Determination of the Effect of Patient-Specific Bladder Filling Protocol for Operated Prostate Cancer Patients on Bladder Volume and Doses Using Cone Beam Computed Tomography Dose Calculations

Abstract

One of the problems in prostate cancer radiotherapy is the effect of bladder volume changes on irradiated target volume and bladder itself. The aim of this study is to develop a patient-specific bladder filling protocol (PSP) by the help of a bladder scanner (BS) in operated patients and to compare the volumetric and dosimetric parameters with that of the standard filling protocol (SP). For the PSP the patient was asked to empty his bladder and drink 500 mL of water. The changes in bladder volume was measured with BS every 5 minutes. The time to reach a 150-200 cc-volume was recorded. That specific time was used for CT simulation and treatment. For the SP after emptying the bladder and drinking 500 mL of water the patient was asked for urgency. Ten patients in each protocol received 72 Gy postprostatectomy salvage RT with VMAT technique. Catphan phantom was loaded into the Monaco 3.30 planning system after creating a CT-to-ED calibration graph with the aid of cone-beam computed tomography (CBCT) images. CBCT images were taken at five different fractions. OAR,CTV and PTV’s were contoured on CBCT images. Treatment plans done with the planning computed tomography (PCT) images were transferred to CBCTs without any mechanical or dosimetric changes. With the help of CBCT doses average treatment DVHs of each patient were obtained. PCT and BS correlation, changes in bladder volume and doses, standard deviations (SD), and their maximum and minimum values for PCT and CBCT were calculated for both protocols. Pearson correlation coefficient for PCT-BS was 0.74.For PSP average bladder volume and SDs, average bladder doses and SDs for PCT and CBCT were 136.92±25.4 cc and 130.38±25.1 cc, and 159.69±6.5 cGy and 160.79±10.1 cGy respectively. For SP the same values were 205.76±87.5 cc and 170.58±55.2 cc, 151.36±25.1 cGy and 163.54±18.3 cGy respectively. Average absolute difference of bladder volumes between PCT and CBCT were 48.47% for SP and 9.91% for PSP (p=0.007). Average absolute difference of bladder doses between PCT and CBCT for SP and PSP were 12.97% and 4.27% respectively (p=0.130). The dose difference between PCT and CBCT was not statistically significant for PSP (p=0.799). There was a trend for significance for SP (p=0.074). No statistically significant difference was found for bladder volumes between PCT and CBCT in both protocols (p=0.139 and p=0.139). With the PSP optimal bladder filling was obtained and maintained throughout the whole treatment course and it was reproducible in every fraction. Average CBCT doses were close to PCT doses, absolute difference of the bladder volumes and doses between PCT and CBCT, interfractional volume and dose changes were very small compared to SP. The findings of this study were reflected to our clinical practice and the determination of the changes in bladder volume and the time for obtaining optimal bladder volume by BS before and throughout the treatment in prostate cancer patients has become our routine clinical practice.