Dose accumulation for personalized stereotactic MR-guided adaptive radiation therapy in prostate cancer

Comparison of online adaptive and non-adaptive magnetic resonance image-guided radiation therapy in prostate cancer using dose accumulation

Background and purpose

Conventional image-guided radiotherapy (conv-IGRT) is standard in prostate cancer (PC) but does not account for inter-fraction anatomical changes. Online-adaptive magnetic resonance-guided RT (OA-MRgRT) may improve organ-at-risk (OARs) sparing and clinical target volume (CTV) coverage. The aim of this study was to analyze accumulated OAR and target doses in PC after OA-MRgRT and conv-IGRT in comparison to pre-treatment reference planning (refPlan).

Material and methods

Ten patients with PC, previously treated with OA-MRgRT at the 1.5 T MR-Linac (20x3Gy), were included. Accumulated OA-MRgRT doses were determined by deformably registering all fraction's MR-images. Conv-IGRT was simulated through rigid registration of the planning computed tomography with each fraction's MR-image for dose mapping/accumulation. Dose-volume parameters (DVPs), including CTV D50% and D98%, rectum, bladder, urethra, Dmax and V56Gy for OA-MRgRT, conv-IGRT and refPlan were compared using the Wilcoxon signed-rank test. Clinical relevance of accumulated dose differences was analyzed using a normal-tissue complication-probability model.

Results

CTV-DVPs were comparable, whereas OA-MRgRT yielded decreased median OAR-DVPs compared to conv-IGRT, except for bladder V56Gy. OA-MRgRT demonstrated significantly lower median rectum Dmax over conv-IGRT (59.1/59.9 Gy, p = 0.006) and refPlan (60.1 Gy, p = 0.012). Similarly, OA-MRgRT yielded reduced median bladder Dmax compared to conv-IGRT (60.0/60.4 Gy, p = 0.006), and refPlan (61.2 Gy, p = 0.002). Overall, accumulated dose differences were small and did not translate into clinically relevant effects.

Conclusion

Deformably accumulated OA-MRgRT using 20x3Gy in PC showed significant but small dosimetric differences comparted to conv-IGRT. Feasibility of a dose accumulation methodology was demonstrated, which may be relevant for evaluating future hypo-fractionated OA-MRgRT approaches.

Ultra-Hypofractionated Prostate Radiotherapy With Online Adaptive Technique: A Case Report

Ultra-hypofractionated radiotherapy (UHF RT) is revolutionizing the treatment approach for low- and intermediate-risk prostate cancer patients. This study reports the planning process of UHF RT utilizing the cone beam computed tomography (CBCT)-based online adaptive radiotherapy (OART) treatment with the Ethos system, focusing on a comparative analysis between OART and image-guided radiotherapy (IGRT) plans. We also assessed the pre-planning capabilities of the Ethos system against the CyberKnife (CK) (Accuray, Sunnyvale, CA) system. A 66-year-old patient, diagnosed with prostatic acinar adenocarcinoma confirmed via biopsy and presenting with elevated prostate-specific antigen (PSA) levels, underwent UHF OART treatment using the Ethos system. The planning encompassed delineating the gross target volume (GTV) as the prostate, while the clinical target volume (CTV) comprised the prostate and proximal seminal vesicle. The planning target volume (PTV) was derived from the CTV with a 5 mm external margin except for a 3 mm posterior margin. A simultaneous integrated boost (SIB) technique was employed, delivering 40 Gy in five fractions (8 Gy per fraction) to the gross tumor volume (GTV) and 36.25 Gy in five fractions (7.25 Gy per fraction) to the remaining part of the planning target volume (PTV), with treatments scheduled biweekly. We compared OART and IGRT plans and conducted a comparative analysis between Ethos planning and the CK system for pre-planning assessment. When comparing Ethos planning and CK plans, Ethos demonstrated slightly better target coverage and organ-at-risk (OAR) sparing. However, CK plans showed superior containment of low-dose spillage, particularly at 50% and 25% iso-doses, due to non-coplanar beam arrangements. Our results demonstrated that OART plans yielded superior target coverage and improved OAR sparing compared to IGRT plans. Notably, the entire OART process, from planning to delivery, was accomplished within 27 minutes. The Ethos OART system's ability to adapt to daily anatomical changes, efficient workflow, and superior OAR-sparing capabilities make it a promising option for prostate cancer treatment using UHF RT.

