Evaluation of 2D ion chamber arrays for patient specific quality assurance using a static phantom at a 0.35 T MR-Linac

INTRODUCTION

Patient specific quality assurance (QA) in MR-Linacs can be performed with MR-compatible ion chamber arrays. However, the presence of a static magnetic field can alter the angular response of such arrays substantially. This works investigates the suitability of two ion chamber arrays, an air-filled and a liquid-filled array, for patient specific QA at a 0.35 T MR-Linac using a static phantom.

METHODS

In order to study the angular response, the two arrays were placed in a static, solid phantom and irradiated with 9.96 × 9.96 cm2 fields every 10° beam angle at a 0.35 T MR-Linac. Measurements were compared to the TPS calculated dose in terms of gamma passing rate and relative dose to the central chamber. 20 patient specific quality assurance plans were measured using the liquid-filled array.

RESULTS

The air-filled array showed asymmetric angular response changes of central chamber dose of up to 18% and down to local 3 mm / 3% gamma rates of 20%, while only minor differences within 3% (excluding parallel irradiation and beams through the couch edges) were found for the liquid-filled ion chamber array without rotating the phantom. Patient plan QA using the liquid-filled array yielded a median local 3 mm / 3% 3D gamma passing rate of 99.8% (range 96.9%-100%).

CONCLUSION

A liquid-filled ionization chamber array in combination with a static phantom can be used for efficient patient specific plan QA in a single measurement set-up in a 0.35 T MR-Linac, while the air-filled ion chamber array phantom shows large angular response changes and has its limitations regarding patient specific QA measurements.

Accumulated dose implications from systematic dose-rate transients in gated treatments with Viewray MRIdian accelerators

The combination of magnetic resonance (MR) imaging and linear accelerators (linacs) into MR-Linacs enables continuous MR imaging and advanced gated treatments of patients. Previously, a dose-rate transient (∼8% reduced dose rate during the initial 0.5 s of each beam) was identified for a Viewray MRIdian MR-Linac (Klavsenet al2022Radiation Measurement106759). Here, the dose-rate transient is studied in more detail at four linacs of the same type at different hospitals. The implications of dose-rate transients were examined for gated treatments. The dose-rate transients were investigated using dose-per pulse measurements with organic plastic scintillators in three experiments: (i) A gated treatment with the scintillator placed in a moving target in a dynamic phantom, (ii) a gated treatment with the same dynamic conditions but with the scintillator placed in a stationary target, and (iii) measurements in a water-equivalent material to examine beam quality deviations at a dose-per-pulse basis. Gated treatments (i) compared with non-gated treatments with a static target in the same setup showed a broadening of accumulated dose profiles due to motion (dose smearing). The linac with the largest dose-rate transient had a reduced accumulated dose of up to (3.1 ± 0.65) % in the center of the PTV due to the combined dose smearing and dose-rate transient effect. Dose-rate transients were found to vary between different machines. Two MR-Linacs showed initial dose-rate transients that could not be identified from conventional linearity tests. The source of the transients includes an initial change in photon fluence rate and an initial change in x-ray beam quality. For gated treatments, this caused a reduction of more than 1% dose delivered at the central part of the beam for the studied, cyclic-motion treatment plan. Quality assurance of this effect should be considered when gated treatment with the Viewray MRIdian is implemented clinically.

Leaf-individual calibration for a double stack multileaf collimator in photon radiotherapy

Background and Purpose

In online adaptive stereotactic body radiotherapy treatments, linear accelerator delivery accuracy is essential. Recently introduced double stack multileaf collimators (MLCs) have new facets in their calibration. We established a radiation-based leaf-individual calibration (LIMCA) method for double stack MLCs.

Materials and Methods

MLC leaf positions were evaluated from four cardinal angles with test patterns at measurement positions throughout the radiation field on EBT3 radiochromic film for each single stack. The accuracy of the method and repeatability of the results were assessed. The effect of MLC positioning errors was characterized for a measured output factor curve and a clinical patient plan.

