated. The relationships of BM size, distance-to-isocenter, and barycenter shift with the difference in target coverage were evaluated. Clinical outcomes were assessed using logistic regression and Kaplan-Meier analysis. The median delivery time was 3.78 min (range, 1.83-9.25). The median post-treatment 3D error was 0.5 mm (range, 0.1-2.7) and the maximum rotational error was 0.3° (range, 0.0-1.3). In single BM patients, the GTV D95% was never reduced by >5%, whereas PTV D95% reductions >1% occurred in only 11 cases (29%). In multiple BM patients, dose deficits >5% and >1% occurred in 2 GTV (2%) and 34 PTV (37%), respectively. The differences in target coverage showed a moderate-to-strong correlation only with barycenter shift. Local failure of at least one treated BM occurred in 13 (21%) patients and the 1-year and 2-year local control rates for all lesions were 94% and 90%, respectively. The implemented workflow ensured that the degradation of target and brain dose metrics in delivered treatments was negligible. Along with encouraging clinical outcomes, these findings warrant a reduction in the PTV margins at our institution.

Cost-effectiveness analysis of 3 radiation treatment strategies for patients with multiple brain metastases

Background

Patients diagnosed with multiple brain metastases often survive for less than 2 years, and clinicians must carefully evaluate the impact of interventions on quality of life. Three types of radiation treatment are widely accepted for patients with multiple brain metastases: Whole brain radiation therapy (WBRT), hippocampal avoidance whole-brain radiation therapy (HA-WBRT), and stereotactic radiosurgery (SRS). WBRT, the standard option, is less costly than its newer alternatives but causes more severe adverse effects such as memory loss. To determine whether the cost-effectiveness ratio of HA-WBRT and SRS are superior to WBRT, we used published data to simulate cases of multiple brain metastases.

Methods

We designed a Markov model using data from previously published studies to simulate the disease course of patients with 5 to 15 brain metastases and determine the cost-effectiveness of HA-WBRT and SRS relative to WBRT. Incremental cost-effectiveness ratios (ICERs) were calculated and compared against a willingness-to-pay threshold of $100 000 per quality-adjusted life year.

Results

SRS met the threshold for cost-effectiveness, with ICERs ranging $41 198-$54 852 for patients with 5 to 15 brain metastases; however, HA-WBRT was not cost-effective, with an ICER of $163 915 for all simulated patients. Model results were robust to sensitivity analyses.

Conclusions

We propose that SRS, but not HA-WBRT, should be offered to patients with multiple brain metastases as a treatment alternative to standard WBRT. Incorporating these findings into clinical practice will help promote patient-centered care and decrease national healthcare expenditures, thereby addressing issues around health equity and access to care.

Spatiotemporal fractionation schemes for stereotactic radiosurgery of multiple brain metastases

BACKGROUND

Stereotactic radiosurgery (SRS) is an established treatment for patients with brain metastases (BMs). However, damage to the healthy brain may limit the tumor dose for patients with multiple lesions.

PURPOSE

In this study, we investigate the potential of spatiotemporal fractionation schemes to reduce the biological dose received by the healthy brain in SRS of multiple BMs, and also demonstrate a novel concept of spatiotemporal fractionation for polymetastatic cancer patients that faces less hurdles for clinical implementation.

METHODS

Spatiotemporal fractionation (STF) schemes aim at partial hypofractionation in the metastases along with more uniform fractionation in the healthy brain. This is achieved by delivering distinct dose distributions in different fractions, which are designed based on their cumulative biologically effective dose (BED α / β ${\rm{BED}}_{{{\alpha}}/{{\beta}}}$ ) such that each fraction contributes with high doses to complementary parts of the target volume, while similar dose baths are delivered to the normal tissue. For patients with multiple brain metastases, a novel constrained approach to spatiotemporal fractionation (cSTF) is proposed, which is more robust against setup and biological uncertainties. The approach aims at irradiating entire metastases with possibly different doses, but spatially similar dose distributions in every fraction, where the optimal dose contribution of every fraction to each metastasis is determined using a new planning objective to be added to the BED-based treatment plan optimization problem. The benefits of spatiotemporal fractionation schemes are evaluated for three patients, each with >25 BMs.

RESULTS

For the same tumor BED10 and the same brain volume exposed to high doses in all plans, the mean brain BED2 can be reduced compared to uniformly fractionated plans by 9%-12% with the cSTF plans and by 13%-19% with the STF plans. In contrast to the STF plans, the cSTF plans avoid partial irradiation of the individual metastases and are less sensitive to misalignments of the fractional dose distributions when setup errors occur.

CONCLUSION

Spatiotemporal fractionation schemes represent an approach to lower the biological dose to the healthy brain in SRS-based treatments of multiple BMs. Although cSTF cannot achieve the full BED reduction of STF, it improves on uniform fractionation and is more robust against both setup errors and biological uncertainties related to partial tumor irradiation.

