ESTRO 36 Abstract Book

S406 ESTRO 36 2017 _______________________________________________________________________________________________

rates. Irradiations were performed with a 60 Co PICKER unit in a secondary standard calibration laboratory. The samples were divided into groups of two and each group was placed at a different distance (56.65 - 427 cm) from the 60 Co source at a 5cm depth within a water phantom. Irradiation times varied in order to deliver the same dose of 1 Gy at the center of all cuvettes with dose rates in the range of 0.018 – 1.0 Gy/min. For the high dose rate study, a similar methodology was employed. Four couples of PRESAGE cuvettes were placed within a slab in a solid water phantom and irradiated at different dose-rates by varying the dose delivery rate of an ELEKTA Versa HD FFF linac from 2.5 up to 19 Gy/min. Dose delivery of 1 Gy for all dose rates was verified by ion chamber measurements. Irradiation induced optical density (OD) change was measured from pre- and post-irradiation scans with a digital spectrophotometer operated at 633 nm. Mean OD change for each group was normalized to the value for the highest dose rate in each study. Results Results presented in figure 1 show a trend of increasing PRESAGE dose sensitivity with decreasing dose rate with the over-response reaching up to 16% at 0.018 Gy/min. Although in a first approach such low dose rates could be considered extremely low in external radiotherapy, recent studies have shown that in advanced radiotherapy techniques (e.g. VMAT) dose rate varies drastically across dose distributions delivered and a considerable contribution of the delivered dose could come from very low dose rates (0.01 - 0.1Gy/min). Regarding the high dose rate study, all responses agree within experimental uncertainties, indicating that PRESAGE sensitivity is not significantly affected. Figure 1: Dose rate dependence of PRESAGE response for both studies included in this work. Error bars correspond to 1 standard deviation of all experimental uncertainties involved. Conclusion Results of this study indicate a significant over-response of this PRESAGE formulation in very low dose rates that should be considered when they are used in applications involving wide range of dose delivery rates. Acknowledgement: This work was financially supported by the State Scholarships Foundation of Greece through the program ‘Research Projects for Excellence IKY/SIEMENS’. PO-0775 Contributions to detector response in arbitrary photon fields S. Wegener 1 , O.A. Sauer 1 1 University Hospital, Radiation Oncology, Würzburg, Germany Purpose or Objective Due to their small active volumes, diodes are often the detectors of choice for many commissioning tasks including the measurement of output factors, especially in small fields. However, high-atomic number material in the chip, detector shielding or other components and a finite active volume size have been found to alter the signal compared to the dose ratios measured in water in the absence of such a detector. As a consequence, correction factors need to be applied to correct the obtained signals. Using three experimental setups (fig. 1), the different contributions to the detector signals were separated and

analyzed: the response to scatter, the primary beam and the combination of both. Material and Methods Signal ratios were obtained for three different experimental setups (fig. 1): First, the standard open field geometry. Secondly, fields in which the central part of the beam was blocked out by a 4 mm aluminum pole and the detector was positioned in the dose minimum below. Finally, the detector in air instead of water with a PMMA cap fitted on top. A range of typically used detectors were analyzed, namely a microDiamond, a PinPoint ionization chamber, an EDGE diode, as well as three shielded and three unshielded diode detectors. EBT3 Gafchromic film served as reference. Measurements were carried out on a PRIMUS linac at a photon beam quality of 6 MV with field sizes between 0.8 and 10 cm. Responses in the blocked field and the PMMA setup were combined to calculate the response in the open square fields. The results were interpolated to a general matrix from which responses in any field could be calculated. Examples of such fields were measured for comparison.

Results A higher detector overresponse and increasing detector to detector differences were observed when the primary beam was blocked out, whereas almost identical response was seen for all detectors in the primary beam. A combination of the responses in those two setups in a detector-dependent ratio reproduced the values obtained in the open field geometry with less than 1% deviation for all detectors studied and all quadratic field sizes. For rectangular and offset fields the agreement is still almost within 1 %. Only when the detector was close to the field edge larger deviations occurred (fig. 2).

Conclusion Detector responses in open fields could be calculated from the response to scatter and in the primary beam with 1% agreement in all studied square fields and for all studied detectors. The calculation was extended to rectangular and non-symmetric fields yielding results in agreement with the measurements for a wide range of fields. This method suggests a way to calculate correction factors for arbitrary fields.