Accurate dosimetry provides a basis for valid patient radiotherapy treatment planning. Significant challenges for small field dosimetry are; source occlusion, absence of lateral electron equilibrium and spatial dose averaging effects. Unshielded Si diodes and diamond detectors have commonly been used for small fields. Despite having small sensitive volumes, their physical dimensions limit the minimum field size in which they can be used. The Air Core FOD™ is a plastic scintillation dosimeter with a sensitive volume of 1mm3 and 2mm external diameter. Small field output factors are measured with this dosimeter and compared with those measured using a PTW Diode E (type 60012) and a PTW Diamond detector (type 60003).
Measurements were performed in a 6 MV SRS photon beam (Varian Novalis Tx), using stereotactic cones with field diameters from 30 to 4mm and MLC fields from 100 to 5mm. In-water output ratios (Scp) were measured in a water tank. In-air output ratios (Sc) were measured using different brass build-up designs and equivalent heights.
In-water diode and diamond measurements agreed for field sizes greater than 10mm. For smaller fields the diamond detector gives a substantially lower Scp than the diode. This discrepancy is not only due to dose averaging but also from non-water equivalent dosimeter densities [Scott 2009]. The Air-Core FOD™, having small physical dimensions and a density close to water, is capable of accurate measurements in all field sizes. Brass build-up caps with dmax equivalent height were found to be sufficient to eliminate electron contamination for 6 MV photon beams.