Verification of Qfix Encompass™ couch modeling using the Acuros XB algorithm and HypeArc™ using a high-spatial-resolution two-dimensional diode array

We modeled the Qfix Encompass™ immobilization system and further verified the calculated dose distribution of the AcurosXB (AXB) dose calculation algorithm using SRS MapCHECKⓇ (SRSMC) in the HyperArc™ (HA) clinical plan. An Encompass system with a StereoPHAN™ QA phantom was scanned by SOMATOM go.Sim and imported to an Eclipse™ treatment planning system to create a treatment plan for Encompass modeling. The Encompass modeling was performed in the StereoPHAN with a pinpoint ion chamber for 6 MV and 6 MV flattening filter free (6 MV FFF), and 2 × 2 cm2, 4 × 4 cm2, and 6 × 6 cm2 irradiation field sizes. The dose calculation algorithm used was AXB ver. 15.5 with a 1.0 mm calculation grid size. The Hounsfield unit (HU) values of the Encompass modeling were set to 400, -100, -200, and -300 for Encompass, and -400, -600, -700, and -800 for the Encompass base. We evaluated the dose distribution after Encompass modeling by SRSMC using gamma analysis in 12 patients. We adopted HU values of -200 for Encompass, -800 for Encompass base for 6 MV, and -200 for Encompass and -700 for Encompass. Base for 6 MV FFF was adopted as the HU values for the Encompass modeling based on the measurement results. The proposed Encompass modeling resulted in a mean pass rate evaluation >98% for both 6 MV and 6 MV FFF when the 1%/1 mm criterion was used, demonstrating that the proposed HU value can be adopted to calculate more accurate dose distributions.

A novel weight optimized dynamic conformal arcs with TrueBeam™ Linac for very small tumors (≤1 cc) with single isocenter of multiple brain metastases (2≤, ≥4) in stereotactic radiosurgery: A comparison with volumetric modulated arc therapy

Introduction

We evaluated whether improved increase delivery efficiency of weight optimized dynamic conformal arc (WO-DCA) therapy in comparison to volumetric modulated arc therapy (VMAT) with single isocenter for SRS treatment of very small volume and multiple brain metastases (BMs).

Materials and Methods

20 patients having a less than 1 cc volume and 2≤, ≥4 of multiple BMs, redesigned for 20 Gy in 1 fraction using WO-DCA and VMAT techniques with double full coplanar and three partial noncoplanar arcs. Plan qualities were compared using tumor coverage, conformity index (CI), gradient index (GI), V4Gy, V10Gy, and V12Gy volumes of brain, monitor units (MUs), and percent of quality assurance pass rate (QA%).

Results

Both techniques satisfied clinical requirements in coverage and CI. VMAT had a significantly higher MU and mean GI than WO-DCA (for MUs; 2330 vs. 1991; P < 0.001, and for GI; 4.72 vs. 3.39; P < 0.001). WO-DCA was found significantly lower V4Gy (171.11 vs. 232.80 cm3, P < 0.001), V10Gy (25.82 vs. 29.71 cm3, P < 0.05), and V12Gy (14.35 vs. 17.28 cm3, P < 0.05) volumes than VMAT. WO-DCA was associated with markedly increase QA pass rates for all plans (97.65% vs. 92.64%, P < 0.001).

Conclusions

WO-DCA may be the first choice compared to the VMAT in reducing the dose in the brain and minimizing small-field dosimetric errors for very small SRS treatment of brain metastases in the range of ≤ 1 cc and 2≤, ≥4.

Optimization of isocenter position for multiple brain metastases single-isocenter stereotactic radiosurgery to minimize dosimetric variations due to rotational uncertainty

PURPOSE

This paper studied a novel calculation framework that can determine the optimal value isocenter position of single isocenter SRS treatment plan for multiple brain metastases, in order to minimize the dosimetric variations caused by rotational uncertainty.

MATERIALS AND METHODS

21 patients with 2-4 GTVswho received SRS treatment for multiple brain metastases in our institution were selected for the retrospective study. The PTVwas obtained by expanding GTV 1 mm isotropic margin. We studied a stochastic optimization framework, which determined the optimal value isocenter location by maximizing the average target dose coverageC_target,meanwith a rotation error of no more than 1°. We evaluated the performance of the optimal isocenter by comparing theC_target,meanand average dice similarity coefficient (DSC)with the optimal value and the center of mass (CM) respectively as the treatment isocenter. The extra PTV margin to achieve 100% target dose coverage was calculated by our framework.

RESULTS

Compared to the CM method, the optimal value isocenter method increased the averageC_target,meanof all targets from 97.0% to 97.7%and the average DSC from 0.794to 0.799. Throughout all the cases, the average extra PTV margin to obtain full target dose coverage was 0.7 mmwhen using the optimal value isocenter as the treatment isocenter.

CONCLUSION

We studied a novel computational framework using stochastic optimization to determine the optimal isocenter position of SRS treatment plan for multiple brain metastases. At the same time, our framework gave the extra PTV margin to obtain full target dose coverage.

Stereotactic Radiosurgery as Treatment for Brain Metastases: An Update

Stereotactic radiosurgery (SRS) is a mainstay treatment option for brain metastasis (BM). While guidelines for SRS use have been outlined by professional societies, consideration of these guidelines should be weighed in the context of emerging literature, novel technology platforms, and contemporary treatment paradigms. Here, we review recent advances in prognostic scale development for SRS-treated BM patients and survival outcomes as a function of the number of BM and cumulative intracranial tumor volume. Focus is placed on the role of stereotactic laser thermal ablation in the management of BM that recur after SRS and the management of radiation necrosis. Neoadjuvant SRS prior to surgical resection as a means of minimizing leptomeningeal spread is also discussed.

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance

INTRODUCTION

Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment.

MATERIALS AND METHODS

A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac.

RESULTS

Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10^-6 ) and 20.6% (p = 3.0×10^-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r² = 0.866 when fit to plans with a single lesion).

CONCLUSIONS

Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.

Dosimetric Impact of Lesion Number, Size, and Volume on Mean Brain Dose with Stereotactic Radiosurgery for Multiple Brain Metastases

We evaluated the effect of lesion number and volume for brain metastasis treated with SRS using GammaKnife^® ICON™ (GK) and CyberKnife^® M6™ (CK). Four sets of lesion sizes (<5 mm, 5-10 mm, >10-15 mm, and >15 mm) were contoured and prescribed a dose of 20 Gy/1 fraction. The number of lesions was increased until a threshold mean brain dose of 8 Gy was reached; then individually optimized to achieve maximum conformity. Across GK plans, mean brain dose was linearly proportional to the number of lesions and total GTV for all sizes. The numbers of lesions needed to reach this threshold for GK were 177, 57, 29, and 10 for each size group, respectively; corresponding total GTVs were 3.62 cc, 20.37 cc, 30.25 cc, and 57.96 cc, respectively. For CK, the threshold numbers of lesions were 135, 35, 18, and 8, with corresponding total GTVs of 2.32 cc, 12.09 cc, 18.24 cc, and 41.52 cc respectively. Mean brain dose increased linearly with number of lesions and total GTV while V8 Gy, V10 Gy, and V12 Gy showed quadratic correlations to the number of lesions and total GTV. Modern dedicated intracranial SRS systems allow for treatment of numerous brain metastases especially for ≤10 mm; clinical evidence to support this practice is critical to expansion in the clinic.

Single isocenter stereotactic irradiation for multiple brain metastases: current situation and prospects

The prognosis of patients with brain metastases has dramatically improved, and long-term tumor control and reduction of the risk of late toxicities, including neurocognitive dysfunction, are important for patient quality of life. Stereotactic irradiation for multiple brain metastases, rather than whole-brain radiotherapy, can result in high local control rate with low incidence of neurocognitive deterioration and leukoencephalopathy. Recent advances in radiotherapy devices, treatment-planning systems, and image-guided radiotherapy can realize single isocenter stereotactic irradiation for multiple brain metastases (SI-STI-MBM), in which only one isocenter is sufficient to treat multiple brain metastases simultaneously. SI-STI-MBM has expanded the indications for linear accelerator-based stereotactic irradiation and considerably reduced patient burden. This review summarizes the background, methods, clinical outcomes, and specific consideration points of SI-STI-MBM. In addition, the prospects of SI-STI-MBM are addressed.

Radiosurgery for multiple brain metastases using volumetric modulated arc therapy: a single institutional series

Background

Patients with brain metastases (BM) live longer due to improved diagnosis and oncologic treatments. The association of volumetric modulated arc therapy (VMAT) and image-guided radiation therapy (IGRT) with brain radiosurgery (SRS) allows complex dose distributions and faster treatment delivery to multiple lesions.

Materials and methods

This study is a retrospective analysis of SRS for brain metastasis using VMAT. The primary endpoints were local disease-free survival (LDFS) and overall survival (OS). The secondary outcomes were intracranial disease-free survival (IDFS) and meningeal disease-free survival (MDFS).

Results

The average number of treated lesions was 5.79 (range: 2-20) per treatment in a total of 113 patients. The mean prescribed dose was 18 Gy (range: 12-24 Gy). The median LDFS was 46 months. The LDFS in 6, 12, and 24 months was for 86%, 79%, and 63%, respectively. Moreover, brain progression occurred in 50 patients. The median overall survival was 47 months. The OS in 75%, 69%, and 61% patients was 6, 12, and 24 months, respectively. IDFS was 6 and 24 months in 35% and 14% patients, respectively. The mean MDFS was 62 months; it was 6 and 24 months for 87% and 83% of patients. Acute severe toxicity was relatively rare. During follow-up, the rates of radionecrosis and neurocognitive impairment were low (10%).

Conclusion

The use of VMAT-SRS for multiple BM was feasible, effective, and associated with low treatment-related toxicity rates. Thus, treatment with VMAT is a safe technique to plan to achieve local control without toxicity.

