Energy response of optically stimulated luminescent dosimeters for non-reference measurement locations in a 6 MV photon beam

Optically stimulated luminescent dosimeters (OSLDs) are becoming increasingly popular for measuring an absorbed dose in clinical radiotherapy. OSLDs have known energy dependence, and this is accounted for by either calibrating the OSLD with a specific nominal energy, or using a standard energy correction factor to account for differences between the experimental beam photon energy and the photon energy used to establish the OSLD's sensitivity (e.g., 60Co). This work is typically done under reference conditions (e.g., at dmax). The impact of variations in photon spectra on the OSLD response is typically ignored for measurement positions that are different than the reference position. We determined that it is generally necessary to apply an additional non-reference energy correction factor to OSLD measurements made at locations that do not correspond to the reference position, particularly for OSLD measurements made out-of-field, where the photon spectra are softer. We determined this energy correction factor for a range of 6 MV photon spectra using two independent methods: Burlin cavity theory and measurements. The non-reference energy correction factor was found to range from 0.97 to 1.00 for in-field measurement locations and from 0.69 to 0.95 for out-of-field measurement locations. The use of a non-reference energy correction factor can improve the accuracy of OSLDs, especially when used out-of-field.