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SU‐E‐T‐577: A Study of Primary Dose‐To‐Collision Kerma Ratio and Primary Collision Kerma Gradient
Author(s) -
Zhu TC,
Bjarngard BE
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4815005
Subject(s) - kerma , physics , beam (structure) , dosimetry , nuclear medicine , optics , computational physics , medicine
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.