Monitor unit calculation for large wedged high-energy photon beams

With the modern high-energy linear accelerators, the following beam characteristics have to be taken into account in the monitor unit (MU) calculation of a wedged treatment: (i) the field size dependence of wedge factors; (ii) the changes in depth dose and maximum build-up depth (dmax) induced by wedges; and (iii) the field size dependence of dmax. The incorporation of a field size specific wedge factor in an MU calculation is straightforward. Effects (ii) and (iii) however, often cause confusion and inconsistency in the choices of the reference depth for wedge factors and the normalization depth for wedged depth dose, and consequently can lead to inconsistent MU calculation formalism with additional efforts of up to 7% in the delivered dose. In this note, we illustrate a derivation of an exact central axis MU calculation for wedged treatments, which correctly accounts for the effects mentioned above.

Tests of an electron monitor for routine quality control measurements of electron energies

The depth dose for electrons is sensitive to energy and the AAPM Task Group 24 has recommended that tests be performed at monthly intervals to assure electron beam energy constancy by verifying the depth for the 80% dose to within +/- 3 mm. Typically, this is accomplished by using a two-depth dose ratio technique. Recently, a new device, the Geske monitor, has been introduced that is designed for verifying energy constancy in a single reading. The monitor consists of nine parallel plate detectors that alternate with 5-mm-thick absorbers made of an aluminum alloy. An evaluation of the clinical usefulness of this monitor for the electron beams available on a Varian Clinac 20 has been undertaken with respect to energy discrimination. Beam energy changes of 3 mm of the 80% dose give rise to measurable output changes ranging from 1.7% for 20-MeV electron beams to 15% for 6-MeV electron beams.