Delineating the inner bladder surface using uniform contractions from the outer surface under variable XXXX conditions.

@entity1 @entity1494
OBJECTIVE: To evaluate the methods to delineate the inner bladder (IB) surface using a uniform contraction from the outer bladder (OB) surface, assuming the bladder wall (BW) is either of constant thickness, constant volume or variable volume. METHODS: 14 prostate intensity-modulated radiotherapy @entity1 with 2 planning CTs were identified. For both CTs, OB was delineated using model-based segmentation. IB was delineated manually. Then, using uniform contractions from OB, the position of IB was approximated using a: 2.5-mm contraction, @entity1 -specific contraction, @entity1 -specific constant wall volume method and variable wall volume method. The structures created using those strategies were compared against the manual IB contours using geometric and dosimetric indices. RESULTS: In the presence of variable @entity1494 , use of a generic or @entity1 -specific constant contraction resulted in a significant overestimation of IB volume (+12 and +13   cm(3), respectively; p   <   0.001) that was inversely correlated with the difference in urine volume between the scans (R(2)   >   0.86). Mean differences across 95% of IB surfaces were  <=2   mm for methods using either constant or variable wall volume. Mean dose-volume histogram (DVH) differences were <1   cm(3) across the whole BW DVH when using the method that assumed a variable wall volume. CONCLUSION: The variable volume BW model provided the best approximation of the IB surface position under varying filling conditions, based on geometric and dosimetric indices. ADVANCES IN KNOWLEDGE: Use of the equation derived in this research provides a quick and accurate method to delineate the hollow BW on serial imaging for the purposes of dose reconstruction.
