Multi-center evaluation of dose conformity in stereotactic body radiotherapy

Evaluating dose coverage and conformity in stereotactic ablative body radiotherapy (SABR) plans

PURPOSE

Accepted conformity metrics in stereotactic ablative body radiotherapy (SABR) have significant limitations. This work aimed to develop a spatial assessment methodology that improves and automates checks of dose prescription and dose gradient from planning target volume (PTV) edge.

METHODS

A Python-based script was developed to determine linear distances from the PTV edge to specified isodose, every 15 degrees on all axial slices and along the central axis in the coronal plane. A new "Internal PTV contour" distance metric is introduced as a size and shape indicator. 134 previously treated SABR patients stratified by anatomical site and PTV volume were analysed to establish baselines and tolerances for automation acceptability.

RESULTS

In the axial plane, median distance (MD) from PTV edge to the 100 % isodose was 0.13 mm (range: -0.67 to 0.53 mm), and for the 90 % isodose was 2.37 mm (1.36 to 3.40 mm). Lung and non-Lung dose gradient criteria was established by fitting a second order polynomial to the MD as a function of "Internal PTV contour". This resulted in acceptability criteria of MD + 1 mm for 80 % isodose and MD + 2 mm for the 50 % isodose. For the coronal plane, MD to the 100 % isodose was 0.49 mm (-1.24 to 2.14 mm) and for the 90 % was 1.73 mm (-0.49 to 4.13 mm).

CONCLUSIONS

Our in-house script enables a high-quality spatial assessment of PTV dose coverage and gradient, with the new 'Internal PTV contour' distance metric correlating well with dose gradient.

Predictors of pathologic fracture and local recurrence following stereotactic body radiation therapy to 505 non-spine bone metastases

PURPOSE

Stereotactic Body Radiation Therapy (SBRT) is increasingly applied to treat non-spine bone metastases (NSBM) though data remains limited on this approach. In this retrospective study, we report outcomes and predictors of local failure (LF) and pathological fracture (PF) post-SBRT for NSBM using a mature single-institution database.

METHODS

Patients with NSBM treated with SBRT between 2011 and 2021 were identified. The primary objective was to assess the rates of radiographic LF. Secondary objectives were to assess the rates of in-field PF, overall survival (OS), and late grade ≥ 3 toxicity. Competing risks analysis was used to assess rates of LF and PF. Univariable regression and multivariable regression (MVR) were performed to investigate predictors of LF and PF.

RESULTS

A total of 373 patients with 505 NSBM were included in this study. Median follow-up was 26.5 months. The cumulative incidence of LF at 6, 12, and 24 months were 5.7%, 7.9%, and 12.6%, respectively. The cumulative incidence of PF at 6, 12, and 24 months were 3.8%, 6.1%, and 10.9%, respectively. Lytic NSBM (HR = 2.18; p < 0.01), a lower biologically effective dose (HR = 1.11 per 5 Gy10 decrease; p = 0.04), and a PTV ≥ 54 cc (HR = 4.32; p < 0.01) predicted for a higher risk of LF on MVR. Lytic NSBM (HR = 3.43; p < 0.01), mixed (lytic/sclerotic) lesions (HR = 2.70; p = 0.04), and rib metastases (HR = 2.68; p < 0.01) predicted for a higher risk of PF on MVR.

CONCLUSION

SBRT is an effective modality to treat NSBM with high rates of radiographic local control with an acceptable rate of PF. We identify predictors of both LF and PF that can serve to inform practice and trial design.

