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 ve sicles, bladder, and rectum throughout the course of external beam rad iotherapy. The frequency and range of motion of these organs are quant ified. Methods and Materials: Ten patients with localized carcinoma of the prostate had conventional simulation followed immediately by a tr eatment 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 spatiall y registered to the initial scan using image correlation software that brings into congruence the bony pelvis of the different scans. The lo cation of the prostate, seminal vesicles, bladder, and rectum on subse quent scans were compared to TPCT0, as well as to each other. Results: Prostate volumes were observed to vary by an average of +/- 10% durin g the course of radiation therapy, while the seminal vesicle volumes v aried 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 les s than 1 mm in the left-right direction, while motion ranging from 0 t o +/- 1 cm was observed in the anterior-posterior and superior-inferio r directions. The individual standard deviations of these motions vari ed from approximately 1-5 mm. These variations were correlated to chan ges in the dimensions of the bladder and rectum. Conclusions: Changes in the location of the prostate, seminal vesicles, and normal tissue v olumes during the course of radiation therapy occur and have dosimetri c consequences that may impact tumor control and normal tissue complic ation probabilities. Conformal therapy for prostate cancer will requir e the incorporation of knowledge of the anatomic relationships of thes e structures as a function of time. Therefore, these uncertainties mus t be taken into account when designing treatment plans and in consider ing dose escalation trials.