A multi-institutional comparison of retrospective deformable dose accumulation for online adaptive magnetic resonance-guided radiotherapy

Background and Purpose

Application of different deformable dose accumulation (DDA) solutions makes institutional comparisons after online-adaptive magnetic resonance-guided radiotherapy (OA-MRgRT) challenging. The aim of this multi-institutional study was to analyze accuracy and agreement of DDA-implementations in OA-MRgRT.

Material and Methods

One gold standard (GS) case deformed with a biomechanical-model and five clinical cases consisting of prostate (2x), cervix, liver, and lymph node cancer, treated with OA-MRgRT, were analyzed. Six centers conducted DDA using institutional implementations. Deformable image registration (DIR) and DDA results were compared using the contour metrics Dice Similarity Coefficient (DSC), surface-DSC, Hausdorff-distance (HD95%), and accumulated dose-volume histograms (DVHs) analyzed via intraclass correlation coefficient (ICC) and clinical dosimetric criteria (CDC).

Results

For the GS, median DDA errors ranged from 0.0 to 2.8 Gy across contours and implementations. DIR of clinical cases resulted in DSC > 0.8 for up to 81.3% of contours and a variability of surface-DSC values depending on the implementation. Maximum HD95%=73.3 mm was found for duodenum in the liver case. Although DVH ICC > 0.90 was found after DDA for all but two contours, relevant absolute CDC differences were observed in clinical cases: Prostate I/II showed maximum differences in bladder V28Gy (10.2/7.6%), while for cervix, liver, and lymph node the highest differences were found for rectum D2cm3 (2.8 Gy), duodenum Dmax (7.1 Gy), and rectum D0.5cm3 (4.6 Gy).

Conclusion

Overall, high agreement was found between the different DIR and DDA implementations. Case- and algorithm-dependent differences were observed, leading to potentially clinically relevant results. Larger studies are needed to define future DDA-guidelines.

Impact of daily plan adaptation on accumulated doses in ultra-hypofractionated magnetic resonance-guided radiation therapy of prostate cancer

Background and purpose

Ultra-hypofractionated online adaptive magnetic resonance-guided radiotherapy (MRgRT) is promising for prostate cancer. However, the impact of online adaptation on target coverage and organ-at-risk (OAR) sparing at the level of accumulated dose has not yet been reported. Using deformable image registration (DIR)-based accumulation, we compared the delivered adapted dose with the simulated non-adapted dose.

Materials and methods

Twenty-three prostate cancer patients treated at two clinics with 0.35 T magnetic resonance-guided linear accelerator (MR-linac) following the same treatment protocol (5 ×  7.5 Gy with urethral sparing and daily adaptation) were included. The fraction MR images were deformably registered to the planning MR image. Both non-adapted and adapted fraction doses were accumulated with the corresponding vector fields. Two DIR approaches were implemented. PTV* (planning target volume minus urethra+2mm) D95%, CTV* (clinical target volume minus urethra) D98%, and OARs (urethra+2mm, bladder, and rectum) D0.2cc, were evaluated. Statistical significance was inferred from a two-tailed Wilcoxon signed-rank test (p < 0.05).

Results

Normalized to the baseline, the accumulated PTV* D95% increased significantly by 2.7 % ([1.5, 4.3]%) through adaptation, and the CTV* D98% by 1.2 % ([0.1, 1.7]%). For the OARs after adaptation, accumulated bladder D0.2cc decreased by 0.4 % ([-1.2, 0.4]%), urethra+2mmD0.2cc by 0.8 % ([-1.6, -0.1]%), while rectum D0.2cc increased by 2.6 % ([1.2, 4.9]%). For all patients, rectum D0.2cc was still below the clinical constraint. Results of both DIR approaches differed on average by less than 0.2 %.

