AbstractID: 8974 Title: A Fluence-based Algorithm for MU Calculation of Megavoltage Electron Beams
An empirical algorithm to calculate MU for electron beams have been developed, with improved accuracy for a broad range of
treatment distances, depth, cone size, and especially field size defined by the cutout. This algorithm adopts the concept of electron
head-scatter factor He(c,r,SSD) to characterize the variation of the incident electron fluence with source-to-detector distance SDD,
cone size (c), and radius (r) of circular cutout. This variation is attributed to electron scattering in the accelerator head, applicators, and
the Cerrobend inserts. Two new quantities, PSF(r) and FDDf(r,d), as functions of equivalent beam radius r and depth d, are
introduced to account for electron phantom scattering in a water phantom per incident fluence. Phantom scatter factor PSF is defined
as BF(r)/H(r), where the blocking factor BF is the ratio of doses measured in water at reference depth for a circular field with r and an
open field, and H is the ratio of doses measured in air using a diode detector without any buildup for the same conditions. Equivalent
radius for an arbitrary field can be calculated using a sector-integration of PSF(r). FDDf is determined by the ratio of
FDD(r,d)·PSF(r)/FDDb(d), where FDD and FDDb are the fractional depth dose for field radius r and infinite large broad beam,
respectively. FDDf is the radius integration of the pencil beam kernel k, 2π∫k(r)rdr. Both PSF and FDDf are independent of the cone
sizes. – This algorithm is ideally suitable for calculating MU for treatment-planning system for electrons based on Monte-Carlo or
other pencil-beam methods.