A MULTIROD COLLIMATOR FOR NEUTRON THERAPY

Purpose: To design, construct, and commission a multirod collimator fo r producing irregularly shaped fields in neutron radiation therapy. To demonstrate the reliability and applicability of this device to routi ne use with a superconducting cyclotron for neutron therapy. Methods a nd Materials: A multirod collimator has been designed, constructed, an d thoroughly tested to investigate its radiological properties; neutro n transmission characteristics, beam profiles, and penumbral widths as a function of field size and depth in a phantom, and the spatial reso lution of the rod array, have been measured. A wide variety of irregul arly shaped fields, used routinely in neutron radiation therapy, have been produced, including fields that incorporate partial transmission blocks. The performance of the collimator has been closely monitored o ver a period of 20 months to accurately assess reliability. Results: T he multirod collimator has been in routine use for 32 months, and duri ng this time a total of 7025 neutron fields has been treated. For the latter 20 months of this period, detailed performance records show tha t collimator failure has caused 28.4 h of downtime during the patient treatment day. Only 5.25 h of this downtime was experienced in the las t 12 months (0.22% of the available treatment time). The results of co llimator attenuation and beam profile measurements show that the radio logical properties of the collimator are comparable to those of other collimator systems used for neutron radiation therapy. Isodose measure ments in a water phantom show that the spatial resolution of the rods is superior to that of the leaves used in neutron multileaf collimator s. The ability of the multirod collimator to produce many irregularly shaped fields commonly encountered in neutron radiation therapy has be en demonstrated. Shaped fields for prostate, head and neck, soft tissu e sarcomas, lung, thyroid, rectum, bladder, colon, breast, pancreas, a cid gynecological tumors have been produced. For some prostate cases, the device has been used to produce partial transmission blocks. Concl usions: A novel multirod collimator has been designed, constructed, an d successfully applied in the routine treatment of neutron radiation t herapy patients.