Predicting the optimal bladder volume to minimize the dose to the organs at risk in carcinoma cervix treated with modern radiotherapy

Abstract

Purpose/Objective(s) Whilst planning external beam radiotherapy (EBRT) for carcinoma cervix, bladder volume is an important consideration in deciding dose to other organs at risk (OARs). Though conformal techniques like volumetric-modulated radiotherapy (VMAT) have reduced the dose received by the OARs, there is still scope for improvement by utilizing other techniques. Planning with a full bladder is one such strategy that pushes a part of the bladder wall and the bowel away from the planning target volume (PTV). This is complicated by the fact that holding urine becomes increasingly more difficult along the course of pelvic radiotherapy. This makes the bladder volume threshold important to reduce the doses received by the OARs. We aim to assess the optimal bladder volume to spare OARs in patients with carcinoma cervix treated with VMAT. Materials/Methods The data of 81 patients of carcinoma cervix stage IB2-IVB treated with EBRT using VMAT followed by brachytherapy was prospectively collected in this study. The patients were advised to void and drink 500 ml of water, 30 minutes following which, a simulation scan was acquired. The target was contoured according to the Japanese Clinical Oncology Group guidelines and OARs in line with Radiation Therapy Oncology Group guidelines. Constraints to the bowel, bladder, rectum, and sigmoid were V45 < 195cc, V45 < 35%, V45 < 45% and V45 < 45% respectively. A dose of 50.4 Gy in 28 fractions was prescribed to the PTV using VMAT. The generated plans were analyzed and acceptable plans were finalized for treatment with higher priority to PTV coverage while minimizing the OAR doses. The bladder volumes at simulation and the doses received by the OARS were correlated using Pearson's correlation. A receiver operating characteristic (ROC) curve was constructed to find the cut-off value of the bladder volume which facilitated achieving the dose constraints to the significantly correlated OARs with acceptable specificity and sensitivity. Results A correlational analysis of the bladder volume with the OAR dose constraints revealed that there was a significant negative weak correlation with the V45 Gy of the bladder and the bowel. A ROC curve was constructed assessing bladder volume against the achievability of the bladder dose constraint. The area under the curve was 0.78 showing a fairly accurate relationship between the variables. A bladder volume of 270cc is noted to be the best cut-off to achieve the bladder dose constraint with a sensitivity and specificity of approximately 75% and 80% respectively. The curve for bladder volume and achievability of bowel constraints yielded a curve with poor accuracy and an area under the curve of 0.64 and so further analysis for the same was abandoned given the low sensitivity and specificity of the attained cut-off (approximately 65%). Conclusion Increasing bladder volume has reduced the doses received by bowel and bladder. A volume of 270cc is to be aimed at to achieve the V45 Gy of the bladder in patients of carcinoma cervix being treated with VMAT.