The challenge of planning vertebral body SBRT: Optimizing target volume coverage

Optimal combination of collimator angles for dual-arc volumetric modulated arc therapy planning in stereotactic body radiotherapy for spinal metastases

In planning the treatment of spinal metastases using stereotactic body radiotherapy (SBRT), the optimal blocking of the spinal cord to match the leaf travel can be achieved with a first-arc collimator angle of approximately 90°. We aim to clarify the optimal second-arc collimator angles when the first-arc collimator angle is fixed to 90° in dual-arc volumetric modulated arc therapy (VMAT). For this retrospective study, we considered 37 spinal segments with spinal metastases and created dual-arc VMAT plans. In the plans, 24 Gy in 2 fractions were prescribed, and the first-arc collimator angle was fixed to 90° while varying the second-arc collimator angle in increments of 15° from 0° to 90°. All the plans were normalized such that the planning organ-at-risk volume for the spinal cord D0.035 cc = 17 Gy and satisfied other dose constraints. D95% for the planning target volume (PTV), V100% for the overlap between the PTV and 10 mm expansion of the spinal cord, modified gradient index, monitor unit, and 3%/1 mm gamma passing rates were compared between different second-arc collimator angles using the Wilcoxon signed-rank test and Bonferroni correction. PTV D95% and overlap V100% were the highest for a second-arc collimator angle of 45° and decreased as the angle approached either 0° or 90°. The maximum mean differences of PTV D95% and overlap V100% were -2.66% (90° vs 45°, p < 0.0024) and -5.49% (90° vs 45°, p < 0.0024), respectively. Moreover, the second-arc collimator angle of 45° was the least suitable in terms of the modified gradient index. The required monitor unit increased from the second-arc collimator angle of 15° to 45°, and the 3%/1 mm gamma passing rates reached over 95% for the evaluated second-arc collimator angles of 15°, 30°, and 45°. We found that in the dual-arc VMAT plan for spine SBRT, second-arc collimator angles other than 90° were suitable, and 45° was the optimal angle in terms of target coverage including the area around the spinal cord.

Dummy run for planning of isotoxic dose-escalated radiation therapy for glioblastoma used in the PRIDE trial (NOA-28; ARO-2024-01; AG-NRO-06)

Background

The PRIDE trial (NOA-28; ARO-2024-01; AG-NRO-06; NCT05871021) is designed to determine whether a dose escalation with 75.0 Gy in 30 fractions can enhance the median overall survival (OS) in patients with methylguanine methyltransferase (MGMT) promotor unmethylated glioblastoma compared to historical median OS rates, while being isotoxic to historical cohorts through the addition of concurrent bevacizumab (BEV). To ensure protocol-compliant irradiation planning with all study centers, a dummy run was planned and the plan quality was evaluated.

Methods

A suitable patient case was selected and the computed tomography (CT), magnetic resonance imaging (MRI) and O-(2-[18F]fluoroethyl)-L-tyrosine (FET) positron emission tomography (PET) contours were made available. Participants at the various intended study sites performed radiation planning according to the PRIDE clinical trial protocol. The treatment plans and dose grids were uploaded as Digital Imaging and Communications in Medicine (DICOM) files to a cloud-based platform. Plan quality and protocol adherence were analyzed using a standardized checklist, scorecards and indices such as Dice Score (DSC) and Hausdorff Distance (HD).

Results

Median DSC was 0.89, 0.90, 0.88 for PTV60, PTV60ex (planning target volume receiving 60.0 Gy for the standard and the experimental plan, respectively) and PTV75 (PTV receiving 75.0 Gy in the experimental plan), respectively. Median HD values were 17.0 mm, 13.9 mm and 12.1 mm, respectively. These differences were also evident in the volumes: The PTV60 had a volume range of 219.1-391.3 cc (median: 261.9 cc) for the standard plans, while the PTV75 volumes for the experimental plans ranged from 71.5-142.7 cc (median: 92.3 cc). The structures with the largest deviations in Dice score were the pituitary gland (median 0.37, range 0.00-0.69) and the right lacrimal gland (median 0.59, range 0.42-0.78).

