Effects of inaccurate small field dose measurements on calculated treatment doses

Abstract

Given the difficulty and potential time- or financial-costs associated with accurate small field dosimetry, this study aimed to establish the clinical necessity of obtaining accurate small field output factor measurements and to evaluate the effects on planned doses that could arise if accurate measurements are not used in treatment planning dose calculations. Isocentre doses, in heterogeneous patient anatomy, were calculated and compared for 571 beams from 48 clinical radiotherapy treatments, using a clinical radiotherapy treatment planning system, with reference to two different sets of beam configuration data. One set of beam configuration data included field output factors (total scatter factors) from precisely positioned and response-corrected diode measurements and the other included field output factors measured using a conventional technique that would have been better suited to larger field measurements. Differences between the field output factor measurements made with the two different techniques equated to 14.2 % for the 6 $\times$ 6 mm$^2$ field, 1.8 % for the 12 $\times$ 12 mm$^2$ field, and less than 0.5 % for the larger fields. This led to isocentre dose differences of up to 3.3 % in routine clinical fields smaller than 9 mm across and and up to 11 % in convoluted fields smaller than 15 mm across. If field widths smaller than 15 mm are used clinically, then accurate measurement (or-remeasurement) of small field output factors in the treatment planning system’s beam data is required in order to achieve dose calculation accuracy within 3 %. If such measurements are not completed, then errors in excess of 10 % may occur if very small, narrow, concave or convoluted treatment fields are used.