Results

All positions in the motor step - position calibration file were optimized in the established LIMCA method. The resulting double stack mean accuracy for all angles was 0.2 ± 0.1 mm for X1 (left bank) and 0.2 ± 0.2 mm for X2 (right bank). The accuracy of the leaf position evaluation was 0.2 mm (95% confidence level). The MLC calibration remained stable over four months. Small MLC leaf position errors (e.g. 1.2 mm field size reduction) resulted in important dose errors (-5.8 %) for small quadratic fields of 0.83 × 0.83 cm2. Single stack position accuracy was essential for highly modulated treatment plans.

Conclusions

LIMCA is a new double stack MLC calibration method that increases treatment accuracy from four angles and for all moving leaves.

Comparison of different dose accumulation strategies to estimate organ doses after stereotactic magnetic resonance-guided adaptive radiotherapy

INTRODUCTION

Re-irradiation is frequently performed in the era of precision oncology, but previous doses to organs-at-risk (OAR) must be assessed to avoid cumulative overdoses. Stereotactic magnetic resonance-guided online adaptive radiotherapy (SMART) enables highly precise ablation of tumors close to OAR. However, OAR doses may change considerably during adaptive treatment, which complicates potential re-irradiation. We aimed to compare the baseline plan with different dose accumulation techniques to inform re-irradiation.

PATIENTS & METHODS

We analyzed 18 patients who received SMART to lung or liver tumors inside prospective databases. Cumulative doses were calculated inside the planning target volumes (PTV) and OAR for the adapted plans and theoretical non-adapted plans via (1) cumulative dose volume histograms (DVH sum plan) and (2) deformable image registration (DIR)-based dose accumulation to planning images (DIR sum plan). We compared cumulative dose parameters between the baseline plan, DVH sum plan and DIR sum plan using equivalent doses in 2 Gy fractions (EQD2).

RESULTS

Individual patients presented relevant increases of near-maximum doses inside the proximal bronchial tree, spinal cord, heart and gastrointestinal OAR when comparing adaptive treatment to the baseline plans. The spinal cord near-maximum doses were significantly increased in the liver patients (D2% median: baseline 6.1 Gy, DIR sum 8.1 Gy, DVH sum 8.4 Gy, p = 0.04; D0.1 cm³ median: baseline 6.1 Gy, DIR sum 8.1 Gy, DVH sum 8.5 Gy, p = 0.04). Three OAR overdoses occurred during adaptive treatment (DIR sum: 1, DVH sum: 2), and four more intense OAR overdoses would have occurred during non-adaptive treatment (DIR sum: 4, DVH sum: 3). Adaptive treatment maintained similar PTV coverages to the baseline plans, while non-adaptive treatment yielded significantly worse PTV coverages in the lung (D95% median: baseline 86.4 Gy, DIR sum 82.4 Gy, DVH sum 82.2 Gy, p = 0.006) and liver patients (D95% median: baseline 87.4 Gy, DIR sum 82.1 Gy, DVH sum 81.1 Gy, p = 0.04).

CONCLUSION

OAR doses can increase during SMART, so that re-irradiation should be planned based on dose accumulations of the adapted plans instead of the baseline plan. Cumulative dose volume histograms represent a simple and conservative dose accumulation strategy.

