Determination of Correction Factors for Small Field Radiotherapy Using 6 and 18MV Energies of a Linac

Background and Estimating dosimetric parameters for small fields at non-reference conditions lead to significant errors if they are done based on the conventional protocols used for large fields and reference conditions. The aim of this study was to determine and compare small fields correction factors (KNR and KNCSF) based on a novel protocol by using different types of detectors. Specific circular cones were constructed and attached to a medical linac head to produce small field sizes down to 30 and 5mm diameters. Then, the KNR and KNCSF correction factors based on a recent protocol (TG155), proposed for small field dosimetry formalism, were determined for different active detectors (a pinpoint chamber, EDP20 and EDP10 diodes) in a homogeneous and a nonhomogeneous phantom at the above fields when irradiated to 6 and 18 MV beams of a Varian linac. The KNR correction factors for the circle field of 30mm estimated for the pinpoint chamber, EDP20 and EDP10 diodes were 0.993, 1.020 and 1.054 at 6 MV; and 0.992, 1.054 and 1.005 at 18 MV, respectively. The KNCSF correction factor for the 5mm circle field estimated for the pinpoint chamber, EDP20 and EDP10 diodes were 0.994, 1.023 and 1.040 at 6MV; and 1.000, 1.014 and 1.022 at 18MV, respectively. Our results indicated that implementing the correction factors estimated for the small fields based on the new proposed dosimetry protocol increases the dosimetric precision and accuracy of such fields in radiotherapy practices. Also, considering the perturbations happening between the boundaries of non-homogeneous area could increase the accuracy of the dosimetry procedures in such conditions.