ESTRO 2020 Abstract Book

S704 ESTRO 2020

Purpose or Objective Small field dose measurements require correction factors due to variations in their dosimetric characteristics affecting detector response. The IAEA TRS-483 report has tables of small field correction factors for different detectors. These correction factors are specified for a reference depth, and depend on machine type, energy, and field size at measured depth. This work evaluates the variability of readings obtained using various detectors when these correction factors are applied, the suitability of the correction factors for depths other than the specified depth, and the validation of a simple method to estimate the correction factors of newer detectors not included in the protocol. Material and Methods Output factors were measured using an isocentric setup for depths of 10 and 5 cm for four small MLC defined fields. Measurements were performed on a Varian Edge with HDMLC for 6X, 6FFF, and 10FFF. Though the published IAEA linac correction factors are specified for use at 10 cm depth, the protocol indicates that data obtained at 5 cm depth was used to determine the factors for detectors not showing field size dependence above 3 cm. The four detectors used in this study (IBA CC04, PTW microdiamond, PTW PinPoint, SNC Edge) had existing IAEA correction factors and do not show this dependence. Therefore, the correction factors were applied to measurements performed at 10 cm and 5 cm. Two newer detectors used in the measurements (IBA RAZOR and RAZOR Nano chambers) did not have IAEA correction factors. The average corrected readings of the four detectors were used to calculate approximate correction factors for these new detectors. Results The application of the IAEA TRS-483 small field correction factors reduced the variability of the output factor measurements. The factors applied to 1x1 cm 2 field reduced the variability within measurements to <1% for all energies. Specifically, the reductions were from 2.1% to 0.3%, 1.9% to 0.9%, and 3.2% to 0.5% for 6X, 6FFF, and 10FFF, respectively. The correction factors for 6FFF did not reduce the variability as much as for the other energies. The reduction in variability was found to be nearly the same for factors measured at 5 cm and 10 cm; thus, confirming the applicability of the correction factors for other depths in energy independent detectors (Fig.1). The corrected measurements for the detectors included in the protocol can be used to determine an approximate correction factor in line with the IAEA protocol or to evaluate other published factors for newer detectors not included in the protocol tables (Fig.2).

Conclusion The IAEA TRS-483 protocol provides a method to standardise the small field measurements and minimise variability between the response of different detectors. The correction factors can also be applied for measurements at 5 cm depth for energy independent detectors. Detectors included in the protocol can be used to validate or to estimate correction factors for small field measurements with new detectors.

PO-1333 Absolute dosimetry at a 0.35T MR-Linac with a combined polymer gel (PG)-TLD system A. Schwahofer 1 , P. Mann 1 , K. Spindeldreier 2 , C. Karger 1 1 German Cancer Research Center DKFZ, Department of Medical Physics in radiation therapy, Heidelberg, Germany ; 2 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective In a previous work (Mann et al., PMB 2019, Vol 64 [1]) it has been shown that high-precision absolute dosimetry in a 3D volume can be performed with a combined method of polymer gels (PG) and thermoluminescence detectors (TL). In this case, the TL detectors, rather than the TPS or an independent IC measurement, was used for the purpose of PG renormalization. This new PG-TLD system will now be applied to an MR-Linac with 0.35T. The goal is to perform absolute dosimetry in various phantom geometries on the MR-Linac. Material and Methods To verify the signal response reproducibility, calibration irradiations for TLD600 and TLD700 were first performed on both the conventional LINAC and the MR-Linac (0.35T) with 6MV. Thereafter, the more stable TL material was selected for the subsequent irradiation trials. The combination of the in-house produced PAGAT PG and TL detectors was used with a cylindrical phantom (see details in [1]) that can be filled with two different materials: (I) air-filled phantom for simulating lung cases and (II) water- filled phantom for simulating abdominal or pelvis regions. For each scenario, two plans were calculated: (a) two opposing beam directions with field size 10 x 10 cm 2 and (b) a target volume based 3D conformal planning with three equidistant incident beams. This resulted in a total of 4 irradiations. Results The signal response reproducibility of the TL detectors was 0.49 % / 0.85 % for TLD600/TLD7000 at the MR-LINAC