TH‐D‐210A‐08: On the Importance of Correcting Anatomical Deformations in Prostate Cancer Patients

Purpose: To assess the effects of target, rectum and bladder anatomical deformations on targeting accuracy of prostate and postprostatectomy cancer patients undergoing IG‐IMRT. Methods: Localization with online kV‐CBCT was performed. The target and OARs positional/volumetric changes were evaluated and couch shifts were applied. For patients with large target/OARs volumetric changes compared to planning CT, arising from medications, diet, and/or ongoing RT, repeated localization CB scans were performed following an interventional procedure, shifts were then evaluated, and the IMRT treatment was subsequently delivered. The interventional procedure involves the insertion of a rectal catheter or rectal deflation, and/or bladder filling. A total of 160 pre‐/post‐intervention shifts from 14 patients in the lateral/LR, vertical/AP, and longitudinal/SI directions were compared. The percentage of shifts larger than 5 mm in all directions was also compared. CTV‐to‐PTV expansion margins were estimated based on the pre‐ and post‐intervention localization data. Results: Systematic/S and random/s shifts from pre‐versus post‐intervention data (in mm) were: LR, 0.2±2.8 vs. 0.4±2.9; AP, −0.7±5.3 vs. −1.1±3.6; SI, 0.6±3.7 vs. −0.5±2.5. The mean 3D shift distance was 6.4±3.1 vs. 4.8±2.4 with a p‐value < 0.05. The percentage of preintervention shifts larger than 5 mm were 7.5%, 31.3%, and 16.3% in the LR, AP, and SI directions, respectively, compared to 8.8%, 15.0%, and 6.3% for post‐intervention. Large anatomical variations were observed for rectum and/or bladder, suggesting that localization without intervention may not be sufficient to ensure accurate targeting and sparing of rectum/bladder. Conclusion: Localization data from pre‐ and post‐intervention procedures show that for treatments that do not include intervention to correct for rectum/bladder anatomical variations, the CTV‐to‐PTV margin required is larger by 5 mm, and more rectum/bladder volumes are potentially at risk of radiation‐induced acute or late toxicity.