A multirod collimator for neutron therapy.

PURPOSE: To design, construct, and commission a multirod collimator for producing irregularly shaped fields in neutron radiation therapy. To demonstrate the reliability and applicability of this device to routine use with a superconducting cyclotron for neutron therapy. METHODS AND MATERIALS: A multirod collimator has been designed, constructed, and thoroughly tested to investigate its radiological properties; neutron transmission characteristics, beam profiles, and penumbral widths as a function of field size and depth in a phantom, and the spatial resolution of the rod array, have been measured. A wide variety of irregularly 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 over a period of 20 months to accurately assess reliability. RESULTS: The multirod collimator has been in routine use for 32 months, and during this time a total of 7025 neutron fields has been treated. For the latter 20 months of this period, detailed performance records show that 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 last 12 months (0.22% of the available treatment time). The results of collimator attenuation and beam profile measurements show that the radiological properties of the collimator are comparable to those of other collimator systems used for neutron radiation therapy. Isodose measurements in a water phantom show that the spatial resolution of the rods is superior to that of the leaves used in neutron multileaf collimators. The ability of the multirod collimator to produce many irregularly shaped fields commonly encountered in neutron radiation therapy has been demonstrated. Shaped fields for prostate, head and neck, soft tissue sarcomas, lung, thyroid, rectum, bladder, colon, breast, pancreas, and gynecological tumors have been produced. For some prostate cases, the device has been used to produce partial transmission blocks. CONCLUSIONS: A novel multirod collimator has been designed, constructed, and successfully applied in the routine treatment of neutron radiation therapy patients.

Cryogenic aspects of the operation of a superconducting cyclotron-based neutron therapy facility.

The Harper Hospital superconducting cyclotron, which is used for neutron radiation therapy, is a unique device. It is the first superconducting cyclotron to be installed in a hospital. The novel magnet cryostat can be rotated through 360 degrees without spilling liquid and whilst remaining vented to a low pressure return line for collection of the boil-off gas. The mode of operation of the cryogenic magnet is described in detail. Some of the problems associated with the cryogenic nature of the cyclotron including those problems encountered in operating a helium liquefaction system in a hospital are discussed. At the present time the magnet is kept cold by filling the cryostat with approximately 75 L of liquid helium each day before patient treatments begin. This is a time-consuming process. The possibility of modifying the helium gas recovery and liquefaction system so that a continuous liquid helium supply could be delivered to the magnet cryostat is discussed.

Transmission measurements in multi-rod arrays: a design study for a multi-rod collimator.

The results of transmission measurements for neutrons, cobalt-60 gamma-rays, and 10 and 15 MV photons made with close-packed arrays of tungsten rods are presented. These results indicate that tungsten rod arrays of reasonable thickness can provide for primary or secondary collimation of all these radiation beams. Development work on a collimation system utilizing the multi-rod concept which is capable of producing irregularly shaped fields and suitable for use in photon or neutron radiation therapy is described.