Does a patient-specific bladder-filling protocol affect bladder volume and dose in postprostatectomy radiotherapy?

Purpose: Our aim is to develop a patient-specific bladder-filling protocol (PSP) using an ultrasound-based bladder scanner (BS) and compare the volumetric and dosimetric parameters with those of the standard filling protocol (SP) in postprostatectomy patients. Methods: Twenty postprostatectomy patients who received salvage radiotherapy (72 Gy/36 fx) were included. For PSP, the patient was asked to drink 500 mL of water after emptying his bladder. Bladder volume was measured using BS every 10 min. Each patient's unique time to reach a 150–200 cc volume was used for simulation and treatment. For the SP, the patient was asked about the feeling of having a full bladder. Organs at risk (OAR) were contoured on cone-beam computed tomography (CBCT) scans that were transferred to the treatment planning system (TPS). Treatment plans were applied to CBCTs. Changes in bladder volume and doses for planning computed tomography (PCT) and CBCT were determined. Results: In the SP, there was no significant difference in mean bladder volume for PCT and CBCT (p = 0.139); however, there was a trend for significance in the mean bladder dose (p = 0.074). In PSP, there was no significant difference in the mean bladder volume or dose for PCT and CBCT (p = 0.139 and p = 0.799, respectively). There was a significant difference in terms of mean CBCT bladder volume between the two protocols (p = 0.007), whereas no significant difference was detected in terms of bladder dose (p = 0.130). Conclusion: With PSP, optimal bladder filling was obtained and maintained throughout the whole treatment course, and it was reproducible in every fraction. © 2023, The Author(s), under exclusive licence to Springer Nature B.V.This project was approved and funded within the context of Ege University Scientific Research Projects (Project number: 2015-TIP-071). First, a pilot study was performed on 20 postprostatectomy patients to test the accuracy of the BS (CUBEscan Biocon-700), and to find the correlation between the bladder volumes derived from the PCT scans and from the BS. The bladder volumes derived from the PCT scans of postprostatectomy patients in whom the ideal dose–volume constraints were obtained were evaluated, and from our experience, the ideal bladder volume was accepted as 150–200 cc. These patients were simulated and treated according to our department’s SP, in which the patient was instructed to restrict fluid intake 2 h before simulation, present with an empty rectum and empty bladder, drink 500-mL water within 15 min, and wait 30 min before CT simulation and before each treatment to maintain a comfortably full bladder. A bladder ultrasound scan was performed immediately before the PCT scan, while the patient was on the CT couch and bladder volume was recorded. Then the bladder was contoured on PCT scans, and bladder volume was recorded. Bladder volumes of 20 patients derived from the PCT scans and BS measurements were compared and a strong correlation was found between the measurements achieved by BS and PCT (Pearson’s correlation coefficient r = 0.74), indicating a high accuracy of the BS.Ege Üniversitesi: 2015-TIP-07