GUIDELINES FOR DEFINING TARGET VOLUMES IN RADIATION-THERAPY OF PROSTATE-CANCER - A REVIEW AND PERSPECTIVE

No definite objective guidelines for selecting patients for pelvic nod al or seminal-vesicle irradiation exist in radical radiotherapy for pr ostate cancer. Developing such guidelines will aid in decreasing unnec essary irradiation of normal tissues in patients in whom involvement o f pelvic nodes and seminal vesicles is unlikely. The development of ob jective criteria will also be of help in successful planning of dose-e scalation studies with 3D conformal radiotherapy. Some recent advances are useful in the development of such guidelines. Based on whole-moun t, morphometric studies from radical prostatectomy specimens, a new un derstanding of the zonal anatomy of the prostate has emerged. The thre e glandular zones are the transitional, central, and peripheral zones; the fourth zone is a nonglandular, fibromuscular one situated anterio rly. The majority of stage A tumors arise in the transitional zone, wh ereas stage B tumors arise from the peripheral zone. A tumor arising f rom the transitional or nontransitional zone usually does not cross th e boundary between zones unless the tumor volume exceeds about 4 cm(3) . The tumor volume influences the following: (a) formation of pelvic n odal metastases; (b) seminal-vesicle involvement; (c) capsular extensi on, and (d) the extent of the Gleason pattern 4-5 component. In genera l, tumors smaller than 4 cm(3) tend not to involve regional nodes or s eminal vesicles; they have a negligible Gleason pattern 4-5 component, and this is a favorable prognostic factor. Tumor volume estimates can be obtained from transrectal ultrasound. In addition, transrectal ult rasound has significant potential in the identification of patients wi th gross seminal-vesicle involvement. Transrectal ultrasound slightly underestimates tumor volumes, and about 20% of tumors may not be visib le sonographically, but these shortcomings can be remedied by measurem ent of the prostate specific antigen level. The level of this antigen, a very useful tumor marker in prostate cancer, depends on the tumor c ell burden. Studies show that nodal and seminal-vesicle involvement is unlikely if the prostate specific antigen level (determined by a mono clonal antibody method) is less than 10 ng/ml, and that it is highly l ikely with levels above 20 ng/ml. In patients with prostate specific a ntigen levels between 10 and 20 ng/ml, ultrasonographic findings as we ll as the stage and grade of the tumor can help the radiotherapist to determine the extent of the target volume. For treatment planning, dec ision trees are developed based on stage, grade, tumor volume determin ation, and prostate specific antigen levels. A significant difference in the volumes of irradiation to the rectum and bladder with or withou t pelvic nodal/seminal vesicle irradiation is illustrated with dose-vo lume histograms. The margin around the target in the definition of the planning target (i.e., the margin from prostate to block) due to day- to-day set-up variations and physiologic movement must be about 1-2 cm . Prospective use of the objective criteria developed here can help in the selection of patients for pelvic nodal and/or seminal-vesicle irr adiation. Such selection can be helpful in customizing of target volum e definition in the radical radiotherapy of prostate cancer. These obj ective criteria should form an integral part of 3D conformal dose esca lation studies.