Conclusions

Online adaptation in MRgRT improved target coverage and OARs sparing at the level of accumulated dose.

Dosimetric predictors for genitourinary toxicity in MR-guided stereotactic body radiation therapy (SBRT): Substructure with fraction-wise analysis

BACKGROUND

MR-guided radiation therapy (MRgRT) systems provide superior soft tissue contrast than x-ray based systems and can acquire real-time cine for treatment gating. These features allow treatment planning margins to be reduced, allowing for improved critical structure sparing and reduced treatment toxicity. Despite this improvement, genitourinary (GU) toxicity continues to affect many patients.

PURPOSE

(1) To identify dosimetric predictors, potentially in combination with clinical parameters, of GU toxicity following SBRT by leveraging MRgRT to accurately monitor daily dose, beyond predicted dose calculated during planning. (2) Improve awareness of toxicity-sensitive bladder substructures, specifically the trigone and urethra.

METHODS

Sixty-nine prostate cancer patients (NCT04384770 clinical trial) were treated on a ViewRay MRIdian MRgRT system, with 40 Gy prescribed to 95% of the PTV in over five fractions. Overall, 17 (24.6%) prostate patients reported acute grade 2 GU toxicity. The CTV, PTV, bladder, bladder wall, trigone, urethra, rectum, and rectal wall were contoured on the planning and daily treatment MRIs. Planning and daily treatment DVHs (0.1 Gy increments), organ doses (min, max, mean), and organ volumes were recorded. Daily dose was estimated by transferring the planning dose distributions to the daily MRI based on the daily setup alignment. Patients were partitioned into a training (55) and testing set (14). Dose features were pre-filtered using a t-test followed by maximum relevance minimum redundancy (MRMR) algorithm. Logistic regression was investigated with regularization to select dosimetric predictors. Specifically, two approaches: time-group least absolute shrinkage and selection (LASSO), and interactive grouped greedy algorithm (IGA) were investigated. Shared features across the planning and five treatment fractions were grouped to encourage consistency and stability. The conventional flat non-temporally grouped LASSO was also evaluated to provide a solid benchmark. After feature selection, a final logistic regression model was trained. Dosimetric regression models were compared to a clinical regression model with only clinical parameters (age, baseline IPSS, prostate gland size, ADT usage, etc.) and a hybrid model, combining the best performing dosimetric features with the clinical parameters, was evaluated. Final model performance was evaluated on the testing set using accuracy, sensitivity, and specificity determined by the optimal threshold of the training set.

RESULTS

IGA had the best testing performance with an accuracy/sensitivity/specificity of 0.79/0.67/0.82, selecting 12 groups covering the bladder (V19.8 Gy, V20.5 Gy), bladder wall (19.7 Gy), trigone (15.9, 18.2, 43.3 Gy), urethra (V41.4 Gy, V41.7 Gy), CTV (V41.9 Gy), rectum (V8.5 Gy), and rectal wall (1.2, 44.1 Gy) dose features. Absolute bladder V19.8 Gy and V20.5 Gy were the most important features, followed by relative trigone 15.9  and 18.2 Gy. Inclusion of clinical parameters in the hybrid model with IGA did not significantly change regression performance.

CONCLUSION

Overall, IGA feature selection resulted in the best GU toxicity prediction performance. This exploratory study demonstrated the feasibility of identification and analysis of dosimetric toxicity predictors with awareness to sensitive substructures and daily dose to potentially provide consistent and stable dosimetric metrics to guide treatment planning. Further patient accruement is warranted to further assess dosimetric predictor and perform validation.

