title: A modified method of calculating the lateral build-up ratio for small electron fields.
abstract: This note outlines an improved method of calculating dose per monitor unit values for small electron fields using Khan's lateral build-up ratio (LBR). This modified method obtains the LBR directly from the ratio of measured, surface normalized, electron beam percentage depth dose curves. The LBR calculated using this modified method more accurately accounts for the change in lateral scatter with decreasing field size. The LBR is used along with Khan's dose per monitor unit formula to calculate dose per monitor unit values for a set of small fields. These calculated dose per monitor unit values are compared to measured values to within 3.5% for all circular fields and electron energies examined. The modified method was further tested using a small triangular field. A maximum difference of 4.8% was found.
authors.affiliation: Physics Department, St Luke's Hospital, Highfield Road, Rathgar, Dublin 6, Republic of Ireland.
authors.forename: E
authors.initials: E
authors.lastname: Tyner
authors.forename: P
authors.initials: P
authors.lastname: McCavana
authors.forename: B
authors.initials: B
authors.lastname: McClean
mesh_terms: D000465:Algorithms; D001822:Body Burden; D003198:Computer Simulation; D004307:Dose-Response Relationship, Radiation; D004583:Electrons / Q000627:therapeutic use*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D008954:Models, Biological; D011835:Radiation Protection / Q000379:methods*; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D012542:Scattering, Radiation