Adapting a practical EPR dosimetry protocol to measure output factors in small fields with alanine

Abstract

AbstractPurposeModern radiotherapy techniques often deliver small radiation fields. In this work, a practical Electron Paramagnetic Resonance (EPR) dosimetry protocol is adapted and applied to measure output factors (OF) in small fields of a 6 MV radiotherapy system. Correction factors and uncertainties are presented and OFs are compared to the values obtained by following TRS‐483 using an ionization chamber (IC).MethodsIrradiations were performed at 10 cm depth inside a water phantom positioned at 90 cm source to surface distance with a 6 MV flattening filter free photon beam of a Halcyon radiotherapy system. OFs for different nominal field sizes (1 × 1, 2 × 2, 3 × 3, 4 × 4, normalized to 10 × 10 cm2) were determined with a PinPoint 3D (PTW 31022) IC following TRS‐483 as well as with alanine pellets with a diameter of 4 mm and a height of 2.4 mm. EPR readout was performed with a benchtop X‐band spectrometer. Correction factors due to volume averaging and due to positional uncertainties were derived from 2D film measurements.ResultsOFs obtained from both dosimeter types agreed within 0.7% after applying corrections for the volume averaging effect. For the used alanine pellets, volume averaging correction factors of 1.030(2) for the 1 × 1 cm2 field and <1.002 for the larger field sizes were determined. The correction factor for positional uncertainties of 1 mm was in the order of 1.018 for the 1 × 1 cm2 field. Combined relative standard uncertainties uc for the OFs resulting from alanine measurements were estimated to be below 1.5% for all field sizes. For IC measurements, uc was estimated to be below 1.0%.ConclusionsA practical EPR dosimetry protocol is adaptable for precisely measuring OFs in small fields down to 1 × 1 cm2. It is recommended to consider the effect of positional uncertainties for field sizes <2 × 2 cm2.