Refined gypsum effective attenuation coefficients for Co-60, 6 MV, 10 MV and 18 MV x-rays

We report new effective linear attenuation coefficients (μ_eff) for refined gypsum for Co-60 and 18 MV x-rays. 6 MV and 10 MV μ_eff agree with published data. For a 100 cm² field size (FS), tissue defect x = 1.35 cm, target depth d = 1.65 cm beneath the tissue defect x, μ_eff is 0.123 cm⁻¹ (Co-60) and 0.0934 cm⁻¹ (6 MV). For 100 cm², x = 1.35 cm, and d = 5.65 cm beneath x, μ_eff is 0.072 cm⁻¹ (10 MV), and 0.0614 cm⁻¹ (18 MV). For d, μ_eff decreases about 10% from 25 to 400 cm². For a given FS, μ_eff decreases with d by 3%–5% for Co-60, and 3% for 6 MV, 10 MV and 18 MV, but depends on (d − x). For d, when x is large (8 cm), depending on energy and FS, μ_eff is 2%–4% less than when x is small (2 cm). These data were used in a treatment-planning computer to design compensator filters for a step phantom. Compensation was to within 10% in the compensation plane (CP). Above and below CP, computer-calculated ratios of doses with and without filters were 0.75–1.13. Chamber dose ratios with and without filters were 0.75–1.12.