Optimising the MR-Linac as a standard treatment modality

The magnetic resonance linear accelerator (MR-Linac) offers a new treatment paradigm, providing improved visualisation of targets and organs at risk while allowing for daily adaptation of treatment plans in real time. Online MR-guided adaptive treatment has reduced treatment uncertainties; however, the additional treatment time and resource requirements may be a concern. We present our experience of integrating an MR-Linac into a busy department and provide recommendations for improved clinical and resource efficiency. Furthermore, we discuss potential future technological innovations that can further optimise clinical productivity in a busy department.

Acute toxicity comparison of magnetic resonance-guided adaptive versus fiducial or computed tomography-guided non-adaptive prostate stereotactic body radiotherapy: A systematic review and meta-analysis

BACKGROUND

Stereotactic body radiotherapy (SBRT) is gaining wider adoption for prostate cancer management but there remain significant toxicity risks when delivering prostate SBRT with standard techniques. Magnetic resonance-guided daily adaptive SBRT (MRg-A-SBRT) offers technological advantages in precision of radiation dose delivery, but the toxicity profile associated with MRg-A-SBRT compared to more standardly used fiducial or computed tomography-guided non-adaptive prostate SBRT (CT-SBRT) remains unknown.

METHODS

A meta-analysis to compare acute toxicity rates associated with MRg-A-SBRT and CT-SBRT for prostate cancer was performed in compliance with PRISMA guidelines. MEDLINE (PubMed) and Google Scholar were searched for prospective studies of prostate SBRT that were published between January 1, 2018 and August 31, 2022. Random effects and fixed effects models were used to estimate pooled toxicity rates, and meta-regression was performed to compare toxicity between MRg-A-SBRT and CT-SBRT study groups.

RESULTS

Twenty-nine prospective studies were identified that met the inclusion criteria and included a total of 2547 patients. The pooled estimates for acute grade 2 or higher (G2+) genitourinary (GU) and gastrointestinal (GI) toxicity for MRg-A-SBRT were 16% (95% confidence interval [CI], 10%-24%) and 4% (95% CI, 2%-7%) and for CT-SBRT they were 28% (95% CI, 23%-33%) and 9% (95% CI, 6%-12%), respectively. On meta-regression, the odds ratios for acute G2+ GU and GI toxicities comparing MRg-A-SBRT and CT-SBRT were 0.56 (95% CI, 0.33-0.97, p = .04) and 0.40 (95% CI, 0.17-0.96, p = .04), respectively.

CONCLUSION

MRg-A-SBRT is associated with a significantly reduced risk of acute G2+ GU or GI toxicity compared to CT-SBRT. Longer follow-up will be needed to evaluate late toxicity and disease control outcomes.

PLAIN LANGUAGE SUMMARY

Magnetic resonance imaging-guided daily adaptive prostate stereotactic radiation (MRg-A-SBRT) is a treatment that may allow for delivery of prostate radiation more precisely than other radiotherapy techniques, but it is unknown whether this reduces side effects compared to standardly used computed tomography-guided SBRT (CT-SBRT). In this systematic review and meta-analysis combining data from 29 clinical trials including 2547 patients, it was found that the risk of short-term urinary side effects was reduced by 44% and the risk of short-term bowel side effects was reduced by 60% with MRg-A-SBRT compared to CT-SBRT.

Experimental validation of multi-fraction online adaptations in magnetic resonance guided radiotherapy

Background and purpose

Radiotherapy plan verification is generally performed on the reference plan based on the pre-treatment anatomy. However, the introduction of online adaptive treatments demands a new approach, as plans are created daily on different anatomies. The aim of this study was to experimentally validate the accuracy of total doses of multi-fraction plan adaptations in magnetic resonance imaging guided radiotherapy in a phantom study, isolated from the uncertainty of deformable image registration.

Materials and methods

We experimentally verified the total dose, measured on external beam therapy 3 (EBT3) film, using a treatment with five online adapted fractions. Three series of experiments were performed, each focusing on a category of inter-fractional variation; translations, rotations and body modifications. Variations were introduced during each fraction and adapted plans were generated and irradiated. Single fraction doses and total doses over five online adapted fractions were investigated.

