Planning, computer optimization, and dosimetric verification of a segmented irradiation technique for prostate cancer

Purpose: To develop and verify a multisegment technique for prostate irradi ation that results in better sparing of the rectal wall compared to a conve ntional three-field technique, for patients with a concave-shaped planning target volume (PTV) overlapping the rectal wall. Methods and Materials: Five patients have been selected with various degree s of overlap between PTV and rectal wall. The planned dose to the ICRU refe rence point is 78 Gy, The new technique consists of five beams, each having an open segment covering the entire PTV and several smaller segments in wh ich the rectum is shielded, Segment weights are computer-optimized using an algorithm based on simulated annealing, The score function to be minimized consists of dose-volume constraints for PTV, rectal wall, and femoral head s, The resulting dose distribution is verified for each patient by using po int measurements and line scans made with an ionization chamber in a water tank and by using film in a cylindrical polystyrene phantom, Results: The final number of segments in the five-field technique ranges fr om 7 to 9 after optimization. Compared to the standard three-held technique , the maximum dose to the rectal wail decreases by approximately 3 Gy for p atients with a large overlap and 1 Gy for patients with no overlap, resulti ng in a reduction of the normal tissue complication probability (NTCP) by a factor of 1.3 and 1.2, respectively. The mean dose to the PTV is the same for the two techniques, but the dose distribution is slightly less homogene ous with the five-field technique (Average standard deviation of five patie nts is 1.1 Gy and 1.7 Gy for the three-field and five-field technique, resp ectively), Ionization chamber measurements show that in the PTV, the calcul ated dose is in general within 1% of the measured dose. Outside the PTV, sy stematic dose deviations of up to 3% exist, Film measurements show that for the complete treatment, the position of the isodose lines in sagittal and coronal planes is calculated fairly accurately, the maximum distance betwee n measured and calculated isodoses being 4 mm, Conclusions: We developed a relatively simple multisegment "step-and-shoot" technique that can be delivered within an acceptable time frame at the tre atment machine (Extra time needed is approximately 3 minutes). The techniqu e results in better sparing of the rectal wall compared to the conventional three-field technique. The technique can be planned and optimized relative ly easily using automated procedures and a predefined score function, Dose calculation is accurate and can be verified for each patient individually. (C) 2001 Elsevier Science Inc.