Measured water transmission curves and calculated zero field size tumor maximum ratios for 4, 6, and 15 MV x-rays

A multihole diverging Cerrobend plug for megavoltage energies was used to measure water transmission values at different locations in a 20 x 20 cm field at 100 cm source-to-axis distance (SAD) for 4, 6, and 15 MV therapy photon beams. The transmission curves in water were measured at 25 locations across the 20 x 20 field, and each location was separated by 5 cm at the isocenter. Each transmission value was made using a 0.3175 cm diameter (0.079 cm2 area) hole of 20 cm length at the central axis (CAX). The small field measured transmission curve in water was used to derive the zero field size tumor maximum ratio (TMR) and the primary photon exposure spectrum as a function of energy at depth. The exposure spectrum was used to find an effective photon energy and linear attenuation coefficient at depth and at different locations in the field. These values were found to vary with location in the field.