Evaluation of changes in the size and location of the prostate, seminal vesicles, bladder, and rectum during a course of external beam radiation therapy

Purpose: To document the size and location of the prostate, seminal vesicles, bladder, and rectum throughout the course of external beam radiotherapy. The frequency and range of motion of these organs are quantified. Methods and Materials: Ten patients with localized carcinoma of the prostate had conventional simulation followed immediately by a treatment planning computed tomography scan (TPCT0). Once treatment was scan initiated, each patient had a weekly CT (TPCT1-N) before or after his daily treatment. Anatomical structures from CT were delineated on a computer workstation for analysis. The serial CT sets were spatially registered to the initial scan using image correlation software that brings into congruence the bony pelvis of the different scans. The location of the prostate, seminal vesicles, bladder, and rectum on subsequent scans were compared to TPCT0, as well as to each other. Results: Prostate volumes were observed to vary by an average of +/- 10% during the course of radiation therapy, while the seminal vesicle volumes varied by as much as 100%. Bladder and rectal volumes varied by +/- 30%. Compared to TPCT0, movement of the prostate was demonstrated in all patients. Quantitation of the center-of-mass (CM) showed motion of less than 1 mm in the left-right direction, while motion ranging from 0 to +/- 1 cm was observed in the anterior-posterior and superior-inferior directions. The individual standard deviations of these motions varied from approximately 1-5 mm. These variations were correlated to changes in the dimensions of the bladder and rectum. Conclusions: Changes in the location of the prostate, seminal vesicles, and normal tissue volumes during the course of radiation therapy occur and have dosimetric consequences that may impact tumor control and normal tissue complication probabilities. Conformal therapy for prostate cancer will require the incorporation of knowledge of the anatomic relationships of these structures as a function of time. Therefore, these uncertainties must be taken into account when designing treatment plans and in considering dose escalation trials.