Results

The online adapted measurements and calculations showed a good agreement for single fractions and multi-fraction treatments for the dose profiles, gamma passing rates, dose deviations and distances to agreement. The gamma passing rate using a 2%/2 mm criterion ranged from 99.2% to 99.5% for a threshold dose of 10% of the maximum dose (Dmax) and from 96.2% to 100% for a threshold dose of 90% of Dmax, for the total translations, rotations and body modifications.

Conclusions

The total doses of multi-fraction treatments showed similar accuracies compared to single fraction treatments, indicating an accurate dosimetric outcome of a multi-fraction treatment in adaptive magnetic resonance imaging guided radiotherapy.

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.

Applicability and usage of dose mapping/accumulation in radiotherapy

Dose mapping/accumulation (DMA) is a topic in radiotherapy (RT) for years, but has not yet found its widespread way into clinical RT routine. During the ESTRO Physics workshop 2021 on "commissioning and quality assurance of deformable image registration (DIR) for current and future RT applications", we built a working group on DMA from which we present the results of our discussions in this article. Our aim in this manuscript is to shed light on the current situation of DMA in RT and to highlight the issues that hinder consciously integrating it into clinical RT routine. As a first outcome of our discussions, we present a scheme where representative RT use cases are positioned, considering expected anatomical variations and the impact of dose mapping uncertainties on patient safety, which we have named the DMA landscape (DMAL). This tool is useful for future reference when DMA applications get closer to clinical day-to-day use. Secondly, we discussed current challenges, lightly touching on first-order effects (related to the impact of DIR uncertainties in dose mapping), and focusing in detail on second-order effects often dismissed in the current literature (as resampling and interpolation, quality assurance considerations, and radiobiological issues). Finally, we developed recommendations, and guidelines for vendors and users. Our main point include: Strive for context-driven DIR (by considering their impact on clinical decisions/judgements) rather than perfect DIR; be conscious of the limitations of the implemented DIR algorithm; and consider when dose mapping (with properly quantified uncertainties) is a better alternative than no mapping.

Knowledge-based adaptive planning quality assurance using dosimetric indicators for stereotactic adaptive radiotherapy for pancreatic cancer

INTRODUCTION

We aimed to develop knowledge-based tools for robust adaptive radiotherapy (ART) planning to determine on-table adaptive DVH metric variations or planning process errors for stereotactic pancreatic ART. We developed volume-based dosimetric identifiers to identify deviations of ART plans from simulation plans.

MATERIALS AND METHODS

Two patient cohorts who were treated on MR-Linac for pancreas cancer were included in this retrospective study; a training cohort and a validation cohort. All patients received 50 Gy in 5 fractions. PTV-OPT was generated by subtracting the critical organs plus a 5 mm-margin from PTV. Several metrics that potentially can identify failure-modes were calculated including PTV & PTV_OPT V95% and PTV & PTV_OPT D95%/D5%. The difference between each DVH metric in each adaptive plan with the DVH metric in simulation plan was calculated. The 95% confidence interval (CI) of the variations in each DVH metric was calculated for the patient training cohort. Variations in DVH metrics that exceeded the 95% CI for all fractions in training and validation cohort were flagged for retrospective investigation for root-cause analysis to determine their predictive power for identifying failure-modes.

RESULTS

The CIs for the PTV & PTV_OPT V95% and PTV & PTV_OPT D95%/D5% were ± 13%, ± 5%, ± 0.1, ± 0.03, respectively. We estimated the positive predictive value and negative predictive value of our method to be 77% and 89%, respectively, for the training cohort, and 80% for both in the validation cohort.

DISCUSSION

We developed dosimetric indicators for ART planning QA to identify population-based deviations or planning errors during online adaptive process for stereotactic pancreatic ART. This technology may be useful as an ART clinical trial QA tool and improve overall ART quality at an institution.