Conclusions

The deviations revealed the necessity of systematic trainings with appropriate feedback before the start of clinical trials in radiation oncology and the constant monitoring of protocol compliance throw-out the study.

Trial registration

NCT05871021.

Efficacious patient-specific QA for Vertebra SBRT using a high-resolution detector array SRS MapCHECK: AAPM TG-218 analysis

PURPOSE

Patient-specific quality assurance (PSQA) for vertebra stereotactic body radiation therapy (SBRT) presents challenges due to highly modulated small fields with high-dose gradients between the target and spinal cord. This study aims to explore the use of the SRS MapCHECK® (SRSMC) for vertebra SBRT PSQA.

METHODS

Twenty vertebra SBRT treatment plans including prescriptions 20 Gy/1 fraction and 24 Gy/2 fractions were selected for each of Millennium (M)-Multileaf Collimator (MLC), and high-definition (HD)-MLC. All 40 plans were measured using Gafchromic EBT3 film (film) and SRSMC, using the StereoPHAN phantom. Plan complexity was assessed using modulation complexity score (MCS), edge metric (EM) (mm-1), modulation factor (MU/cGy), and average leaf pair opening (ALPO) (mm) and its correlation with gamma-pass rate was investigated. The high dose gradient between the target and the spinal cord was analyzed for film and SRSMC and compared against the treatment planning system (TPS). Applying the methodology proposed by AAPM TG-218, action and tolerance values specific to the SRSMC for vertebra SBRT were determined for β values ranging from 5 to 8.

RESULTS

Film and SRSMC gamma-pass rates showed no correlation (p > 0.05). A moderate negative correlation (R = -0.57, p = 0.01) is present between EM and SRSMC 3%/1 mm gamma-pass rate for HD-MLC plans. Both film and SRSMC accurately measured high dose gradients between the target and the spinal cord (R2 > 0.86, p ≤ 0.05). Notably, dose-gradient of HD-MLC plans is 22% steeper and has a smaller standard deviation to M-MLC plans (p ≤ 0.05). Applying TG-218, the film tolerance limit was 96% with action limit 95% for 5%/1 mm (β = 6) and for the SRSMC tolerance limit was 97% with an action limit of 96% for 4%/1 mm (β = 6).

CONCLUSION

Our findings suggest that universal TG-218 limits may not be suitable for vertebra SBRT PSQA. This study demonstrates that SRSMC is a viable tool for vertebra SBRT PSQA, supported by TG-218 implementation of process-based tolerance and action limits.

Stereotactic body radiation therapy to the spine: contouring the cauda equina instead of the spinal cord is more practical as the organ at risk

Background

Stereotactic body radiotherapy (SBRT) is recognized as a curative treatment for oligometastasis. The spinal cord becomes the cauda equina at the lumbar level, and the nerves are located dorsally. Recently, a consensus has been reached that the cauda equina should be contoured as an organ at risk (OAR). Here, we examined the separate contouring benefits for the spinal canal versus the cauda equina only as the OAR.

Materials and methods

A medical physicist designed a simulation plan for 10 patients with isolated lumbar metastasis. The OAR was set with three contours: the whole spinal canal, cauda equina only, and cauda equina with bilateral nerve roots. The prescribed dose for the planning target volume (PTV) was 30 Gy/3 fx.

Results

For the constrained QAR doses, D90 and D95 were statistically significant due to the different OAR contouring. The maximum dose (Dmax) was increased to the spinal canal when the cauda equina max was set to ≤ 20 Gy, but dose hotspots were observed in most cases in the medullary area. The Dmax and PTV coverage were negatively correlated for the cauda equina and the spinal canal if Dmax was set to ≤ 20 Gy for both.

Conclusions

A portion of the spinal fluid is also included when the spinal canal is set as the OAR. Thus, the PTV coverage rate will be poor if the tumor is in contact with the spinal canal. However, the PTV coverage rate increases if only the cauda equina is set as the OAR.

