Determination of Monitor Doses in Neutron Therapy Using the U-120 Cyclotron

Purpose Analyze the various methods for determining the monitor doses in neutron therapy using the U-120 cyclotron and to choose the monitoring method that provides the highest accuracy in dose delivery to the tumor. Material and methods The distributions of the absorbed dose of the therapeutic beam from the U-120 cyclotron were measured in a tissue-equivalent medium using the differential method, in which two ionization chambers with different sensitivity to neutron radiation were used. A comparison of radiation effects on tissues using various techniques of determining the monitor doses was made. The linear-quadratic model was used to assess responses to ionizing radiation. Results Dosimetry studies revealed that the therapeutic beam of the U-120 cyclotron contains concomitant gamma radiation, the contribution of which to the total neutron-photon dose increases with increasing depth of the irradiated medium. The presence of gamma radiation in the neutron beam dictate the need to find the correct method for monitoring neutron therapy. A comparison of radiation effects on the tumor tissue using different techniques of determining the monitor doses was made. It was found that at equal neutron-photon doses, the neutron dose in the tumor changed depending on its depth. It can lead to an incorrect conclusion about the effectiveness of neutron therapy depending on a single dose as well as in relation to various dose fractionation schedules. Conclusion The analysis of the results obtained showed that the problem can be most accurately solved using a technique in which the monitor coefficient and monitor doses are determined from the distribution of the neutron dose, taking into account the contribution of the gamma radiation dose to the total neutron-photon dose.

Comparisons of 3-Dimensional Conformal and Intensity-Modulated Neutron Therapy for Head and Neck Cancers

Purpose Neutron therapy is a high linear energy transfer modality that is useful for the treatment of radioresistant head and neck (H&N) cancers. It has been limited to 3-dimensioanal conformal-based fast-neutron therapy (3DCNT), but recent technical advances have enabled the clinical implementation of intensity-modulated neutron therapy (IMNT). This study evaluated the comparative dosimetry of IMNT and 3DCNT plans for the treatment of H&N cancers. Materials and Methods Seven H&N IMNT plans were retrospectively created for patients previously treated with 3DCNT at the University of Washington (Seattle). A custom RayStation model with neutron-specific scattering kernels was used for inverse planning. Organ-at-risk (OAR) objectives from the original 3DCNT plan were initially used and were then systematically reduced to investigate the feasibility of improving a therapeutic ratio, defined as the ratio of the mean tumor to OAR dose. The IMNT and 3DCNT plan quality was evaluated using the therapeutic ratio, isodose contours, and dose volume histograms. Results When compared with the 3DCNT plans, IMNT reduces the OAR dose for the equivalent tumor coverage. Moreover, IMNT is most advantageous for OARs in close spatial proximity to the target. For the 7 patients with H&N cancers examined, the therapeutic ratio for IMNT increased by an average of 56% when compared with the 3DCNT. The maximum OAR dose was reduced by an average of 20.5% and 20.7% for the spinal cord and temporal lobe, respectively. The mean dose to the larynx decreased by an average of 80%. Conclusion The IMNT significantly decreases the OAR doses compared with 3DCNT and provides comparable tumor coverage. Improvements in the therapeutic ratio with IMNT are especially significant for dose-limiting OARs near tumor targets. Moreover, IMNT provides superior sparing of healthy tissues and creates significant new opportunities to improve the care of patients with H&N cancers treated with neutron therapy.

Late complication of the oral cavity and oropharynx after interstitial neutron therapy with sources of Cf-252

Radiotherapy is used to treat approximately 80% of patients with cancer of the head and neck. Despite enormous advances in radiotherapy planning and delivery, a significant number of pa-tients will experience radiation-associated toxicities. Many effective management options are available for acute radiotherapy-associated toxicities, but treatment options are much more lim-ited and of variable benefit among patients who develop late sequelae after radiotherapy. The present work is based on observations of 586 patients with oral and oropharynx cancer, for which interstitial neutron therapy was carried out in an independent version or in combination with re-mote radiation therapy. Radiation injuries occurred in 130 (22.2%) patients, in 92 (15.7%) they appeared in the form of radiation ulcers and in 38 (6.4%) – osteoradionecrosis of the lower jaw. The clinical picture and the course of radiation injuries after interstitial neutron therapy with sources of 252Cf did not differ significantly from radiation complications after photon irradiation.