Interfractional dose accumulation for MR-guided liver SBRT: Variation among algorithms is highly patient- and fraction-dependent

BACKGROUND AND PURPOSE

Daily plan adaptations could take the dose delivered in previous fractions into account. Due to high dose delivered per fraction, low number of fractions, steep dose gradients, and large interfractional organ deformations, this might be particularly important for liver SBRT. This study investigates inter-algorithm variation of interfractional dose accumulation for MR-guided liver SBRT.

MATERIALS AND METHODS

We assessed 27 consecutive MR-guided liver SBRT treatments of 67.5 Gy in three (n = 15) or 50 Gy in five fractions (n = 12), both prescribed to the GTV. We calculated fraction doses on daily patient anatomy, warped these doses to the simulation MRI using seven different algorithms, and accumulated the warped doses. Thus, we obtained differences in planned doses and warped or accumulated doses for each algorithm. This enabled us to calculate the inter-algorithm variations in warped doses per fraction and in accumulated doses per treatment course.

RESULTS

The four intensity-based algorithms were more consistent with planned PTV dose than affine or contour-based algorithms. The mean (range) variation of the dose difference for PTV D_95% due to dose warping by these intensity-based algorithms was 10.4 percentage points (0.3 to 43.7) between fractions and 8.6 (0.3 to 24.9) between accumulated treatment doses. As seen by these ranges, the variation was very dependent on the patient and the fraction being analyzed. Nevertheless, no correlations between patient or plan characteristics on the one hand and inter-algorithm dose warping variation on the other hand was found.

CONCLUSION

Inter-algorithm dose accumulation variation is highly patient- and fraction-dependent for MR-guided liver SBRT. We advise against trusting a single algorithm for dose accumulation in liver SBRT.

Dosimetric evaluation of magnetic resonance imaging-guided adaptive radiation therapy in pancreatic cancer by extent of re-contouring of organs-at-risk

PURPOSE

The safety of online contouring and planning for adaptive radiotherapy is unknown. This study aimed to evaluate the dosimetric difference of the organ-at-risk (OAR) according to the extent of contouring in stereotactic magnetic resonance image-guided adaptive RT (SMART) for pancreatic cancer.

MATERIALS AND METHODS

We reviewed the treatment plan data used for SMART in patients with pancreatic cancer. For the online contouring and planning, OARs within 2 cm from the planning target volume (PTV) in the craniocaudal direction were re-controlled daily at the attending physician's discretion. The entire OARs were re-contoured retrospectively for data analysis. We termed the two contouring methods the Rough OAR and the Full OAR, respectively. The proportion of dose constraint violation and other dosimetric parameters was analyzed.

RESULTS

Nineteen patients with 94 fractions of SMART were included in the analysis. The dose constraint was violated in 10.6% and 43.6% of the fractions in Rough OAR and Full OAR methods, respectively (p = 0.075). Patients with a large tumor, a short distance from gross tumor volume (GTV) to OAR, and a tumor in the body or tail were associated with more occult dose constraint violations-large tumor (p = 0.027), short distance from GTV to OAR (p = 0.061), tumor in body or tail (p = 0.054). No dose constraint violation occurred outside 2 cm from the PTV.

CONCLUSION

More occult dose constraint violations can be found by the Full OAR method in patients with pancreatic cancer with some clinical factors in the online re-planning for SMART. Re-contouring all the OARs would be helpful to detect occult dose constraint violations in SMART planning. Since the dosimetric profile of SMART cannot be represented by a single fraction, patient selection for the Full OAR method should be weighted between the clinical usefulness and the time and workforce required.

Magnetic Resonance-Guided Prostate Stereotactic Body Radiation Therapy With Daily Online Plan Adaptation: Results of a Prospective Phase 1 Trial and Supplemental Cohort

Purpose

Stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) for prostate cancer allows for MR-based contouring, real-time MR motion management, and daily plan adaptation. The clinical and dosimetric benefits associated with prostate SMART remain largely unknown.

