PROSTATE TARGET VOLUME VARIATIONS DURING A COURSE OF RADIOTHERAPY

Purpose: The purpose of this study was to measure the mobility of the clinical target volume (CTV) in prostate radiotherapy with respect to the pelvic anatomy during a course of therapy. These data are needed t o properly design the planning target volume (PTV), Methods and Materi als: Seventeen patients were studied. Each patient underwent computed tomography (CT) scanning for treatment planning purposes. Subsequently , three CT scans were obtained at approximately 2-week intervals durin g treatment, The prostate, seminal vesicles, bladder, and rectum were outlined on each CT study. The second through the fourth CT studies we re aligned with the first study using a rigid body transformation base d on the bony anatomy, The transformation was used to compute the cent er of mass position and bounding box of each organ in the subsequent s tudies relative to the first study. Differences in the bounding box li mits and center of mass positions between the first and subsequent stu dies were tabulated and correlated with bladder and rectal volume and positional parameters. Results: The mobility of the CTV was characteri zed by standard deviations of 0.09 cm (left-right), 0.36 cm (cranial-c audal), and 0.41 cm (anterior-posterior). Prostate mobility was not si gnificantly correlated with bladder volume. However, the mobility of b oth the prostate and seminal vesicles was very significantly correlate d with rectal volume. Bladder and rectal volumes decreased between the pretreatment CT scan and the first on-treatment CT scan, but were con stant for all on-treatment CT scans. Conclusion: Margins between the C TV and PTV based on the simple geometric requirement that a point on t he edge of the CTV is enclosed by the PTV 95% of the time are 0.7 cm i n the lateral and cranial-caudal directions, and 1.1 cm in the anterio r-posterior direction. However, minimum dose to the CTV and avoidance of organs at risk are more important considerations when drawing beam apertures. More consistent methods for reproducing prostate position ( e.g., empty rectum) and more sophisticated beam aperture optimization are needed to guarantee consistent coverage of the CTV while avoiding organs at risk. Elsevier Science Inc. (C) 1998 Elsevier Science Inc.