PHYSICAL CHARACTERISTICS OF A CLINICAL D(48.5)+BE NEUTRON THERAPY BEAM PRODUCED BY A SUPERCONDUCTING CYCLOTRON

The Harper Hospital and Wayne State University fast neutron therapy fa cility is the only one in the world to use a compact superconducting c yclotron and multirod collimator. Neutrons are produced by the interac tion of the 48.5-MeV deuteron beam with a thick internal beryllium tar get and the compact accelerator is gantry mounted to allow full 360 de grees rotation of the neutron beam about the therapy couch. The deuter on beam strikes the beryllium target at a glancing angle. A flattening filter is used to flatten the asymmetric neutron beam which results f rom this geometry. Details of the flattening filter design and constru ction are discussed. The physical characteristics of the resulting neu tron therapy beam were measured. The central axis depth-dose values ar e approximately equivalent to those of a 4-MV photon beam. The dose bu ildup curve reaches its maximum value at a depth of 9 mm in a water ph antom and the surface dose is approximately 42%. The beam penumbra pro duced by the multirod collimator has been measured in terms of the dis tance between the 20% and 80% isodose lines. The penumbra width for a 10X10-cm(2) field at a depth of 10 cm in a water phantom is 1.65+/-0.1 cm, and is comparable to that achieved with other high-energy neutron beams. The long-term stability of the dose, monitoring system has bee n measured and found to be satisfactory. The physical characteristics of the neutron beam are comparable with those of other modern fast neu tron therapy facilities.