Comparison of treatment plans for irradiating adenocarcinoma of the prostate.

In a clinic with multiple megavoltage beams (Co-60, 6 MV, 10 MV, 18 MV) available, the optimal dose distributions depends on the radiation technique, patient size, and beam energy. To determine optimal dose distributions for external beam irradiation for patients with localized adenocarcinoma of the prostate, we investigated the dependence of beam energy, treatment technique, and patient size. Various radiation isodose plans were prepared at the central plane of the treatment field using computerized tomography scans of the pelvis for small, medium, and large patients. The dose uniformity in the target volume, the dose received by sensitive tissues such as bladder, rectum, and femoral heads, and the irradiated volume of these normal tissues were used as the criteria for comparing the plans. In all cases, the target volume was within the 95% isodose line with good dose uniformity (+/- 5%, of the prescribed dose). The volume of the rectum and bladder enclosed within the 90% isodose curve depended on the choice of the beam energy and the treatment technique for different patient sizes. For the small size patient the optimal dose distribution was achieved by using 10 MV box and arc or oblique boost. For the medium size patient, using 18 MV X-rays with box technique and oblique boost gave the optimal treatment plan. For the large size patient, using 18 MV or 10 MV X-rays box technique with arc boost were both acceptable.