Prostate position late in the course of external beam therapy: Patterns and predictors

Purpose: To examine prostate and seminal vesicles position late in the cour se of radiation therapy and to determine the effect and predictive value of the bladder and rectum on prostate and seminal vesicles positioning, Methods and Materials: Twenty-four patients with localized prostate cancer underwent a computerized tomography scan (CT1) before the start of radiatio n therapy. After 3-5 weeks of radiation therapy, a second CT scan (CT2) was obtained. All patients were scanned in the supine treatment position with instructions to maintain a full bladder. The prostate, seminal vesicles, bl adder, acid rectum were contoured, CT2 was aligned via fixed bony anatomy t o CT1, The geometrical center and volume of each structure were obtained an d directly compared. Results: The prostate shifted along a diagonal axis extending from an anter ior-superior position to a posterior-inferior position, The dominant shift was to a more posterior-inferior position. On average, bladder and rectal v olumes decreased to 51% (+/-29%) and 82% (+/-45%) of their pretreatment val ues, respectively. Multiple regression analysis (MRA) revealed that bladder movement and volume change and upper rectum movement were independently as sociated with prostate motion (p = 0.016, p = 0.003, and p = 0.052 respecti vely). Conclusion: Patients are often instructed to maintain a full bladder during a course of external beam radiation therapy, in the hopes of decreasing bl adder and small bowel toxicity. However, our study shows that large bladder volumes late in therapy are strongly associated with posterior prostate di splacement. This prostate displacement may result in marginal miss. (C) 200 0 Elsevier Science Inc.