Methods and Materials

A phase 1 trial of prostate SMART was conducted with primary endpoints of safety and feasibility. An additional cohort of patients similarly treated with prostate SMART were included in the analysis. SMART was delivered to 36.25 Gy in 5 fractions to the prostate ± seminal vesicles using the MRIdian linear accelerator system (ViewRay, Inc). Rates of urinary and gastrointestinal toxic effects and patient-reported outcome measures were assessed. Dosimetric analyses were conducted to evaluate the specific benefits of daily plan adaptation.

Results

The cohort included 22 patients (n = 10 phase 1, n = 12 supplemental) treated in 110 fractions. Median follow-up was 7.9 months. Acute grade 2 urinary and gastrointestinal toxic effects were observed in 22.7% and 4.5%, respectively, and 4.5% and 0%, respectively, at last follow-up. No grade 3+ events were observed. Expanded Prostate Cancer Index-26 urinary obstructive scores decreased during SMART (mean, 9.3 points; P = .03) and returned to baseline by 3 months. No other significant changes in patient-reported outcome measures were observed. One-hundred percent of fractions required plan adaptation owing to exceeding organ-at-risk metrics (68%) or suboptimal target coverage (33%) resulting from anatomic changes. Minimum acceptable planning target volume, rectal, bladder, and urethra/bladder neck metrics were violated in 24%, 20%, 24%, and 33% of predicted plans, respectively; 0% of reoptimized plans violated metrics. Underlying causes for deficient dosimetry before reoptimization included changes in bladder filling, seminal vesicle position, prostate volume (median 4.7% increase by fraction 3; range, 0%-56%), and hotspots shifting into urethra/bladder neck.

Conclusions

Prostate SMART results in low risk of acute toxic effects with improvements in target and organ-at-risk dosimetry. The clinical benefits resulting from daily plan adaptation, including urethra/bladder neck protection, warrant further investigation.

Accumulated bladder wall dose is correlated with patient-reported acute urinary toxicity in prostate cancer patients treated with stereotactic, daily adaptive MR-guided radiotherapy

BACKGROUND AND PURPOSE

Magnetic resonance (MR)-guided linear accelerators (MR-Linac) enable accurate estimation of delivered doses through dose accumulation using daily MR images and treatment plans. We aimed to assess the association between the accumulated bladder (wall) dose and patient-reported acute urinary toxicity in prostate cancer (PCa) patients treated with stereotactic body radiation therapy (SBRT).

MATERIALS AND METHODS

One-hundred-and-thirty PCa patients treated on a 1.5T MR-Linac were included. Patients filled out International Prostate Symptom Scores (IPSS) questionnaires at baseline, 1 month, and 3 months post-treatment. Deformable image registration-based dose accumulation was performed to reconstruct the delivered dose. Dose parameters for both bladder and bladder wall were correlated with a clinically relevant increase in IPSS (≥10 points) and/or start of alpha-blockers within 3 months using logistic regression.

RESULTS

Thirty-nine patients (30%) experienced a clinically relevant IPSS increase and/or started with alpha-blockers. Bladder D5cm³, V_10-35Gy (in %), and D_mean and Bladder wall V_10-35Gy (cm³ and %) and D_mean were correlated with the outcome (odds ratios 1.04-1.33, p-values 0.001-0.044). Corrected for baseline characteristics, bladder V_10-35Gy (in %) and D_mean and bladder wall V_10-35Gy (cm³ and %) and D_mean were still correlated with the outcome (odds ratios 1.04-1.30, p-values 0.001-0.028). Bladder wall parameters generally showed larger AUC values.

CONCLUSION

This is the first study to assess the correlation between accumulated bladder wall dose and patient-reported urinary toxicity in PCa patients treated with MR-guided SBRT. The dose to the bladder wall is a promising parameter for prediction of patient-reported urinary toxicity and therefore warrants prospective validation and consideration in treatment planning.