Treatment planning system for the fast neutron therapy at the Institute of Medical Science, University of Tokyo

We have been treating patients with the d(14) + Be fast neutron from the IMS cyclotron since 1976. To control the quality of the neutron therapy, treatment dose given to the patients have been monitored and verified by in-vivo dosimetry. The precision of patient skin dose was proved to be as precise as a few percents. The precision of the patient positioning was also examined by means of double exposure film technique. 80% of the patients received the neutron field within the precision of 3 mm, relative to the anatomical structure. In order to improve the accuracy of neutron therapy, we have developed a treatment planning system on a mini-computer system. The commercial software system (TP-11), which is based on the photon Scatter Air Ratio method, was modified to run under multi-user/task operating system. The neutron plus gamma dose in tissue equivalent media is separated into primary and scattered radiation. The calculated dose distributions agreed well with the measured data within one percent. However, there were discrepancies between them in the penumbrae and deeper regions, where gamma dose component increased. The handling of gamma dose needs more refinement. In case of mixed neutron and photon therapy modality, the effective dose distribution is calculated, taking the constant RBE of fast neutron into account. The corrections of inhomogeneous organs were sometimes made for fast neutron. When available, the X-ray CT image was used to derive the contours of the inhomogeneous organs. As the CT number and the neutron kerma has little correlation, the method of density correction for neutron therapy needs further considerations. (orig.)