Brain Re-Irradiation Robustly Accounting for Previously Delivered Dose

(1) Background: The STRIDeR (Support Tool for Re-Irradiation Decisions guided by Radiobiology) planning pathway aims to facilitate anatomically appropriate and radiobiologically meaningful re-irradiation (reRT). This work evaluated the STRIDeR pathway for robustness compared to a more conservative manual pathway. (2) Methods: For ten high-grade glioma reRT patient cases, uncertainties were applied and cumulative doses re-summed. Geometric uncertainties of 3, 6 and 9 mm were applied to the background dose, and LQ model robustness was tested using α/β variations (values 1, 2 and 5 Gy) and the linear quadratic linear (LQL) model δ variations (values 0.1 and 0.2). STRIDeR robust optimised plans, incorporating the geometric and α/β uncertainties during optimisation, were also generated. (3) Results: The STRIDeR and manual pathways both achieved clinically acceptable plans in 8/10 cases but with statistically significant improvements in the PTV D98% (p < 0.01) for STRIDeR. Geometric and LQ robustness tests showed comparable robustness within both pathways. STRIDeR plans generated to incorporate uncertainties during optimisation resulted in a superior plan robustness with a minimal impact on PTV dose benefits. (4) Conclusions: Our results indicate that STRIDeR pathway plans achieved a similar robustness to manual pathways with improved PTV doses. Geometric and LQ model uncertainties can be incorporated into the STRIDeR pathway to facilitate robust optimisation.

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.

Intra-fraction motion of pelvic oligometastases and feasibility of PTV margin reduction using MRI guided adaptive radiotherapy

Purpose

This study assesses the impact of intra-fraction motion and PTV margin size on target coverage for patients undergoing radiation treatment of pelvic oligometastases. Dosimetric sparing of the bowel as a function of the PTV margin is also evaluated.

Materials and methods

Seven patients with pelvic oligometastases previously treated on our MR-linac (35 Gy in 5 fractions) were included in this study. Retrospective adaptive plans were created for each fraction on the daily MRI datasets using PTV margins of 5 mm, 3 mm, and 2 mm. Dosimetric constraint violations and GTV coverage were measured as a function of PTV margin size. The impact of intra-fraction motion on GTV coverage was assessed by tracking the GTV position on the cine MR images acquired during treatment delivery and creating an intra-fraction dose distribution for each IMRT beam. The intra-fraction dose was accumulated for each fraction to determine the total dose delivered to the target for each PTV size.

Results

All OAR constraints were achieved in 85.7%, 94.3%, and 100.0% of fractions when using 5 mm, 3 mm, and 2 mm PTV margins while scaling to 95% PTV coverage. Compared to plans with a 5 mm PTV margin, there was a 27.4 ± 12.3% (4.0 ± 2.2 Gy) and an 18.5 ± 7.3% (2.7 ± 1.4 Gy) reduction in the bowel D_0.5cc dose for 2 mm and 3 mm PTV margins, respectively. The target dose (GTV V_{35 Gy}) was on average 100.0 ± 0.1% (99.6 - 100%), 99.6 ± 1.0% (97.2 - 100%), and 99.0 ± 1.4% (95.0 - 100%), among all fractions for the 5 mm, 3 mm, and 2 mm PTV margins on the adaptive plans when accounting for intra-fraction motion, respectively.

Conclusion

A 2 mm PTV margin achieved a minimum of 95% GTV coverage while reducing the dose to the bowel for all patients.

Practical considerations of single-fraction stereotactic ablative radiotherapy to the lung

Stereotactic ablative radiotherapy (SABR) is a well-established treatment for patients with medically inoperable early-stage non-small cell lung cancer (NSCLC) and pulmonary oligometastases. The use of single-fraction SABR in this setting is supported by excellent local control and safety profiles which appear equivalent to multi-fraction SABR based on the available data. The resource efficiency and reduction in hospital outpatient visits associated with single-fraction SABR have been particularly advantageous during the COVID-19 pandemic. Despite the increased interest, single-fraction SABR in subgroups of patients remains controversial, including those with centrally located tumours, synchronous targets, proximity to dose-limiting organs at risk, and concomitant severe respiratory illness. This review provides an overview of the published randomised evidence evaluating single-fraction SABR in primary lung cancer and pulmonary oligometastases, the common clinical challenges faced, immunogenic effect of SABR, as well as technical and cost-utility considerations.