A sub-analysis of multi-center planning radiosurgery for intracranial metastases through automation (MC-PRIMA) comparing UK and international centers

OBJECTIVES

A sub-analysis of the MC-PRIMA study was performed to compare the plan quality of stereotactic radiosurgery (SRS) to multiple brain metastases (MBM) between UK and other international centres.

METHODS AND MATERIALS

Six centres from the UK and nineteen from other international centres autoplanned using Multiple Brain Mets™ (AutoMBM; Brainlab, Munich, Germany) software for a five MBM study case from a prior planning competition that was originally organized by the Trans-Tasmania Radiation Oncology Group (TROG). Twenty-three dosimetric metrics and the resulting composite plan score per the TROG planning competition were compared between the UK and other international centres. Planning experience and planning time from each planner were recorded and statistically compared.

RESULTS

Planning experiences between two groups are equal. Except for mean dose to the hippocampus, all other 22 dosimetric metrics were comparable between two groups. The inter-planner variations in these 23 dosimetric metrics and the composite plan score were also statistically equivalent. Planning time is slightly longer in the UK group (mean = 86.8 min) with a mean difference of 50.3 min.

CONCLUSIONS

AutoMBM effectively achieves standardization of the plan quality of SRS to MBM within UK and further against the other international centres. Significant planning efficiency gain by AutoMBM both among the UK and other international centres may help to increase the capacity of SRS service by alleviating the clinical and technical loadings.

Stereotactic body radiotherapy for spinal oligometastases: a review on patient selection and the optimal methodology

Stereotactic body radiotherapy (SBRT) has excellent local control and low toxicity for spinal metastases and is widely performed for spinal oligometastases. However, its additional survival benefit to standard of care, including systemic therapy, is unknown because the results of large-scale randomized controlled trials regarding SBRT for oligometastases have not been reported. Consequently, the optimal patient population among those with spinal oligometastases and the optimal methodology for spine SBRT remain unclear. The present review article discusses two topics: evidence-based optimal patient selection and methodology. The following have been reported to be good prognostic factors: young age, good performance status, slow-growing disease with a long disease-free interval, minimal disease burden, and mild fluorodeoxyglucose accumulation in positron emission tomography. In addition, we proposed four measures as the optimal SBRT method for achieving excellent local control: (i) required target delineation; (ii) recommended dose fraction schedule (20 or 24 Gy in a single fraction for spinal oligometastases and 35 Gy in five fractions for lesions located near the spinal cord); (iii) optimizing dose distribution for the target; (iv) dose constraint options for the spinal cord.

Personalized Automation of Treatment Planning for Linac-Based Stereotactic Body Radiotherapy of Spine Cancer

Purpose/Objective(s)

Stereotactic ablative body radiotherapy (SBRT) for vertebral metastases is a challenging treatment process. Planning automation has recently reported the potential to improve plan quality and increase planning efficiency. We performed a dosimetric evaluation of the new Personalized engine implemented in Pinnacle3 for full planning automation of SBRT spine treatments in terms of plan quality, treatment efficiency, and delivery accuracy.

Materials/Methods

The Pinnacle3 treatment planning system was used to reoptimize six patients with spinal metastases, employing two separate automated engines. These two automated engines, the existing Autoplanning and the new Personalized, are both template-based algorithms that employ a wishlist to construct planning goals and an iterative technique to replicate the planning procedure performed by skilled planners. The boost tumor volume (BTV) was defined as the macroscopically visible lesion on RM examination, and the planning target volume (PTV) corresponds with the entire vertebra. Dose was prescribed according to simultaneous integrated boost strategy with BTV and PTV irradiated simultaneously over 3 fractions with a dose of 30 and 21 Gy, respectively. Dose-volume histogram (DVH) metrics and conformance indices were used to compare clinically accepted manual plans (MP) with automated plans developed using both Autoplanning (AP) and Personalized engines (Pers). All plans were evaluated for planning efficiency and dose delivery accuracy.

