Evaluation of hybrid DIR performance using controlling structures and points of interest in MR-guided adaptive radiotherapy for prostate cancer patients.

[BACKGROUND] MR-guided adaptive radiotherapy (ART) allows for daily plan optimization based on patient-specific anatomy. Accumulated doses, driven by deformable image registration (DIR), of daily fractions can provide cumulative dose metrics and insights into toxicity and tumor control. In prostate ART, inter- and intra-factional deformations, particularly due to bladder and rectum, pose a challenge to accurate DIR generation.

[PURPOSE] To quantify geometric and dosimetric accuracy of a proposed prostate MR-to-MR DIR approach to support MR-guided ART dose accumulation.

[METHODS] We evaluated DIR accuracy in 25 patients treated with 30 Gy in five fractions on a 1.5 T MR-linac using an adaptive workflow. For all patients, a reference MR was used for planning, with three images collected at each fraction: adapt MR for adaptive planning, verify MR for pretreatment position verification and beam-on for capturing anatomy during radiation delivery. We assessed three DIR approaches: intensity-based, intensity-based with controlling structures (CS), and intensity-based with controlling structures and points of interest (CS + P). DIRs were performed between the reference and fraction images and within fractions (adapt-to-verify and adapt-to-beam-on). For the evaluation, we propagated CTV, bladder, and rectum contours using the DIRs and compared each to manually delineated contours using Dice similarity coefficient, mean distance to agreement, and dose-volume metrics.

[RESULTS] CS and CS + P improved geometric agreement between manual and propagated contours over intensity-only DIR. For example, mean distance to agreement (DTA) for reference-to-beam-on intensity-only DIR was 0.131 ± 0.009 cm (CTV), 0.46 ± 0.08 cm (bladder), and 0.154 ± 0.013 cm (rectum). For the CS, the DTA values were 0.018 ± 0.002, 0.388 ± 0.14, and 0.036 ± 0.013 cm. Finally, for CS + P, these values were 0.015 ± 0.001, 0.025 ± 0.004, and 0.021 ± 0.002 cm. Dosimetrically, comparing CS and CS + P for reference to beam-on DIRs resulted in a change of CTV D98% from [-29 cGy, 19 cGy] to [-18 cGy, 26 cGy], bladder D5cc from [-51 cGy, 544 cGy] to [-79 cGy, 36 cGy], and rectum D1cc from [-106 cGy, 72 cGy] to [-52 cGy, 74 cGy].

[CONCLUSION] CS improved geometric and dosimetric accuracy over intensity-only DIR, with CS + P providing further performance improvement, particularly for bladder. However, session image segmentation remains a challenge, which may be addressed with automated contouring.