Adaptive radiotherapy for bladder cancer using deformable image registration of empty and full bladder

Abstract

A common objective of various adaptive radiotherapy (ART) strategies for bladder cancer is to reduce irradiation of normal tissue, thereby reduce the risk of radiation induced toxicity, and maintain or improve the target coverage. Bladder radiotherapy, typically involves generous margins (up to 20 mm) for bladder planning target volume (PTV). The goal of this retrospective study is to define, evaluate and optimize new patient-specific anisotropic PTVs (a-PTVs) using deformable image registration (DIR) between empty and full bladder computed tomography (CT) scans. This will provide an ART that incorporates the extreme deformations of the bladder, and is applicable from the first day of treatment. Deformation vector fields (DVFs), measured from the deformable image registration between empty and full bladder CTs, were scaled and constrained to construct the a-PTVs. For each patient, four a-PTVs were constructed such that a-PTV1 was the largest and a-PTV4 was the smallest. All the a-PTVs were defined such that they covered at least the bladder volume plus 5 mm margin. These a-PTVs were retrospectively evaluated and compared to the current clinical standard (conv-PTV), with 10 mm uniform margins, using 5 bladder cancer patients and a total of 100 fractions. It was found that the smaller a-PTV, a-PTV4 and a-PTV3, were appropriate in 87% of the fractions, while a-PTV2 and a-PTV1 were required in 12% of the fractions respectively. The use of the a-PTVs reduced the PTV volume by 32% (28-36%) as compared to conv-PTV. In conclusion, the results of this pilot study indicate that the use of a-PTVs could result in substantial decrease in the course averaged planning target volume. This reduction in the PTV is likely to decrease the radiation related toxicity and benefit bladder cancer patients. Currently, more patients are being investigated to strengthen these findings, and also dosimetric analysis is underway.