SU-E-T-577: A Study of Primary Dose-To-Collision Kerma Ratio and Primary Collision Kerma Gradient

Abstract

Purpose: This study examines the relationship between the magnitude of primary dose-to-collision kerma ratio (β) and the primary collision kerma gradients (dKcp/dx, dKcp/dy, dKcp/dz). Methods: A convolution algorithm is used to calculate the primary dose for primary kerma with various external beam conditions (single open, wedge, IMRT beams, and composite beams). EGS4 Monte-Carlo simulation is used to generate primary point spread kernels in water for 6 and 15 MV Mohan Spectrum. The primary point-spread kernel is normalized such that its volume integral is one. Results: For a single external photon beam, we have shown that when dKp/dz>0, then β < 1, when dKcp/dz = 0, then β = 1, and when dKcp/dz <0, then β > 1. However, this relationship does not apply for composite photon beams, where β is not 1 when dKp/dz = 0 for an AP/PA parallel opposed open beam pair in the middle plane. For a 60° EDW beam, β is approximately the same as that of the corresponding open beam of the same energy and does not change with lateral gradients. For an IMRT beam with abrupt intensity modulation, then β < 1 in the peak Kcp region and β > 1 in the valley Kcp region. Conclusion: The primary dose-to-collision kerma ratio is primarily dependent on the longitudinal gradient (dKcp/dz) in the direction of the radiation incidence within the beam collimation. Near the edge of the beam (e.g., buildup and penumbra), β < 1 in the peak Kcp regions and β > 1 in the valley Kcp regions.