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Purpose-To investigate the impact of bladder filling state on dosimetry and determine the best bladder dosimetric parameter in vaginal-cuff brachytherapy.Materials and Methods-Twenty women received vaginal cylinder high-dose-rate (HDR) brachytherapy with each fraction followed by a planning CT scan on a prospective clinical trial. The bladder was full for fraction 2 and empty for fraction 3. Dose volume histogram (DVH) and dose surface histogram (DSH) values were generated for the bladder, rectum, and urethra. The midline maximum bladder point (MBP) and the midline maximum rectal point (MRP) were recorded. Paired t-tests, Pearson correlations, and regression analyses were performed.Results-The volume and surface area of bladder irradiated were significantly smaller when the bladder was empty than when full. Of several DVH and DSH parameters evaluated, the bladder D2cc, V50, V70 and SA50 significantly predicted the difference in empty versus full filling states. The V70 and D2cc were significantly correlated with the MBP. Bladder filling did not alter the volume or surface area of rectum irradiated. However, an empty bladder did result in the nearest point of bowel being significantly closer to the vaginal cylinder than when the bladder was full.Conclusions-In order to minimize radiation dose to the bladder, patients receiving vaginal-cuff HDR brachytherapy should be treated with an empty bladder if feasible. The MBP correlates well with the volumetric assessments of bladder dose and provides a non-invasive method for reporting maximum bladder point dose using 3D imaging. The MBP can therefore be used as a surrogate for complex dosimetry in the clinic.