Implementing corrections of isocentric shifts for the stereotactic irradiation of cerebral targets: Clinical validation

Purpose: Any Linac will show geometric imprecisions, including non‐ideal alignment of the gantry, collimator and couch axes, and gantry sag or wobble. Their angular dependence can be quantified and resulting changes of the dose distribution predicted (Wack, JACMP 20(5), 2020). We analyzed whether it is feasible to correct geometric shifts during treatment planning. The successful implementation of such a correction procedure was verified by measurements of different stereotactic treatment plans.

Methods: Isocentric shifts were quantified for two Elekta Synergy Agility Linacs using the QualiForMed ISO‐CBCT+ module, yielding the shift between kV and MV isocenters, the gantry flex and wobble as well as the positions of couch and collimator rotation axes. Next, the position of each field's isocenter in the Pinnacle treatment planning system was adjusted accordingly using a script. Fifteen stereotactic treatment plans of cerebral metastases (0.34 to 26.53 cm³) comprising 9–11 beams were investigated; 54 gantry and couch combinations in total. Unmodified plans and corrected plans were measured using the Sun Nuclear SRS‐MapCHECK with the Stereophan phantom and evaluated using gamma analysis.

Results: Geometric imprecisions, such as shifts of up to 0.8 mm between kV and MV isocenter, a couch rotation axis 0.9 mm off the kV isocente,r and gantry flex with an amplitude of 1.1 mm, were found. For eight, mostly small PTVs D98 values declined more than 5% by simulating these shifts. The average gamma (2%/2 mm, absolute, global, 20% threshold) was reduced from 0.53 to 0.31 (0.32 to 0.30) for Linac 1 (Linac 2) when including the isocentric corrections. Thus, Linac 1 reached the accuracy level of Linac 2 after correction.

Conclusion: Correcting for Linac geometric deviations during the planning process is feasible and was dosimetrically validated. The dosimetric impact of the geometric imperfections can vary between Linacs and should be assessed and corrected where necessary.

Managing Central Nervous System Spread of Lung Cancer: The State of the Art

Brain metastases (BrM) are common in both non-small-cell lung cancer and small-cell lung cancer. Substantial progress in BrM management has occurred in the past decade related to advances in both radiation and medical oncology. Recent and ongoing radiation trials have focused on increasing the candidacy for focal therapy of BrM with stereotactic radiosurgery; reducing the toxicity and improving patient selection for whole brain radiotherapy; and, in small-cell lung cancer, evaluating brain magnetic resonance imaging surveillance without prophylactic cranial irradiation, hippocampal avoidance in prophylactic cranial irradiation and whole brain radiotherapy, and the role of upfront stereotactic radiosurgery for BrM. In medical oncology, the development of multiple tyrosine kinase inhibitors with encouraging CNS activity and emerging data on the CNS activity of immune checkpoint inhibitors in some patients have opened the door to novel systemic and multidisciplinary treatment strategies for the management of BrM. Future research will focus on more robust characterizations of the CNS activity of targeted therapy and immunotherapies, as well as optimal integration and patient selection for multidisciplinary strategies involving CNS-active drugs, radiation therapy, and CNS surveillance.

Outcomes in Patients With 4 to 10 Brain Metastases Treated With Dose-Adapted Single-Isocenter Multitarget Stereotactic Radiosurgery: A Prospective Study

Purpose

To examine the effectiveness and safety of single-isocenter multitarget stereotactic radiosurgery using a volume-adapted dosing strategy in patients with 4 to 10 brain metastases.

Methods and Materials

Adult patients with 4 to 10 brain metastases were eligible for this prospective trial. The primary endpoint was overall survival. Secondary endpoints were local recurrence, distant brain failure, neurologic death, and rate of adverse events. Exploratory objectives were neurocognition, quality of life, dosimetric data, salvage rate, and radionecrosis. Dose was prescribed in a single fraction per RTOG 90-05 or as 5 Gy × 5 fractions for lesions ≥3 cm diameter, lesions involving critical structures, or single-fraction brain V_12Gy >20 mL.

Results

Forty patients were treated with median age of 61 years, Karnofsky performance status 90, and 6 brain metastases. Twenty-two patients survived longer than expected from the time of protocol SRS, with 1 living patient who has not reached that milestone. Median overall survival was 8.1 months with a 1-year overall survival of 35.7%. The 1-year local recurrence rate was 5% (10 of 204 of evaluable lesions) in 12.5% (4 of 32) of the patients. Distant brain failure was observed in 19 of 32 patients with a 1-year rate of 35.8%. Grade 1-2 headache was the most common complaint, with no grade 3-5 treatment-related adverse events. Radionecrosis was observed in only 5 lesions, with a 1-year rate of 1.5%. Rate of neurologic death was 20%. Neurocognition and quality of life did not significantly change 3 months after SRS compared with pretreatment.

Conclusions

These results suggest that volume-adapted dosing single-isocenter multitarget stereotactic radiosurgery is an effective and safe treatment for patients with 4 to 10 brain metastases.

