ESTRO 2020 Abstract book

S753 ESTRO 2020

estimate a calibration coefficient N D,w,Q for a given user quality Q when a calibration coefficient at the reference quality Q 0 is known:

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 compared to 0.48 % / 0.83 % (TLD600/TLD700) at the conventional LINAC. TLD600 were therefore selected for the following phantom measurements. Using the TL dose information for PG renormalization, high 3D gamma passing rates were achieved using the 3%/2mm criteria: 91.0 % (Ia), 92.6 % (Ib), 94.3 % (IIa), 97.4 % (IIb), respectively. Conclusion This study shows that the TL signal reproducibility is independent of the magnetic field (applies to 0.35T). Dependencies in higher magnetic fields still have to be investigated. Furthermore, the previously established renormalization method for PG was applied to measurements at a MR-LINAC and was verified as suitable for evaluations of homogeneous dose distribution in the target volume. PO-1334 Feasibility of the kQQ0 formalism for Farmer- type chambers in medium-energy kV x-ray beams M. Pinto 1 , M. Pimpinella 1 1 Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti ENEA Casaccia Resea, Dosimetry, Santa Maria di Galeria Roma, Italy Purpose or Objective Calibration of clinical dosimeters in terms of the quantity absorbed dose to water, in medium-energy x-rays (MEnX- rays, generating potential > 80 kV), is currently based on traceability to air kerma ( K a ) standards, with subsequent estimation of calibration coefficients in terms of absorbed dose to water ( D w ) based on dosimetry Codes of Practice such as the IAEA TRS 398. The recent availability of D w primary standard in the domain of MEnX-rays offers the chance to obtain a D w calibration coefficient with direct traceability to a D w primary standard and therefore, as for other established radiation modalities using external beams, the opportunity to use the kQQ 0 formalism to

However, it is not clear whether the kQQ 0 formalism can be effectively adopted for Farmer-type chambers in MEnX- rays beams. This study addresses such problem with a combined experimental and computer simulation approach based on the Monte Carlo method. Material and Methods Three types of commercial, 0.6 cm 3 Farmer chambers (NE2571, 5 units; IBA FC65G, 4 units; PTW 30013, 2 units) were calibrated in terms of absorbed dose to water with traceability to the Italian standard of absorbed dose to water in MEnX-rays. For four filtered radiation beams realized at ENEA-INMRI in the MEnX-rays series, these experimental calibrations and the accompanying Monte Carlo simulations run using the app egs_chamber from the EGSnrc code allowed to estimate and compare kQQ 0 factors, where the reference quality Q 0 was set to the CCRI-250 quality. Results Results for all three chamber models are shown in the figure, where experimental determinations for estimates of kQQ0 for individual of each chamber model are compared to the Monte Carlo determinations (open circles). Combined standard uncertainties on kQQ0 experimental factors are 2.0% (k=1, not shown on graph) and are largely dominated by the uncertainty in the determination of D w . Conclusion The Monte Carlo-calculated and the average experimentally kQQ 0 factors agree within 1.2%, 1.1% and 1.0% (NE2571, PTW 30013 and FC65-G chambers), which is within the uncertainties estimated (0.1% for the Monte Carlo calculations, type A only, and 2.0% for the experimental determinations of the kQQ 0 factors, k=1). Based on the chamber-to-chamber kQQ 0 maximum variability observed (1.6%, 0.3%, 1.7% for NE2571, PTW 30013 and FC65-G chambers), the kQQ 0 formalism may be considered applicable also in the domain of MEnX-rays. However, this study may need to be repeated after further advancements on the Italian national standard of absorbed dose to water will offer smaller uncertainties in the estimate of D w . Acknowledgment : This research has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme PO-1335 Characterization of novel 3D printed plastic scintillation dosimeters N. Lynch 1 , T. Monajemi 1,2,3 , J. Robar 1,2,3 1 Dalhousie University, Department of Physics & Atmospheric Science, Halifax, Canada ; 2 Dalhousie University, Department of Radiation Oncology, Halifax, Canada ; 3 Nova Scotia Health Authority, Department of Medical Physics, Halifax, Canada