The value of nonuniform margins for six-field conformal irradiation of localized prostate cancer.

Abstract 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 radiotherapy (SFCRT) of the prostate,it is possible to adequately treat the clinical target volume (CTV) and minimize dose to normal structures. Methods and Materials: Serial computed tomography (CT) scans of prostate patients were taken at 0.5 cm intervals for treatment planning purposes. The initial treatment planning scans were performed with the rectum empty and the bladder full. Subsequent scans were taken at the end fo the first week of treatment with the bladder full, but with no attempt to empty the rectum, to mimic the typical treatment situation. The gross tumor volume (GTV), consisting of the prostate and seminal vesicles, as well as the CTV, were defined on teh CT images with the aid of a urethrogram to define the inferior border (apex) of the prostate. Variable blocking margins were designed around the CTV using the University of Michigan three dimensional (3D) treatment planning system (UM-PLAN). Isodose distributions displayed on axial, sagittal, coronal, and oblique slices were used to evaluate the adequacy of the various margins applied. Nonuniform margins varying from 0.75 cm posteriorly to 2.0 anteriorly and inferiorly were compared to uniform margins of 1.0, 1.5 and 2.0 cm for each patients. Dose volume histograms (DVH) were used to compare doses to the GTV, CTV, rectum, and bladder. Results: In a series of 10 patients scanned with the above protocol, treatment plans 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 tot eh rectum and bladder, but the use of 1.0 cm uniform margins resulted in inadequate coverage of the CTV in 40% of the cases. The 1.5 and 2.0 cm uniform margins adequately covered the CTV but resulted in significantly higher doses to the bladderd and rectum. Conclusion: The use of nonuniform margins, when combined with CT scans performed with the rectum empty and bladder full, can improve tumor control probability while minimizing the risk of morbidity to adjacent critical structures.