Deep learning‐based dose prediction for MR‐guided prostate SIB: Supporting rapid feasibility assessment and adaptive editing margin selection

BACKGROUND: The template-based treatment planning workflow in the Unity magnetic resonance-guided linear accelerator (MR-Linac) helps facilitate the treatment planning process; however, dose optimization and calculation remain time-consuming in complex cases. In addition, the deformable image registration (DIR) algorithm used during adaptive planning may produce inaccurate results, thereby making efficient contour editing essential for online adaptation, especially in cases involving multiple organs at risk adjacent to the target. PURPOSE: This study aims to resolve key clinical challenges in treatment planning and online adaptive planning for prostate simultaneous integrated boost (SIB) treatment on the MR-Linac using deep learning (DL)-based dose prediction. Multiple DL-based dose prediction models are developed and evaluated for rapid dosimetric feasibility assessment, and the top-performing model is used to guide the development of a novel contouring strategy to streamline the adaptive planning workflow. METHODS: Six DL models were developed and evaluated for 3D dose prediction using 80 reference and adaptive plans from 20 patients previously treated with SIB intensity-modulated radiation therapy (SIB-IMRT) on the Unity MR-Linac. To address DIR-introduced contour inaccuracies during adaptation, a contour editing strategy based on a minimal manual editing margin (MMEM) around the target structures was introduced. The MMEM was determined using six margin sizes (5-30 mm, in 5-mm increments) evaluated by the top-performing DL model. Twenty adaptive plans with DIR inaccuracies in bladder, rectum, and bowel bag from 12 patients were analyzed for MMEM determination, and 10 adaptive plans were further evaluated for feasibility in the Monaco treatment planning system. RESULTS: The dosimetric differences between model predictions and the treatment planning system-calculated ground truth were small for most models (≤1.5 Gy or 2%), and the average Dice Similarity Coefficients across the 10-100% isodose volumes exceeded 0.9, demonstrating strong spatial agreement. Among the evaluated models, the attention-gated (AG) U-Net achieved the best overall performance and was used for the MMEM determination. Based on the MMEM and feasibility analyses, a 10-mm editing margin is recommended for the rectum, with the use of the semi-automatic segmentation tool for the bladder and no manual edits for the bowel bag. CONCLUSION: The AG U-Net enabled accurate dose prediction for rapid dosimetric feasibility assessment in prostate SIB treatment on the Unity MR-Linac. Leveraging this model, an MMEM-based contouring strategy was developed to guide contour modification and improve the efficiency of online adaptive planning.