A retrospective study to establish recommendations for plan quality metrics in Lung SBRT

The increased use of Stereotactic body radiation therapy (SBRT) has warranted a new method of plan evaluation. The crucial component of SBRT is the precise, conformal delivery of radiation dose to the target with rapid dose fall-off in the surrounding normal tissues.In this study, we retrospectively evaluated plan quality in lung SBRT patients by calculating conformity, homogeneity, and gradient parameters using an in-house script. The goal of this study was to establish achievable, size-dependent recommendations for these plan quality metrics such that they may be used as a guideline in our clinic. Seventy-three patients treated with lung SBRT at The University of Toledo Medical Center during the period 2017-2020 were retrospectively reviewed for this study. Plans were evaluated using dosimetric indices from respective The Radiation Therapy Oncology Group (RTOG) and International Commission on Radiation Units and Measurements (ICRU) protocols. Average values for each of the following indices were calculated: RTOG conformity index = 1.12 ± 0.13; Paddick conformity index = 0.82 ± 0.07; gradient index = 4.63 ± 0.71; and Homogeneity index = 0.3 ± 0.07, for all studied lung lesions with a mean volume of 23.2 cc. Our final recommendations are based on clinically approved plans, after having removed statistical outliers that we may not have approved had the metrics been calculated. Additionally, we observed that a sharper dose fall-off and a more homogeneous plan were found using 6 FFF compared to 10 FFF energy. Comparison between our results and RTOG0915 data shows no deviation or minor deviation for the RTOG conformity index and the ratio of 50% prescription isodose volume to the target volume. Furthermore, no statistically significant correlation between RTOG conformity index and target volume was observed which is in agreement with RTOG0915. Using various dosimetric indices to characterize dose distributions in lung SBRT is a powerful tool to assess plan quality. We recommend that these values be calculated for all plans, utilizing a script or program so as to improve clinical workflow.

CT-guided versus MR-guided radiotherapy: Impact on gastrointestinal sparing in adrenal stereotactic body radiotherapy

BACKGROUND AND PURPOSE

To quantify the indication for adaptive, gated breath-hold (BH) MR-guided radiotherapy (MRgRT_BH) versus BH or free-breathing (FB) CT-based image-guided radiotherapy (CT-IGRT) for the ablative treatment of adrenal malignancies.

MATERIALS AND METHODS

Twenty adrenal patients underwent adaptive IMRT MRgRT_BH to a median dose of 50 Gy/5 fractions. Each patient was replanned for VMAT CT-IGRT_BH and CT-IGRT_FB on a c-arm linac. Only CT-IGRT_FB used an ITV, summed from GTVs of all phases of the 4DCT respiratory evaluation. All used the same 5 mm GTV/ITV to PTV expansion. Metrics evaluated included: target volume and coverage, conformality, mean ipsilateral kidney and 0.5 cc gastrointestinal organ-at-risk (OAR) doses (D_0.5cc). Adaptive dose for MRgRT_BH and predicted dose (i.e., initial plan re-calculated on anatomy of the day) was performed for CT-IGRT_BH and MRgRT_BH to assess frequency of OAR violations and coverage reductions for each fraction.

RESULTS

The more common VMAT CT-IGRT_FB, with its significantly larger target volumes, proved inferior to MRgRT_BH in mean PTV and ITV/GTV coverage, as well as small bowel D_0.5cc. Conversely, VMAT CT-IGRT_BH delivered a dosimetrically superior initial plan in terms of statistically significant (p ≤ 0.02) improvements in target coverage, conformality and D_0.5cc to the large bowel, duodenum and mean ipsilateral kidney compared to IMRT MRgRT_BH. However, non-adaptive CT-IGRT_BH had a 71.8% frequency of predicted indications for adaptation and was 2.8 times more likely to experience a coverage reduction in PTV D_95% than predicted for MRgRT_BH.

CONCLUSION

Breath-hold VMAT radiotherapy provides superior target coverage and conformality over MRgRT_BH, but the ability of MRgRT_BH to safely provide ablative doses to adrenal lesions near mobile luminal OAR through adaptation and direct, real-time motion tracking is unmatched.