Long-Term Clinical Results of MR-Guided Stereotactic Body Radiotherapy of Liver Metastases

(1) Background: Magnetic-resonance (MR)-guided stereotactic body radiotherapy (SBRT) allows for ablative, non-invasive treatment of liver metastases. However, long-term clinical outcome data are missing. (2) Methods: Patients received MR-guided SBRT with a MRIdian Linac between January 2019 and October 2021 and were part of an ongoing prospective observational registry. Local hepatic control (LHC), distant hepatic control (DHC), progression free survival (PFS) and overall survival (OS) were estimated with the Kaplan-Meier method. Toxicity was documented according to CTCAE (v.5.0). (3) Results: Forty patients were treated for a total of 54 liver metastases (56% with online plan adaptation). Median prescribed dose was 50 Gy in five fractions equal to a biologically effective dose (BED) (alpha/beta = 10 Gy) of 100 Gy. At 1 and 2 years, LHC was 98% and 75%, DHC was 34% and 15%, PFS was 21% and 5% and OS was 83% and 57%. Two-year LHC was higher in case of BED > 100 Gy (100% vs. 57%; log-rank p = 0.04). Acute grade 1 and 2 toxicity (mostly nausea) occurred in 26% and 7% of the patients, with no grade ≥ 3 event. (4) Conclusions: To our knowledge, this is the largest cohort of MR-guided liver SBRT. Long-term local control was promising and underscores the aim of achieving >100 Gy BED. Nonetheless, distant tumor control remains challenging.

Pulmonary magnetic resonance-guided online adaptive radiotherapy of locally advanced: the PUMA trial

BACKGROUND

Patients with locally-advanced non-small-cell lung cancer (LA-NSCLC) are often ineligible for surgery, so that definitive chemoradiotherapy (CRT) represents the treatment of choice. Nevertheless, long-term tumor control is often not achieved. Intensification of radiotherapy (RT) to improve locoregional tumor control is limited by the detrimental effect of higher radiation exposure of thoracic organs-at-risk (OAR). This narrow therapeutic ratio may be expanded by exploiting the advantages of magnetic resonance (MR) linear accelerators, mainly the online adaptation of the treatment plan to the current anatomy based on daily acquired MR images. However, MR-guidance is both labor-intensive and increases treatment times, which raises the question of its clinical feasibility to treat LA-NSCLC. Therefore, the PUMA trial was designed as a prospective, multicenter phase I trial to demonstrate the clinical feasibility of MR-guided online adaptive RT in LA-NSCLC.

METHODS

Thirty patients with LA-NSCLC in stage III A-C will be accrued at three German university hospitals to receive MR-guided online adaptive RT at two different MR-linac systems (MRIdian Linac®, View Ray Inc. and Elekta Unity®, Elekta AB) with concurrent chemotherapy. Conventionally fractioned RT with isotoxic dose escalation up to 70 Gy is applied. Online plan adaptation is performed once weekly or in case of major anatomical changes. Patients are followed-up by thoracic CT- and MR-imaging for 24 months after treatment. The primary endpoint is twofold: (1) successfully completed online adapted fractions, (2) on-table time. Main secondary endpoints include adaptation frequency, toxicity, local tumor control, progression-free and overall survival.

DISCUSSION

PUMA aims to demonstrate the clinical feasibility of MR-guided online adaptive RT of LA-NSCLC. If successful, PUMA will be followed by a clinical phase II trial that further investigates the clinical benefits of this approach. Moreover, PUMA is part of a large multidisciplinary project to develop MR-guidance techniques.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT05237453 .

To fly or not to fly: Stereotactic MR-guided adaptive radiotherapy effectively treats ultracentral lung tumors with favorable long-term outcomes

BACKGROUND

Stereotactic radiotherapy of ultracentral lung tumors (ULT) is challenging as it may cause overdoses to sensitive mediastinal organs with severe complications. We aimed to describe long-term outcomes after stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) as an innovative treatment of ULT.

PATIENTS & METHODS

We analyzed 36 patients that received SMART to 40 tumors between 02/2020 - 08/2021 inside prospective databases. ULT were defined by planning target volume (PTV) overlap with the proximal bronchial tree or esophagus. We calculated Kaplan Meier estimates for overall survival (OS) and progression-free survival (PFS), and competing risk estimates for the incidence of tumor progression and treatment-related toxicities. ULT patients (N = 16) were compared to non-ULT patients (N = 20).

RESULTS

Baseline characteristics were similar between ULT and non-ULT, but ULT were larger (median PTV: ULT 54.7 cm³, non-ULT 19.2 cm³). Median follow-up was 23.6 months. ULT and non-ULT showed a similar OS (2-years: ULT 67%, non-ULT 60%, p = 0.7) and PFS (2-years: ULT 37%, non-ULT 34%, p = 0.73). Progressions occurred mainly at distant sites (2-year incidence of distant progression: ULT 63%, non-ULT 61%, p = 0.77), while local tumor control was favorable (2-year incidence of local progression: ULT 7%, non-ULT 0%, p = 0.22). Treatment of ULT led to significantly more toxicities ≥ grade (G) 2 (ULT: 9 (56%), non-ULT: 1 (5%), p = 0.002). Most toxicities were moderate (G2). Two ULT patients developed high-grade toxicities: 1) esophagitis G3 and bronchial bleeding G4 after VEGF treatment, 2) bronchial bleeding G3. Estimated incidence of high-grade toxicities was 19% (3-48%) in ULT, and no treatment-related death occurred.

CONCLUSION

Our small series supports SMART as potentially effective treatment of ULT. SMART with careful fractionation could reduce severe complications, but treatment of ULT remains a high-risk procedure and needs careful benefit-risk-assessment.

Stereotactic magnetic resonance-guided online adaptive radiotherapy of adrenal metastases combines high ablative doses with optimized sparing of organs at risk

Purpose/Objective

To evaluate the potential of stereotactic magnetic resonance-guided online adaptive radiotherapy (SMART) to fulfill dose recommendations for stereotactic body radiotherapy (SBRT) of adrenal metastases and spare organs at risk (OAR).

Materials and methods

In this subgroup analysis of a prospective registry trial, 22 patients with adrenal metastases were treated on a 0.35 T MR-Linac in 5-12 fractions with fraction doses of 4-10 Gy. Baseline plans were re-calculated to the anatomy of the day. These predicted plans were reoptimized to generate adapted plans. Baseline, predicted and adapted plans were compared with regard to PTV objectives, OAR constraints and published dose recommendations.

Results

The cohort comprised patients with large GTV (median 36.0 cc) and PTV (median 66.6 cc) and predominantly left-sided metastases. 179 of 181 fractions (98.9 %) were adapted because of PTV and/or OAR violations. Predicted plans frequently violated PTV coverage (99.4 %) and adjacent OAR constraints (bowel: 32.9 %, stomach: 32.8 %, duodenum: 10.4 %, kidneys: 10.8 %). In the predicted plans, the volume exposed to the maximum dose was exceeded up to 16-fold in the duodenum and up to 96-fold in the spinal cord. Adapted plans significantly reduced OAR violations by 96.4 % for the bowel, 98.5 % for the stomach, 85.6 % for the duodenum and 83.3 % for the kidneys. Plan adaptation improved PTV coverage from 82.7 ± 8.1 % to 90.6 ± 4.9 % (p < 0.001). Furthermore, recently established target volume thresholds could easily be fulfilled with SMART. No toxicities > grade II occurred.

Conclusion

SMART fulfills established GTV and PTV dose recommendations while simultaneously sparing organs at risk even in a challenging cohort.

A body mass index-based method for "MR-only" abdominal MR-guided adaptive radiotherapy

PURPOSE

Dose calculation for MR-guided radiotherapy (MRgRT) at the 0.35 T MR-Linac is currently based on deformation of planning CTs (defCT) acquired for each patient. We present a simple and robust bulk density overwrite synthetic CT (sCT) method for abdominal treatments in order to streamline clinical workflows.

METHOD

Fifty-six abdominal patient treatment plans were retrospectively evaluated. All patients had been treated at the MR-Linac using MR datasets for treatment planning and plan adaption and defCT for dose calculation. Bulk density CTs (4M-sCT) were generated from MR images with four material compartments (bone, lung, air, soft tissue). The relative electron densities (RED) for bone and lung were extracted from contoured CT structure average REDs. For soft tissue, a correlation between BMI and RED was evaluated. Dose was recalculated on 4M-sCT and compared to dose distributions on defCTs assessing dose differences in the PTV and organs at risk (OAR).