Results

For similar spinal cord sparing, automated plans reported a significant improvement of target coverage and dose conformity. On average, Pers plans increased near-minimal dose D98% by 10.4% and 8.9% and target coverage D95% by 8.0% and by 4.6% for BTV and PTV, respectively. Automated plans provided significantly superior dose conformity and dose contrast by 37%–47% and by 4.6%–5.7% compared with manual plans. Overall planning times were dramatically reduced to about 15 and 23 min for Pers and AP plans, respectively. The average beam-on times were found to be within 3 min for all plans. Despite the increased complexity, all plans passed the 2%/2 mm γ-analysis for dose verification.

Conclusion

Automated planning for spine SBRT through the new Pinnacle3 Personalized engine provided an overall increase of plan quality in terms of dose conformity and a major increase in efficiency. In this complex anatomical site, Personalized strongly reduce the tradeoff between optimal accurate dosimetry and planning time.

A dosimetric comparative analysis of Brainlab elements and Eclipse RapidArc for spine SBRT treatment planning

Purpose:This is a dosimetric study comparing stereotactic body radiotherapy (SBRT) plans of spine tumors using Brainlab Elements Spine planning module against Eclipse RapidArc plans. Dose conformity, dose gradient, dose fall-off, and patient-specific quality assurance (QA) metrics were evaluated.Methods:Twenty patients were immobilized in supine position using half Vac-Lok. A prescription dose of 16 Gy in a single fraction was planned for Varian TrueBeam. Conformal arc plans were generated with Pencil beam (PB), MonteCarlo (MC) in Elements, and RapidArc with Acuros XB algorithm in Eclipse using identical treatment geometry.Results:Eclipse, Elements PB, and Elements MC generated dosimetrically conformal plans having Inverse Paddick Conformity Index (IPCI) <1.3. All plans satisfied the dose constraints to target and OARs. Elements PB had a sharper gradient than Elements MC with average GI of 3.67(95% CI: 3.52-3.82) and 4.06 (95% CI: 3.93-4.20) respectively. Eclipse plans were more homogeneous with mean HI= 1.22 (95% CI: 1.20-1.23) that is lower than others. Average maximum clinical target volume (CTV) doses were higher in Elements MC with 22.31Gy (95% CI: 21.87-22.74), while PB plans have 21.15Gy (95% CI: 20.36-21.96), respectively. Elements MC and PB plans had lower average dose to 0.35 cc of spinal cord (D0.35cc) of 7.60Gy (95% CI: 7.18-8.02) and 8.42Gy (95% CI: 7.83-9.01). All plans had >95% points passing the gamma QA criteria at 3%/2 mm.Conclusion:All treatment plans achieved clinically acceptable target coverage >95% and meet spinal cord dose limits. Smart optimization in Brainlab Elements spine module produced dosimetrically superior plans by better spinal cord sparing.

What is plan quality in radiotherapy? The importance of evaluating dose metrics, complexity, and robustness of treatment plans

Plan evaluation is a key step in the radiotherapy treatment workflow. Central to this step is the assessment of treatment plan quality. Hence, it is important to agree on what we mean by plan quality and to be fully aware of which parameters it depends on. We understand plan quality in radiotherapy as the clinical suitability of the delivered dose distribution that can be realistically expected from a treatment plan. Plan quality is commonly assessed by evaluating the dose distribution calculated by the treatment planning system (TPS). Evaluating the 3D dose distribution is not easy, however; it is hard to fully evaluate its spatial characteristics and we still lack the knowledge for personalising the prediction of the clinical outcome based on individual patient characteristics. This advocates for standardisation and systematic collection of clinical data and outcomes after radiotherapy. Additionally, the calculated dose distribution is not exactly the dose delivered to the patient due to uncertainties in the dose calculation and the treatment delivery, including variations in the patient set-up and anatomy. Consequently, plan quality also depends on the robustness and complexity of the treatment plan. We believe that future work and consensus on the best metrics for quality indices are required. Better tools are needed in TPSs for the evaluation of dose distributions, for the robust evaluation and optimisation of treatment plans, and for controlling and reporting plan complexity. Implementation of such tools and a better understanding of these concepts will facilitate the handling of these characteristics in clinical practice and be helpful to increase the overall quality of treatment plans in radiotherapy.