ESTRO 2022 - Abstract Book

S1346

Abstract book

ESTRO 2022

PO-1564 Detector specific output correction factors in small fields for different 2D detector arrays

E. Gershkevitsh 1 , I. Mendez 2 , B. Casar 2

1 North Estonia Medical Centre, Radiotherapy, Tallinn, Estonia; 2 Institute of Oncology Ljubljana, Radiophysics, Ljubljana, Slovenia Purpose or Objective IAEA TRS-483 has greatly improved the methodology of small field dosimetry of megavoltage photon beams by introducing and providing detector specific output correction (DSOC) factors for many different detector types. However, TRS-483 does not contain DSOC factors for 2D detector arrays designed for measurements in small fields. The purpose of this study was to determine DSOC factors for five different commercially available 2D detector arrays using W1 scintillator (Standard Imaging, WI, USA) and EBT3 Gafchromic (Ashland, NJ, USA) films as reference detectors. Materials and Methods Five different 2D detector arrays (IBA Matrixx Resolution, IBA MyQA SRS, PTW Octavius 1600 SRS, PTW Octavius 1500 and SunNuclear SRS MapCheck) were studied. The arrays were sandwiched between Virtual water (Med-Cal Inc., WI) slabs positioning the effective point of measurements at 10 cm depth, SSD=90 cm. 7 cm slabs were placed below the detector arrays for backscatter. The detector arrays were irradiated on TrueBeam linear accelerator equipped with stereotactic conical collimators (ø 4, 5, 7.5, 10, 12.5, 15, 17.5 mm) with 6 WFF, 6 FFF and 10 FFF beams. The measured output factors were compared to the relevant reference values obtained in water phantom with W1 scintillator and in Virtual water with EBT3 Gafchromic films for the same set-up as it was used for 2D detector arrays. Results The measurement results were corrected for dose-rate dependence which some of the detector arrays have exhibited. Max variation of 9% was observed for 10 FFF beam in the dose-rate range 400 - 2400 MU/min (PTW Octavius 1600 SRS). The DSOC factors for 6 FFF beam are shown in Table 1. DSOC factors were within ±3% for most of the studied detector arrays, for the cone size 12.5 mm and above. Largest DSOC factors were found for the smallest cone sizes. However, PTW Octavius 1500 array had a correction factor higher than 5% even for the largest 17.5 mm cone. The highest congruence with output factors obtained with reference detectors showed IBA MyQA SRS 2D detector array having DSOC factors within 2% for all cone sizes. Table 1 – Detector specific output correction factors for 6 FFF beam generated by TrueBeam linear accelerator for five different 2D detector arrays (values in bold are those that fall within ± 3%). DSOC factors were obtained by using W1 scintillator and EBT3 Gafchromic films as reference detectors.

Conclusion The detector specific output correction factors were determined for five commercially available 2D detector arrays. They can be used to correct the readings for output factors in small fields. Some of the arrays exhibit dose-rate dependence which need to be accounted for when there is a difference between calibration and measurement field sizes.

PO-1565 Using novel silica bead TL dosimeters to determine output factors for a kV radiotherapy unit

J. Kearton 1 , A. Palmer 1 , V. Goudousis 2 , S. Jafari 1

1 Portsmouth Hospitals University NHS Trust, Medical Physics, Portsmouth, United Kingdom; 2 University of Surrey, Department of Physics, Faculty of Engineering and Physical Sciences, Guildford, United Kingdom Purpose or Objective It is common practice to use generic data published in BJR supplement 25 [BJR 1996] to derive kV radiotherapy treatment output factors (MU per cGy). Due to difficulties in measuring dose for small fields at low energies this is an acceptable data sourse. However, the data is based on HVL and takes no account of the beam characteristics of specific kV treatment units which can lead to uncertainty. The objective of this work was to devise a novel method using small silica bead TL detectors to accurately measure the output factors for kV radiotherapy treatments, and compare results to a traditional ionisation chamber technique.

Materials and Methods