Prostate volumes and organ movement defined by serial computerized tomographic scans during three-dimensional conformal radiotherapy.

The aim of this study was to assess changes in prostate volumes and organ movement during a course of external beam irradiation using serial computerized tomographic (CT) scans and three-dimensional treatment planning software. Ten consenting prostate cancer patients underwent repeat CT scans at biweekly intervals during the course of external beam irradiation. The spacing of 5 mm was used because this spacing mimics our clinical treatment approach. Prostate locations were determined by merging CT images using bony anatomy and comparing the differences in the prostate volumes, the edges (anterior, posterior, superior, inferior, and lateral) and centers of the prostate (EoP and CoP, respectively). Compared to the 10 initial treatment planning CT scans, the prostate volume determined by the repeat CT scans tended to be smaller (approximately 14%, P < 0.001). The prostate volumes determined by repeat CT scans tended to be stable with a mean volume of 86% (S.D. = 18%) of the initial CT. When assessed by changes in the EoP, superior movements appeared to be the most common source for concern for adequate coverage of the prostate, while inferior movement was not seen. When assessed by changes in CoP, movement of > or = 3 mm was noted in 47% of the studies in the superior direction, with the average displacement being approximately 2.0 mm. In this study, the prostate volume tended to be smaller 2 weeks after the start of radiotherapy. Moreover, the prostate volumes defined by the serial CT scans were less reproducible than expected. Superior displacement of the prostate is the most common and significant type of displacement, while inferior movement is least frequent when patients are simulated with their rectums empty. Because of the magnitude of daily setup errors, organ movement, and problems with reproducibility in target definition, additional field edge reductions do not appear to be warranted during the delivery of three-dimensional conformal radiotherapy. Efforts should be directed at improving our ability to reduce organ movement and accurately targeting the prostate.