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 patients 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, patient-specific contraction, patientspecific 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 bladder filling, use of a generic or patient-specific constant contraction resulted in a significant overestimation of IB volume (112 and 113 cm3, respectively; p,0.001) that was inversely correlated with the difference in urine volume between the scans (R2.0.86). Mean differences across 95% of IB surfaces were #2mm for methods using either constant or variable wall volume. Mean dose-volume histogram (DVH) differences were ,1cm3 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 BWon serial imaging for the purposes of dose reconstruction.