Heterogeneous dose calculations for Collaborative Ocular Melanoma Study eye plaques using actual seed configurations and Task Group Report 43 formalism.

Abstract

Collaborative Ocular Melanoma Study (COMS) eye plaques (EPs) contain silastic and Modulay materials that introduce 15-30% dose differences compared with all-water dosimetry. A Task Group Report 43 (TG43) dose rate calculation method is presented that includes silastic and Modulay heterogeneous effects, uses the actual plaque seed configuration, is not restricted to a particular commercial treatment planning system, and does not require purchase of additional software. Dose rate is calculated using TG43 formalism: Dos˙e(EP)(r,θ)=S(K)Λ(G(L)(r,θ)/G(L)(r0,θ0))g(EP)(r)F(EP)(r,θ), with revised radial dose, g(EP)(r), and anisotropy, F(EP)(r,θ), functions specific to (125)I or (103)Pd seeds in COMS EPs. The EP signifies that the functions are specific to COMS EPs. The g(EP)(r) is obtained from Monte Carlo (MC) data for EPs that contain just a single center seed. The F(EP)(r,θ) is obtained by performing a Nelder-Mead Simplex routine to find a least squares solution that minimizes differences between MC dose rate and Do˙seEP(r,θ). The TG43 formalism calculations agree with MC results, for 10-22-mm (125)I and (103)Pd EPs, to within 2% along and near the plaque central axis and within 4% in the penumbra region for depths of 1 mm or greater. Methods and data are provided for COMS plaque calculations using seed models other than (125)I Model 6711 and (103)Pd Model 200. Because actual seed configurations are used in dose rate calculations, this formalism may also be used to estimate dosimetry for nonstandard seed loadings. This manuscript enables the clinical user to perform accurate heterogeneity-corrected dose rate calculations for COMS EPs using TG43 formalism in a spreadsheet or commercial treatment planning system that has a TG43 line source geometry function calculation capabilities.