Description of a 3D conformal neutron and photon radiotherapy technique for prostate cancer.

Several methods have been described to improve the therapeutic ratio of radiotherapy in patients with locally advanced carcinoma of the prostate (LACaP). Studies have shown improved survival and local control in patients treated with neutron irradiation as opposed to photons alone. However, an increased complication rate was observed when conformal field design was not utilized. A 3D conformal mixed neutron/photon technique for LACaP has been developed at this institution. Field shaping for the neutron component is achieved with a multirod collimator. Prior to treatment, all patients undergo a conventional simulation followed by a treatment planning computed tomography (CT) scan. Contours of target and normal structures are entered into the 3D treatment planning system. Beam apertures are designed utilizing beam's eye view display. The photon dose is given in 10 fractions at 1.8 Gy each to the prostate and seminal vesicles (PSVs) and pelvic lymph node (PLN) volumes. The neutron dose is given in 15 fractions at 1.0 Gy per fraction to the PSV volume and 0.6 Gy to the PLN volume. The neutron portion includes a partial transmission block to achieve this dose differential. The treatment planning process, including beam weighting and custom block design, is described. Using these techniques, the neutron dose distributions to the target volumes, bladder, and rectum were comparable to those seen with four-field conformal photon irradiation.