SU‐E‐J‐15: Weekly Bladder V65 and V40 Changes in Correlation to the CBCT Bladder Filling

Abstract

Purpose: Full bladder at time of simulation and daily treatment is requested of prostate/prostate‐bed patients; however, not all patients comply. This study is to analyze the V65 and V40 changes in correlation with CBCT bladder filling. Methods: Daily 2D KV/KV IGRT using implanted radio‐opaque markers and weekly 3D CBCT IGRT direct visualization of target volume were routinely performed. Physicians compared the pre‐treatment CBCT bladder filling to the planned CT bladder filling to identify the poor bladder fillers. Weekly CBCT bladder volumes were contoured of poor bladder fillers, and VMAT/IMRT plans recalculated to obtain the actual bladder dose, using the commercially available DVH Evaluator to obtain the V65 and V40 values promptly and accurately. Results: From Jan 2012‐Nov 2012, a total of 181 prostate/prostate‐bed patients were analyzed. 47 patients were classified as poor‐average bladder fillers. A total of 358 CBCT bladders were contoured to obtain bladder volumes/doses. The 358 CBCT images were grouped into 10 bins to differentiate bladder volumes from baseline. The median change in V65 and V40 bins were both monotonically increasing for this dataset. Despite this evident trend, no statistically significant correlation in IPSS scores was found in relation to the change in V65 or V40. Conclusions: The V65 <= 25% and V40 <= 50% are among the tightest bladder constraints in common clinical use. Since the image registration aligns the prostate and the daily bladder volumes vary, the resulting tightly conformal plans are robust to daily bladder filling variations. No statistically significant correlation to IPSS score changes were identified thus far in relation to the bladder filling changes. Additional follow‐up is required to ensure that there are no long‐term adverse effects based on poor bladder filling during care. Intellectual property rights for the DVH Evaluator software tool, which was used for this analysis