Advances in radiotherapy for brain metastases

As novel systemic therapies yield improved survival in metastatic cancer patients, the frequency of brain metastases continues to increase. Over the years, management strategies have continued to evolve. Historically, stereotactic radiosurgery has been used as a boost to whole-brain radiotherapy (WBRT) but is increasingly being used as a replacement for WBRT. Given its capacity to treat both macro- and micro-metastases in the brain, WBRT has been an important management strategy for years, and recent research has identified technologic and pharmacologic approaches to delivering WBRT more safely. In this review, we outline the current landscape of radiotherapeutic treatment options and discuss approaches to integrating radiotherapy advances in the contemporary management of brain metastases.

Optimization of treatment isocenter location in single-isocenter LINAC-based stereotactic radiosurgery for management of multiple brain metastases

PURPOSE

Single-isocenter linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) has become a promising treatment technique for the management of multiple brain metastases. Because of the high prescription dose and steep dose gradient, SRS plans are sensitive to geometric errors, resulting in loss of target coverage and suboptimal local tumor control. Current planning techniques rely on adding a uniform and isotropic setup margin to all gross tumor volumes (GTVs) to account for rotational uncertainties. However, this setup margin may be insufficient, since the magnitude of rotational uncertainties varies and is dependent upon the distance between a GTV and the isocenter. In this study, we designed a framework to determine the optimal isocenter of a single-isocenter SRS plan for multiple brain metastases using stochastic optimization to mitigate potential errors resulting from rotational uncertainties.

METHODS

Planning target volumes (PTVs), defined as GTVs plus a 1-mm margin following common SRS planning convention, were assumed to be originally treated with a prescription dose and therefore covered by the prescription isodose cloud. The dose distribution, including the prescription isodose, was considered invariant assuming small rotations throughout the study. A stochastic optimization scheme was developed to determine the location of the optimal isocenter, so that the prescription dose coverage of rotated GTVs, equivalent to the intersecting volumes between the rotated GTVs and original PTVs, was maximized for any random small rotations about the isocenter. To evaluate the coverage of GTVs, the expected V 100 % undergoing random rotations was approximated as the sample average V 100 % undergoing a predetermined number of rotations. The expected V 100 % of each individual GTV and total GTVs was then compared between the plans using the optimal isocenter and the center-of-mass (CoM), respectively.

RESULTS

Twenty-two patients previously treated for multiple brain metastases in a single institute were included in this retrospective study. Each patient was initially treated for more than three brain metastases (mean: 7.6; range: 3-15) with the average GTV volume of 0.89 cc (range: 0.03-11.78 cc). The optimal isocenter found for each patient was significantly different from the CoM, with the average Euclidean distance between the optimal isocenter and the CoM being 4.36 ± 2.59 cm. The dose coverage to GTVs was also significantly improved (paired t-test; p < 0.001) when the optimal isocenter was used, with the average V 100 % of total GTVs increasing from 87.1% (standard deviation as std: 11.7%; range: 39.9-98.2%) to 94.2% (std: 5.4%; range: 77.7-99.4%). The volume of a GTV was positively correlated with the expected V 100 % regardless of the isocenter used (Spearman coefficient: ρ = 0.66 ; p < 0.001). The distance between a GTV and the isocenter was negatively correlated with the expected V 100 % when the CoM was used ( ρ = - 0.21 ; p = 0.004), however no significant correlation was found when the optimal isocenter was used ( ρ = - 0.11 ; p = 0.137).

CONCLUSION

The proposed framework provides an effective approach to determine the optimal isocenter of single-isocenter LINAC-based SRS plans for multiple brain metastases. The implementation of the optimal isocenter results in SRS plans with consistently higher target coverage despite potential rotational uncertainties, and therefore significantly improves SRS plan robustness against random rotational uncertainties.

Predicting the effect of indirect cell kill in the treatment of multiple brain metastases via single-isocenter/multitarget volumetric modulated arc therapy stereotactic radiosurgery

Purpose

Due to spatial uncertainty, patient setup errors are of major concern for radiosurgery of multiple brain metastases (m‐bm) when using single‐isocenter/multitarget (SIMT) volumetric modulated arc therapy (VMAT) techniques. However, recent clinical outcome studies show high rates of tumor local control for SIMT‐VMAT. In addition to direct cell kill (DCK), another possible explanation includes the effects of indirect cell kill (ICK) via devascularization for a single dose of 15 Gy or more and by inducing a radiation immune intratumor response. This study quantifies the role of indirect cell death in dosimetric errors as a function of spatial patient setup uncertainty for stereotactic treatments of multiple lesions.