RESULTS

Mean RED of bone was 1.17 ± 0.02, mean RED of lung 0.17 ± 0.05. The correlation between BMI and RED for soft tissue was statistically significant (p < 0.01). PTV dose differences between 4M-sCT and defCT were Dmean: -0.4 ± 1.0%, D1%: -0.3 ± 1.1% and D95%: -0.5 ± 1.0%. OARs showed D2%: -0.3 ± 1.9% and Dmean: -0.1 ± 1.4% differences. Local 3D gamma index pass rates (2%/2mm) between dose calculated using 4M-sCT and defCT were 96.8 ± 2.6% (range 89.9-99.6%).

CONCLUSION

The presented method for sCT generation enables precise dose calculation for MR-only abdominal MRgRT.

Dosimetric Benefit of Adaptive Magnetic Resonance-Guided Stereotactic Body Radiotherapy of Liver Metastases

(1) Background: To assess dosimetry benefits of stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) of liver metastases. (2) Methods: This is a subgroup analysis of an ongoing prospective registry including patients with liver metastases. Patients were treated at the MRIdian Linac between February 2020 and April 2022. The baseline plan was recalculated based on the updated anatomy of the day to generate the predicted plan. This predicted plan could then be re-optimized to create an adapted plan. (3) Results: Twenty-three patients received 30 SMART treatment series of in total 36 liver metastases. Most common primary tumors were colorectal- and pancreatic carcinoma (26.1% respectively). Most frequent fractionation scheme (46.6%) was 50 Gy in five fractions. The adapted plan was significantly superior compared to the predicted plan in regard to planning-target-volume (PTV) coverage, PTV overdosing, and organs-at-risk (OAR) dose constraints violations (91.5 vs. 38.0%, 6 vs. 19% and 0.6 vs. 10.0%; each p < 0.001). Plan adaptation significantly increased median BEDD95 by 3.2 Gy (p < 0.001). Mean total duration of SMART was 72.4 min. (4) Conclusions: SMART offers individualized ablative irradiation of liver metastases tailored to the daily anatomy with significant superior tumor coverage and improved sparing of OAR.

SMART ablation of lymphatic oligometastases in the pelvis and abdomen: Clinical and dosimetry outcomes

PURPOSE

To demonstrate dosimetry benefits and report clinical outcomes of stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) of abdominopelvic lymphatic oligometastases.

PATIENTS & METHODS

Prospective registry data of 26 patients with 31 oligoprogressive lymphatic metastases (1 - 2 lesions) who received SMART between April 2020 and April 2021 was analyzed. Prostate cancer was the most common histology (69%). Most patients (63%) had received previous abdominopelvic radiotherapy (RT). SMART was delivered in 3 - 7 fractions based on planning target volume (PTV) location and previous dose exposures. For SMART, the baseline plan was recalculated on daily 3D MR-imaging (predicted plan), and plan adaptation was mandatory in case of planning objective violations.

RESULTS

Plan adaptation was mostly performed due to violation of planning objectives in the predicted plan (134/140 fractions, 96%) and significantly improved plan dosimetry: 1) PTV coverage was increased (predicted: median 89%, adapted: median 95%, p < 0.001), 2) organs-at-risk (OAR) overdoses were reduced (predicted: 27/140 (19%), adapted: 1/140 (1%), p < 0.001) and 3) PTV overdoses were reduced (predicted: 21/140 (15%), adapted: 1/140 (1%), p < 0.001). After a median follow-up of 9.8 months, one patient had in-field tumor progression and twelve patients had out-field tumor progression (at 6 months: progression-free survival: 63% [46 - 88%], local control rate: 97% [90 - 100%]). Treatment was tolerated well and no grade ≥ 3 toxicity was reported.

