(Sunday, 7/14/2019) 2:00 PM - 3:00 PM
Room: Stars at Night Ballroom 1
Purpose: ICRU Report 36 on microdosimetry provides a general formula to estimate quantities based on idealized distributions for the mean chord length, lÌ… and the random transversal of particles passing through a volume of interest in a straight line. For a convex volume, theoretic considerations imply l=4V/A. V is the volume and A is the surface area. This formula, which is based on a straight and uniform (constant) energy deposition of the particle, has been widely used in the literature regardless of the site size, type and quality of ionizing radiation as well as the density and chemical composition of the dosimeters
Methods: In this study, we revisit the formulation of microdosimetric concepts for a scanning beam of proton. An event by event Monte Carlo simulation is used to compute the first and second moments of the linear-energy distribution as a function of proton energy and depth. We examine the validity of the ICRU formula for lineal energy and mean specific energy for a range of target sizes (1 nm to 1 mm).
Results: We show that the implied value of l ̅ continuously drops as a function of depth and asymptotically saturates to a minimum value for low energy protons, where it exhibits a universal scaling behavior determined by conservation of energy considerations. We further investigate the validity of the Kellerer's equation where a linear relation between y_D=〈y^2 〉/〈y〉 and 〖LET〗_d has been assumed. More specifically, we show that the Kellerer formula based on straight line, constant energy losses in the region of interest breaks down for low energy protons and gives rise to a non-linear relationshiop between y_D and 〖LET〗_d.
Conclusion: This study tests the pros and cons of using a mean chord length and constant stopping power to evaluate microdosimetric quantities of interest in radiation biology.
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