Material and Methods

Nine complex patients with 61 total tumors (2‐16 tumors/patient) were planned using SIMT‐VMAT with geometry similar to HyperArc with a 10MV‐FFF beam (2400 MU/min). Isocenter was placed at the geometric center of all tumors. Average gross tumor volume (GTV) and planning target volume (PTV) were 1.1 cc (0.02–11.5) and 1.9 cc (0.11–18.8) with an average distance to isocenter of 5.4 cm (2.2–8.9). The prescription was 20 Gy to each PTV. Plans were recalculated with induced clinically observable patient setup errors [±2 mm, ±2^o] in all six directions. Boolean structures were generated to calculate the effect of DCK via 20 Gy isodose volume (IDV) and ICK via 15 Gy IDV minus the 20 Gy IDV. Contributions of each IDV to the PTV coverage were analyzed along with normal brain toxicity due to the patient setup uncertainty. Induced uncertainty and minimum dose covering the entire PTV were analyzed to determine the maximum tolerable patient setup errors to utilize the ICK effect for radiosurgery of m‐bm via SIMT‐VMAT.

Results

Patient setup errors of 1.3 mm /1.3° in all six directions must be maintained to achieve PTV coverage of the 15 Gy IDV for ICK. Setup errors of ±2 mm/2° showed clinically unacceptable loss of PTV coverage of 29.4 ± 14.6% even accounting the ICK effect. However, no clinically significant effect on normal brain dosimetry was observed.

Conclusions

Radiosurgery of m‐bm using SIMT‐VMAT treatments have shown positive clinical outcomes even with small residual patient setup errors. These clinical outcomes, while largely due to DCK, may also potentially be due to the ICK. Potential mechanisms, such as devascularization and/or radiation‐induced intratumor immune enhancement, should be explored to provide a better understanding of the radiobiological response of stereotactic radiosurgery of m‐bm using a SIMT‐VMAT plan.

Dosimetric comparison of multiple vs single isocenter technique for linear accelerator-based stereotactic radiosurgery: The Importance of the six degree couch

PURPOSE

Single isocenter technique (SIT) for linear accelerator-based stereotactic radiosurgery (SRS) is feasible. However, SIT introduces the potential for rotational error which can lead to geographical miss. Additional planning treatment volume (PTV) margin is required when using SIT. With the six degrees of freedom (6DoF) couch, rotational error can be minimized. We sought to evaluate the effect of the 6DoF couch on the dosimetry of patients with multiple brain metastases treated with SIT.

MATERIALS AND METHODS

Ten consecutive patients treated with SRS to ≥3 metastases were identified. Original treatments had MIT plans (MITP). The lesions were replanned using SIT. Lesions 5-10 cm from isocenter had an additional 1mm of margin. Patients were replanned with these additional margins to account for inability to correct rotational error (SITPM). Multiple dosimetric variables and time metrics were evaluated. Dosimetry planning time (DPT) and patient treatment time (PTT) were evaluated. Statistics were calculated using the Wilcoxon signed-rank test.

RESULTS

A total of 73 brain metastases receiving SRS, to a median of 6 lesions per patient, were identified. MITPs treated 73 lesions with 63 isocenters. On average, MITPs had a 19.2% higher brain V12 than SITPs (P = 0.017). For creation of SITPM, 30 lesions required 1 mm of additional margin, while none required 2 mm of margin. This increased V12 by 47.8% on average per patient (P = 0.008) from SITP to SITPM. DPT was 5.5 hours for SITP, while median for MITP was 12.5 hours (P = 0.005) PTT was 30 minutes for SITP, while median for MITP was 144 minutes (P = 0.005).

CONCLUSIONS

SITPs are comparable to MITPs if rotational error can be corrected with the use of a 6DoF couch. Increasing margin to account for rotational error leads to a nearly 50% increase in V12, which could result in higher rates of radiation necrosis. Time savings are significant using SIT.

A case of multiple synchronously diagnosed brain metastases from alveolar soft part sarcoma without concurrent lung involvement

Background:

Alveolar soft part sarcoma (ASPS) is a rare soft-tissue sarcoma with a propensity for early hematogenous dissemination to the lungs and frequent brain metastasis. The development of lung metastasis almost invariably precedes intracranial involvement. There are no previously reported cases in which a patient was synchronously diagnosed with ASPS and multiple brain metastasis without lung involvement.

Case Description:

A 29-year-old gentleman was found to have three intracranial lesions following the onset of generalized seizures. Staging studies identified a soft-tissue mass in the left thigh and an adjacent femoral lesion. Biopsy of the soft-tissue mass was consistent with ASPS. The patient then underwent neoadjuvant stereotactic radiotherapy to all three brain lesions, followed by en bloc resection of the dominant lesion. The patient was then started on a programmed death-ligand 1 (PD-L1) inhibitor. Subsequent surgical resection of the primary lesion and femur metastasis demonstrates a histopathologic complete response of the bony metastasis and partial response of the primary lesion. At present, the patient has received 14 cycles of atezolizumab without recurrence of the primary or bony lesions and the irradiated intracranial disease has remained stable without recurrence of the resected dominant lesion.

Conclusion:

While intracranial involvement is relatively common in ASPS, a case with multiple, synchronously diagnosed brain metastasis without concurrent lung metastasis has not been described. The presented case discusses the safety and efficacy of aggressive management of intracranial disease in the setting of atezolizumab. Prospective evaluation of the efficacy of checkpoint inhibitors and the prognostic value of PD-L1 expression in ASPS with brain metastasis are necessary.

Simultaneous stereotactic radiosurgery of multiple brain metastases using single-isocenter dynamic conformal arc therapy: a prospective monocentric registry trial

BACKGROUND

Single-isocenter dynamic conformal arc (SIDCA) therapy is a technically efficient way of delivering stereotactic radiosurgery (SRS) to multiple metastases simultaneously. This study reports on the safety and feasibility of linear accelerator (LINAC) based SRS with SIDCA for patients with multiple brain metastases.

METHODS

All patients who received SRS with this technique between November 2017 and June 2019 within a prospective registry trial were included. The patients were irradiated with a dedicated planning tool for multiple brain metastases using a LINAC with a 5 mm multileaf collimator. Follow-up was performed every 3 months, including clinical and radiological examination with cranial magnetic resonance imaging (MRI). These early data were analyzed using descriptive statistics and the Kaplan-Meier method.

RESULTS

A total of 65 patients with 254 lesions (range 2-12) were included in this analysis. Median beam-on time was 23 min. The median follow-up at the time of analysis was 13 months (95% CI 11.1-14.9). Median overall survival and median intracranial progression-free survival was 15 months (95% CI 7.7-22.3) and 7 months (95% CI 3.9-10.0), respectively. Intracranial and local control after 1 year was 64.6 and 97.5%, respectively. During follow-up, CTCAE grade I adverse effects (AE) were experienced by 29 patients (44.6%; 18 of them therapy related, 27.7%), CTCAE grade II AEs by four patients (6.2%; one of them therapy related, 1.5%), and CTCAE grade III by three patients (4.6%; none of them therapy related). Two lesions (0.8%) in two patients (3.1%) were histopathologically proven to be radiation necrosis.

CONCLUSION

Simultaneous SRS using SIDCA seems to be a feasible and safe treatment for patients with multiple brain metastases.

Distance to isocenter is not associated with an increased risk for local failure in LINAC-based single-isocenter SRS or SRT for multiple brain metastases

PURPOSE

To evaluate the impact of the distance between treatment isocenter and brain metastases on local failure in patients treated with a frameless linear-accelerator-based single-isocenter volumetric modulated arc (VMAT) SRS/SRT for multiple brain metastases.

METHODS AND MATERIALS

Patients treated with SRT for brain metastases (BM) between April 2014 and May 2019 were included in this retrospective study. BM treated with a single-isocenter multiple-target (SIMT) SRT were evaluated for local recurrence-free intervals in dependency to their distance to the treatment isocenter. A Cox-regression model was used to investigate different predictor variables for local failure. Results were compared to patients treated with a single-isocenter-single-target (SIST) approach.

RESULTS

In total 315 patients with a cumulative number of 1087 BM were analyzed in this study of which 140 patients and 708 BM were treated with SIMT SRS/SRT. Median follow-up after treatment was 13.9 months for SIMT approach and 11.9 months for SIST approach. One-year freedom from local recurrence was 87% and 94% in the SIST and SIMT group, respectively. Median distance to isocenter (DTI) was 4.7 cm (range 0.2-10.5) in the SIMT group. Local recurrence-free interval was not associated with the distance to the isocenter in univariable or multivariable Cox-regression analysis. Multivariable analysis revealed only volume as an independent significant predictor for local failure (p-value <0.05).

CONCLUSION

SRS/SRT using single-isocenter VMAT for multiple targets achieved high local metastases control rates irrespective of distance to the isocenter, supporting efficacy of single-isocenter stereotactic radiation therapy for multiple brain metastases.

Management of multiple brain metastases via dual-isocenter VMAT stereotactic radiosurgery

Single-isocenter volumetric modulated arc therapy (VMAT) stereotactic radiosurgery (SRS) techniques to treat multiple brain metastases simultaneously can significantly improve treatment delivery efficiency, patient compliance, and clinic workflow. However, due to large number of brain metastases sharing the same MLC pair causing island blocking, there is higher low- and intermediate-dose spillage to the normal brain and higher dose to organs-at-risk (OAR). To minimize this problem and improve plan quality, this study proposes a dual-isocenter planning strategy that groups lesions based on hemisphere location (left vs right sided) in the brain parenchyma, providing less island blocking reducing the MLC travel distance. This technique offers simplified planning while also increasing patient comfort and compliance by allowing for large number of brain metastases to be treated in 2 groups. Seven complex patients with 5 to 16 metastases (64 total) were planned with a single-isocenter VMAT-SRS technique using a 10MV-FFF beam with a prescription of 20 Gy to each lesion. The isocenter was placed at the approximate geometric center of the targets. Each patient was replanned using the dual-isocenter approach, generating 2 plans and placing each isocenter at the approximate geometric center of the combined targets of each side with corresponding non-coplanar partial arcs. Compared to single-isocenter VMAT, dual-isocenter VMAT plans provided similar target coverage and dose conformity with less spread of intermediate dose to normal brain with reduction of dose to OAR. Reduction in total monitor units and beam on time was observed, but due to the second isocenter setup and verification, overall treatment time was increased. Dual-isocenter VMAT-SRS planning for multiple brain metastases is a simplified approach that provides superior treatment options for patient compliance who may not tolerate longer traditional treatment times as with individual isocenters to each target. This planning technique significantly reduces the amount of low- and intermediate-dose spillage, further sparing OAR and normal brain, potentially improving target accuracy though localization of left vs right-sided tumors for each isocenter set up.