CONCLUSION

SMART improves target volume coverage and yields superior OAR protection compared to non-adaptive radiotherapy, thus representing an innovative approach to challenging cases, such as repeated radiotherapy.

Quality assurance for on-table adaptive magnetic resonance guided radiation therapy: A software tool to complement secondary dose calculation and failure modes discovered in clinical routine

Online adaption of treatment plans on a magnetic resonance (MR)‐Linac enables the daily creation of new (adapted) treatment plans using current anatomical information of the patient as seen on MR images. Plan quality assurance (QA) relies on a secondary dose calculation (SDC) that is required because a pretreatment measurement is impossible during the adaptive workflow. However, failure mode and effect analysis of the adaptive planning process shows a large number of error sources, and not all of them are covered by SDC. As the complex multidisciplinary adaption process takes place under time pressure, additional software solutions for pretreatment per‐fraction QA need to be used. It is essential to double‐check SDC input to ensure a safe treatment delivery. Here, we present an automated treatment plan check tool for adaptive radiotherapy (APART) at a 0.35 T MR‐Linac. It is designed to complement the manufacturer‐provided adaptive QA tool comprising SDC. Checks performed by APART include contour analysis, electron density map examinations, and fluence modulation complexity controls. For nine of 362 adapted fractions (2.5%), irregularities regarding missing slices in target volumes and organs at risks as well as in margin expansion of target volumes have been found. This demonstrates that mistakes occur and can be detected by additional QA measures, especially contour analysis. Therefore, it is recommended to implement further QA tools additional to what the manufacturer provides to facilitate an informed decision about the quality of the treatment plan.

Stability of conventional and machine learning-based tumor auto-segmentation techniques using undersampled dynamic radial bSSFP acquisitions on a 0.35 T hybrid MR-linac system

PURPOSE

Hybrid MRI-linear accelerator systems (MR-linacs) allow for the incorporation of MR images with high soft-tissue contrast into the radiation therapy procedure prior to, during, or post irradiation. This allows not only for the optimization of the treatment planning, but also for real-time monitoring of the tumor position using cine MRI, from which intrafractional motion can be compensated. Fast imaging and accurate tumor tracking are crucial for effective compensation. This study investigates the application of cine MRI with a radial acquisition scheme on a low-field MR-linac to accelerate the acquisition rate and evaluates the effect on tracking accuracy.

METHODS

An MR sequence using tiny golden-angle radial k-space sampling was developed and applied to cine imaging on patients with liver tumors on a 0.35 T MR-linac. Tumor tracking was assessed for accuracy and stability from the cine images with increasing k-space undersampling factors. Tracking was achieved using two different auto-segmentation algorithms: a deformable image registration B-spline similar to that implemented on the MR-linac and a convolutional neural network approach known as U-Net.

RESULTS

Radial imaging allows for increased temporal resolution with reliable tumor tracking, although tracking robustness decreases as temporal resolution increases. Additional acquisition-based artifacts can be avoided by reducing the angle increment using tiny golden-angles. The U-net algorithm was found to have superior auto-segmentation metrics compared to B-spline. U-net was able to track two well-defined tumors, imaged with just 30 spokes per image (10.6 frames per second), with an average Dice coefficient ≥ 83%, Hausdorff distance ≤ 1.4 pixel, and mean contour distance ≤ 0.5 pixel.

CONCLUSIONS

Radial acquisitions are commonplace in dynamic imaging; however, in MR-guided radiotherapy, robust tumor tracking is also required. This study demonstrates the in vivo feasibility of tumor tracking from radially acquired images on a low-field MR-linac. Radial imaging allows for decreased image acquisition times while maintaining robust tracking. The U-net algorithm can track a tumor with higher accuracy in images with undersampling artifacts than a conventional deformable B-spline algorithm and is a promising tool for tracking in MR-guided radiation therapy.