Effect of tissue inhomogeneity on beta dose calculation

Appropriate monoclonal antibodies labeled with beta-emitting nuclides of high specific activity have been suggested for treatment of specific tumors. In a homogeneous medium the radiation dose rate distribution R due to a distributed activity distribution C can be calculated by convolution of the beta dose point kernel of the radionuclide in soft tissue with C. Prototype computer programs using Fast Fourier Transform techniques have been developed to evaluate the three dimensional spatial convolution efficiently. To study the effect of tissue inhomogeneity on R, the authors simulated a soft tissue-bone interface by a polystyrene (PST)-aluminum (A1) interface and considered the backscattering of beta rays from a point source and a plane source of 32P. LiF thermoluminescent dosimeters were used. With the point source at the PST-A1 interface, R at 0-31, 125-156, and 283-314 mg/cm2 separations from the interface were increased by (12 +/- 3)%, (8 +/-2)%, and (3 +/- 2)%, respectively, compared with a PST-PST interface. With the plane source, the increases were (8 +/- 3)%, (6 +/- 3)%, and (5 +/- 5)% for separations of 23-58, 150-184, and 277-311 mg/cm2, respectively. With the point source at a PST-air interface to simulate soft tissue-air interface, R at 0-31, 139-170, and 283-314 mg/cm2 from the interface were decreased by (25 +/- 4)%, (11 +/- 7)%, and (5 +/-2)%, respectively. The changes in R have also been measured with degraded spectra of 32P. Comparison of the experimental data with Monte Carlo calculation and the Two-Group method of calculation will be discussed. 20 references, 6 figures, 2 tables