Whole-Brain Radiation Therapy Versus Stereotactic Radiosurgery for Cerebral Metastases

Whole-brain radiation therapy (WBRT) was frequently used to treat brain metastases in the past. Stereotactic radiosurgery (SRS) is now generally preferred to WBRT for patients with limited brain metastases. SRS can also be used to treat extensive brain metastases (>10-15 metastases), and clinical trials are currently comparing WBRT with SRS for extensive disease. SRS may allow for an increased risk of radiation necrosis or leptomeningeal disease dissemination after treatment. Preoperative SRS and multifraction radiotherapy decrease the risk of these side effects and may soon become standard of care. Combining SRS with immune checkpoint inhibitors may improve patient outcomes.

Initial Experience With Single-Isocenter Radiosurgery to Target Multiple Brain Metastases Using an Automated Treatment Planning Software: Clinical Outcomes and Optimal Target Volume Margins Strategy

Purpose

Our purpose was to assess the clinical outcomes and target positioning accuracy of frameless linear accelerator single-isocenter multiple-target (SIMT) dynamic conformal arc (DCA) stereotactic radiosurgery (SRS) for multiple brain metastases (BM).

Methods and Materials

Between October 2016 and September 2018, 31 consecutive patients ≥18 years old with 204 BM <3 cm in maximum size receiving SIMT DCA SRS were retrospectively evaluated. All plans were created using a dedicated automated treatment planning software (Brainlab, Munich, Germany), and treatments were performed with a Truebeam STx or a Novalis Tx (Brainlab and Varian Medical Systems, CA). The accuracy of setup and interfraction patient repositioning was assessed by Brainlab ExacTrac radiograph 6-dimensional image system and the risk of compromised target dose coverage evaluated. Brain control and overall survival were estimated by Kaplan-Meier method calculated from the time of SRS.

Results

Fourteen patients were treated for 4 to 6 and 17 patients for 7 to 10 BM. The mean gross tumor volume (GTV) was 0.65 cm³ and the mean planning target volume (PTV) was 0.89 cm³. Mean V95 (the volume of the PTV covered by 95% of the prescription dose) and D95 (the prescription dose covering 95% of the PTV) were 99.5% and 21.1 Gy, respectively. With a median clinical follow-up of 11 months (range, 4-26 months), the 1-year survival was 68% and local control was 89%. As a consequence of plan isocenter residual errors, a loss of target coverage, defined as V95 < 95%, occurred in 28 PTVs (10 patients); using a 1 mm GTV-to-PTV margin, adequate dose coverage was maintained for all lesions.

Conclusions

SIMT DCA SRS represents a fast and effective approach for patients with up to 10 BM. The dosimetric effects of residual set-up and intrafraction positioning errors are modest, although a GTV-to-PTV margin of 1 mm is recommended.

Klippel-Trenaunay-Weber Syndrome Associated with Multiple Cerebral Arteriovenous Malformations: Usefulness of Gamma Knife Stereotactic Radiosurgery in This Syndrome

BACKGROUND

Klippel-Trenaunay-Weber syndrome (KTWS) is characterized by the presence of a combined vascular malformation of capillaries, veins, and lymphatic vessels; congenital venous abnormalities; and limb hypertrophy. Its association with neurovascular abnormalities is infrequent, and the presence of intracranial arteriovenous malformations (AVMs) is extremely rare.

CASE DESCRIPTION

We report a case of a 48-year-old male diagnosed with KTWS who spontaneously presented with a cerebral hemorrhage. Computed tomography scan and angio-computed tomography studies revealed bleeding associated with AVM rupture. In the conventional arteriography study, 10 small (<1 cm) AVMs were observed. The patient presented a good clinical recovery. These multiple small lesions were not considered susceptible to surgical or endovascular treatments. Therefore all lesions were treated with Gamma Knife stereotactic radiosurgery since it attains the highest dose drop and minimal irradiation of the healthy parenchyma. One year after the treatment, the lesions have shrunk.

CONCLUSIONS

Cerebral AVMs are extremely rare in KTWS cases; however, their presence can have serious consequences if they are treated. We find it advisable to include brain imaging tests, such as nuclear magnetic resonance imaging, to diagnose and monitor KTWS. Furthermore, a Gamma Knife may be useful when multiple AVMs are present.