ESTRO 2021 Abstract Book

S232

ESTRO 2021

Purpose or Objective The determination of field output factors (OF) in small fields requires applying corrections to the ionometric ratio due to the different conditions from reference field. The objective of this work is to compare the OF values for field size < 8x8 cm 2 , obtained by applying the correction factors published in the Dosimetry of Small Static Fields protocol (TRS-483) for different detectors using the intermediate field method (IFM) or only the reference field (RFM). Likewise, data have been compared with those obtained using intermediate fields without correction, daisy-chainin g (DC), as used to be done before the publication of TRS-483 protocol. Materials and Methods Measurements were made in water, at a source-surface distance of 100 cm and depth 10 cm, for photon energies of 6 MV with and without flattening filter (6 WFF, 6 FFF) and 10 MV with flattening filter (10 WFF) for a TrueBeam STx linac ( Varian Medical System , USA). Different types of detectors were used depending on the field size: ionization chambers (IC) of various sizes, unshielded diode, and radiochromic film (see Table 1). Table 1. Detector used for each nominal field size ( S ) range.

Pressure and temperature corrections were applied to ionometric measurements. A monitor chamber was not used for field sizes < 4x4 cm2, but it was found that the beam variability was <0,5%. For each field size, the OF value was determined always considering the IFM for the detector whose correction factor was the lowest (it corresponds to the values in bold of Table 2). Results Table 2 shows the OF values obtained for the analyzed energies applying the TRS-483 correction factors and

without applying them, in DC case, as well as their discrepancy compared to IFM. Table 2. OF values obtained for the different detectors, methods and energies studied.