ESTRO 37 Abstract book

S921

ESTRO 37

Results The top of the tables shows that MOSFETs behave properly for all field sizes; similar response is observed for diodes. Microdiamond has a good response in fields up to 7x7 cm 2 where it begins to underestimate the OF. ICs PinPoint, SemiFlex, CC13, and FC65-G present an under-response for the smallest fields (0.6x0.6 and 1x1 cm 2 ) because they have a higher active volume. However, these ICs are ideal for dose measurements from larger fields as reported in both tables, with the exception of PinPoint that underestimates the OF from 7x7 cm 2 due to central electrode effect. No difference is observed for dose responses for all detectors between 6 MV and 6MV FFF beams. The OF of FFF beams was lower with respect to the FF beams for field sizes larger than 3x3 cm 2 , but higher for field sizes smaller than 3x3 cm 2 as other authors have already demonstrated for the Versa HD. The result of the double checking described above resulted in the following deviations with respect reference OFs: 1.82% for 0.6x0.6 cm 2 and 0.40% for 1x1 cm 2 in 6 MV and 2.06% and 0.12% in 6MV FFF.

Material and Methods We used three SemiFlex 3D 31021 (PTW, Germany) ionization chambers placed in a Beamscan water phantom (PTW, Germany), with a high-precision TRUFIX positioning system (PTW, Germany) and connected to a DOSE 1 electrometer (IBA, Germany). The operating voltages and the dose required for the initial irradiation of the chambers were those specified by the manufacturer. The linear electron accelerator used was an Elekta Versa HD with Agility collimator (Elekta, Sweden), for the 6 MV and 6 MV FFF energies. The field size used was 10×10 cm^2, with a source-surface distance of 90 cm and the chamber at 10 cm depth. For the calculation of k_pol, we took fifteen measurements of charge with voltages of 400 V and −400 V, and we calculated the arithmetic mean. The value of the correction factor was obtained from equation 12 of Technical Reports Series No. 398 (TRS-398). For k_s we used the method of the two voltages. We measured with voltage pairs 400 V and 100 V, and -400 V and -100 V. We took five measurements of charge and calculated the arithmetic mean. The value of the correction factor was obtained from equation 14 of TRS- 398. For the 6 MV FFF beam, of which dose rate is greater than 6 MV, we verified the values of k_s with the Jaffé plots. All measurements were corrected with the pressure and temperature factor, k_TP, given by equation 11 of TRS- 398. Results The obtained values for k_pol are shown in Table 1. The results show consistent k_pol values, with a variation with respect to 1 lower than 0.4% for the two energies. The deviations between chambers are less than 0.5%.

The obtained values for k_s are shown in Table 2. These values differ from the unit at most 0.5%. On average, k_s is 0.14% lower for negative voltages than for positive. With the method of the two voltages, k_s are 0.2% higher on average for the 6 MV FFF beam than for 6 MV because of the higher dose rate. This is why we used the Jaffé plots from which we obtained values that differed less than 0.1% with respect the obtained results by the previous method.

Conclusion The major difference between detectors response is due to volume detector. MOSFET, MicroDiamond and diodes are good detectors for small field dosimetry. For all the analyzed detectors we found the same general trend regardless the type of filtration used. EP-1723 Polarity and ion recombination corrections for SemiFlex 3D 31021 IC in 6MV and 6MV FFF photon beams M.C. Castellet García 1 , P. Monasor Denia 1 , C. Manjón García 1 , J. López Tarjuelo 1 1 Consorcio Hospitalario Provincial de Castellón, Radiofísica y Protección Radiológica, Castellón, Spain Purpose or Objective It is important for the medical physicist to characterize the novel detectors released to the market. Hence, we studied the polarity (k_pol) and ion recombination (k_s) correction factors for three novel SemiFlex 3D 31021 ionization chambers in 6 MV and 6 MV flattening filter free (FFF) pulsed photon beams.

We performed an estimation of uncertainties for both sets of factors and obtained values lower than 0.07%, so we decided not to consider them. Conclusion The obtained results do not show substantial differences among the SemiFlex 3D 31021 ionization chambers used and show that they are valid for high dose rate beams. The variations obtained for k_pol and k_s are below the technical specifications provided by the manufacturer. Furthermore, the values of these factors can be taken equal to 1. When we compared the method of the two voltages with the Jaffé plots for the calculation of k_s, we obtained compatible results.