MICRODOSIMETRIC SPECIFICATION OF THE RADIATION QUALITY OF A D(48.5)-NEUTRON THERAPY BEAM PRODUCED BY A SUPERCONDUCTING CYCLOTRON(BE FAST)

The proportional counter microdosimetric technique has been employed t o quantify variations in the quality of a d(48.5)+Be fast neutron beam passing through a homogeneous water phantom. Single event spectra hav e been measured as a function of spatial location in the water phantom and field size. The measured spectra have been separated into compone nt spectra corresponding to the gamma, recoil proton and alpha plus he avy recoil ion contribution to the total absorbed dose. The total abso rbed dose normalized to the ''monitor units'' used in daily clinical u se has been calculated from the measured spectra and compared to the d ata measured with calibrated ion chambers. The present measurements ag ree with the ion chamber data to within 5%. The RBE of the neutron bea m is assumed to be proportional to the microdosimetric parameter gamma for the dose ranges pertinent to fractionated neutron therapy. The r elative variations in gamma, assumed to be representative of variatio ns in the RBE are mapped as a function of field size and spatial locat ion in the phantom. A variation in the RBE of about 4% for points with in and 8% for points outside a 10 cmX10 cm field is observed. The vari ations in the RBE within the beam are caused by an increase in the gam ma component with depth. An increase in the RBE of about 4% is observe d with increasing field size which is attributed to a change in the ne utron spectrum. Compared to the uncertainties in the prescribed dose, associated with uncertainties in the clinically used RBE, variation in the RBE between various tissues, and other dosimetric uncertainities caused by factors such as patient inhomogeneities, patient setup error s, patient motion, etc., the measured spatial RBE variations are not c onsidered significant enough to be incorporated into the treatment pla nning scheme. (C) 1996 American Association of Physicists in Medicine.