THE VALUE OF NONUNIFORM MARGINS FOR 6-FIELD CONFORMAL IRRADIATION OF LOCALIZED PROSTATE-CANCER

Purpose: Evaluate the hypothesis that by combining nonuniform margins with a technique for limiting the possible extent of posterior motion of the prostate during the delivery of six-field conformal radiotherap y (SFCRT) of the prostate, it is possible to adequately treat the clin ical target volume (CTV) and minimize dose to normal structures. Metho ds and Materials: Serial computed tomography (CT) scans of prostate pa tients were taken at 0.5 cm intervals for treatment planning purposes. The initial treatment planning scans were performed with the rectum e mpty and the bladder full. Subsequent scans were taken at the end of t he first week of treatment with the bladder full, but with no attempt to empty the rectum, to mimic the typical treatment situation. The gro ss tumor volume (GTV), consisting of the prostate and seminal vesicles , as well as the CTV, were defined on the CT images with the aid of a urethrogram to define the inferior border (apex) of the prostate. Vari able blocking margins were designed around the CTV using the Universit y of Michigan three dimensional (3D) treatment planning system (UM-PLA N). Isodose distributions displayed on axial, sagittal, coronal, and o blique slices were used to evaluate the adequacy of the various margin s applied. Nonuniform margins varying from 0.75 cm posteriorly to 2.0 cm anteriorly and inferiorly were compared to uniform margins of 1.0, 1.5, and 2.0 cm for each patient. Dose volume histograms (DVH) were us ed to compare doses to the GTV, CTV, rectum, and bladder. Results: In a series of 10 patients scanned with the above protocol, treatment pla ns with nonuniform margins were compared with uniform margins of 1.0, 1.5, and 2.0 cm. Dose-volume histograms showed that nonuniform and 1.0 cm uniform margins deliver the lowest doses to the rectum and bladder , but the use of 1.0 cm uniform margins resulted in inadequate coverag e of the CTV in 40% of the cases. The 1.5 and 2.0 cm uniform margins a dequately covered the CTV but resulted in significantly higher doses t o the bladder and rectum. Conclusions: The use of nonuniform margins, when combined with CT scans performed with the rectum empty and bladde r full, can improve tumor control probability while minimizing the ris k of morbidity to adjacent critical structures.