Dosimetric evaluation of different planning techniques based on flattening filter-free beams for central and peripheral lung stereotactic body radiotherapy

This study aimed to dosimetrically compare and evaluate the flattening filter-free (FFF) photon beam-based three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiation therapy (IMRT), and volumetric modulated arc therapy (VMAT) for lung stereotactic body radiotherapy (SBRT). RANDO phantom computed tomography (CT) images were used for treatment planning. Gross tumor volumes (GTVs) were delineated in the central and peripheral lung locations. Planning target volumes (PTVs) was determined by adding a 5 mm margin to the GTV. 3DCRT, IMRT, and VMAT plans were generated using a 6-MV FFF photon beam. Dose calculations for all plans were performed using the anisotropic analytical algorithm (AAA) and Acuros XB algorithms. The accuracy of the algorithms was validated using the dose measured in a CIRS thorax phantom. The conformity index (CI), high dose volume (HDV), low dose location (D_2cm), and homogeneity index (HI) improved with FFF-VMAT compared to FFF-IMRT and FFF-3DCRT, while low dose volume (R_50%) and gradient index (GI) showed improvement with FFF-3DCRT. Compared with FFF-3DCRT, a drastic decrease in the mean treatment time (TT) value was observed with FFF-VMAT for different lung sites between 57.09% and 60.39%, while with FFF-IMRT it increased between 10.78% and 17.49%. The dose calculation with Acuros XB was found to be superior to that of AAA. Based on the comparison of dosimetric indices in this study, FFF-VMAT provides a superior treatment plan to FFF-IMRT and FFF-3DCRT in the treatment of peripheral and central lung PTVs. This study suggests that Acuros XB is a more accurate algorithm than AAA in the lung region.

Can reducing planning safety margins broaden the inclusion criteria for lung stereotactic ablative body radiotherapy?

INTRODUCTION

Stereotactic ablative body radiotherapy (SABR) is currently indicated for inoperable, early-stage non-small cell lung carcinoma (NSCLC). Advancements in image-guidance technology continue to improve treatment precision and enable reductions in planning safety margins. We investigated the dosimetric benefits of margin reduction, its potential to extend SABR to more NSCLC patients and the factors influencing plan acceptability.

METHODS

This retrospective analysis included 61 patients (stage IA-IIIA) treated with conventional radiotherapy. Patients were ineligible for SABR due to tumour size or proximity to organs at risk (OAR). Using Pinnacle auto-planning, three SABR plans were generated for each patient: a regular planning target volume margin plan, a reduced margin plan (gross tumour volume GTV+3 mm) and a non-margin plan. Targets were planned to 48Gy/4 or 50Gy/5 fractions depending on location. Plans were compared in terms of target coverage, OAR doses and dosimetric acceptability based on local guidelines. Predictors of acceptability were investigated using logistic regression analysis.

RESULTS

Compared to regular margin plans, both reduced margin and non-margin plans resulted in significant reductions to almost all dose constraints. Dose conformity was significantly worse in non-margin plans (P < 0.05) and strongly correlated with targets' surface area/volume ratio (R2  = 0.9, P < 0.05). 26% of reduced margin plans were acceptable, compared to 54% of non-margin plans. GTV overlap with OARs significantly affected plan acceptability (OR 0.008, 95% CI 0.001-0.073).

CONCLUSION

Margin reduction significantly reduced OAR doses enabling acceptable plans to be achieved for patients previously excluded from SABR. Indications for lung SABR may broaden as treatment accuracy continues to improve; further work is needed to identify patients most likely to benefit.

Why all radiotherapy planning studies are wrong but some are useful

Dosimetry comparison studies of radiotherapy treatment planning are common, but often their limitations are not fully acknowledged. Useful data for the community may be produced, which is reproducible and reliable for implementation by others. However, this will only be achieved by clear and detailed reporting, and by consideration of delivered doses and clinical significance.