Use of primary off-axis ratios for dose and monitor unit calculations in wedged photon beams with asymmetric collimators

Abstract

The use of asymmetric collimators or heavily blocked fields in the presence of physical wedge filters (wedges) is very common in daily clinical practice. In such cases a reliable dosimetry system to perform manual dose and monitor unit (MU) calculations is necessary in order to be able to manually and independently verify calculations from today's commercial treatment planning systems. In this work, primary wedged off-axis ratios (POAR/sub w/) recently measured experimentally, are used in order to account for changes in the beam quality along the wedged direction of the photon field at the specific treatment depth. POAR/sub w/(x,d) is only a function of off-axis distance and depth, and is the only additional parameter necessary to be obtained in order to perform accurate MU calculations according to the formalism that we present. Our formalism is based on standard parameters measured in the CA of open symmetric photon fields such as output factors (for collimator and phantom scatter), tissue phantom ratios (or percentage depth doses), and wedge factors as functions of field size and depth for the wedge to be used. Measurements of dose for set number of monitor units and specific geometry with asymmetric wedged photon fields of 6 and 18 MV agreed well with our calculations. Comparisons of our manual calculations with predictions of a modern treatment planning system for the same experimental conditions are presented. Our presentation also underlines the importance of the implementation of a comprehensive dosimetry system for complex treatment in the clinical environment.