# Program Information

## A Study of Primary Dose-To-Collision Kerma Ratio and Primary Collision Kerma Gradient

### T C Zhu*, B E Bjarngard, Univ Pennsylvania, Philadelphia, PA

SU-E-T-577 Sunday 3:00PM - 6:00PM Room: Exhibit HallPurpose: This study examines the relationship between the magnitude of primary dose-to-collision kerma ratio (Î²) 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 Î² < 1, when dKcp/dz = 0, then Î² = 1, and when dKcp/dz <0, then Î² > 1. However, this relationship does not apply for composite photon beams, where Î² is not 1 when dKp/dz = 0 for an AP/PA parallel opposed open beam pair in the middle plane. For a 60Â° EDW beam, Î² 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 Î² < 1 in the peak Kcp region and Î² > 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), Î² < 1 in the peak Kcp regions and Î² > 1 in the valley Kcp regions.

Contact Email: