Assessment and management of interfractional variations in daily diagnostic-quality-CT guided prostate-bed irradiation after prostatectomy.

PURPOSE: To quantify interfractional anatomic variations and limitations of the current practice of image-guided radiation therapy (IGRT) for prostate-bed patients and to study dosimetric benefits of an online adaptive replanning scheme that addresses the interfractional variations.

METHODS: Contours for the targets and organs at risk (OARs) from daily diagnostic-quality CTs acquired with in-room CT (CTVision, Siemens) were generated by populating the planning contours using an autosegmentation tool based on deformable registration (ABAS, Elekta) with manual editing for ten prostate-bed patients treated with postoperative daily CT-guided IMRT. Dice similarity coefficient (DSC) obtained by maximizing the overlap of contours for a structure between the daily and plan contours was used to quantify the organ deformation between the plan and daily CTs. Three interfractional-variation-correction schemes, the current standard practice of IGRT repositioning, a previously developed online adaptive RT (ART), and the full reoptimization, were applied to these daily CTs and a number of dose-volume quantities for the targets and organs at risk were compared for their effectiveness to account for the interfractional variations.

RESULTS: Large interfractional organ deformations in prostate-bed irradiation were seen. The mean DSCs for CTV, rectum, and bladder were 86.6 ± 5.1% (range from 61% to 97%), 77.3% ± 7.4% (range from 55% to 90%), and 75.4% ± 11.2% (range from 46% to 96%), respectively. The fractional and cumulative dose-volume quantities for CTV and PTV: V100 (volume received at least 100% prescription dose), and rectum and bladder: V45Gy and V60Gy (volume received at least 45 or 60 Gy), were compared for the repositioning, adaptive, reoptimization, and original plans. The fractional and cumulative dosimetric results were nearly the same. The average cumulative CTV V100 were 88.0%, 98.4%, 99.2%, and 99.3% for the IGRT, ART, reoptimization, and original plans, respectively. The corresponding rectal V45Gy (V60Gy) were 58.7% (27.3%), 48.1% (20.7%), 43.8% (16.1%), and 44.9% (16.8%). The results for bladder were comparable among three schemes. Paired two-tailed Wilcoxon signed-rank tests were performed and it was found that ART and reoptimization provide better target coverage and better OAR sparing, especially rectum sparing.

CONCLUSIONS: The interfractional organ motions and deformations during prostate-bed irradiation are significant. The online adaptive replanning scheme is capable of effectively addressing the large organ deformation, resulting in cumulative doses equivalent to those originally planned.