ESTRO 2021 Abstract Book

S1508

ESTRO 2021

Conclusion Although D w

is the recommended metric to be used for the patient size because of its ability to account for attenuation, it requires an advanced software implemented in the CT scanner system to calculate attenuation for an individual patient. This may be a challenge as such software may not be readily available. Therefore, calculation of D w based on the ED method, which can be easily measured before performing a CT scan, could provide a simple approach to overcome this challenge. PO-1782 Doses from 2.5 MV and 6 MV 2D-imaging in IGRT, measured with MOSFET detectors K. Graczyk 1 , M. Kruszyna-Mochalska 2,1 , B. Pawałowski 3,4 , J. Malicki 2,1 1 Greater Poland Cancer Centre, Medical Physics, Poznań, Poland; 2 University of Medical Sciences, Electroradiology, Poznań, Poland; 3 Greater Poland Cancer Centre, Medical Physics , Poznań, Poland; 4 Poznan University of Technology, Materials Engineering and Technical Physics, Poznań, Poland Purpose or Objective The IGRT is used to precisely position the patient prior to exposure to the therapeutic beam. However, during this procedure, the patient receives an additional dose. The main aim was to consider a new option of imaging using 2.5 MV beam in terms of dose measured with MOSFET detectors and compare to 6MV beam during 2D-MV imaging of the pelvic area for an anthropomorphic phantom. Materials and Methods High sensitivity range MOSFETs (Best Medical Canada) were used for imaging dose measurements. To properly prepare, they were calibrated and characterized. As the reference detector, the Semiflex ionization chamber (PTW, Freiburg) was selected. Available protocols for TrueBeam (Varian Medical Systems) 2D-MV imaging of the pelvic area were chosen: 6 MV High Quality (3MU), 6 MV Low Dose (2MU), 2.5 MV High Quality (3MU). An anthropomorphic Alderson phantom (Radiology Support Devices Inc.) was used with MOSFETs inside four regions of interest: right and left femoral head, rectum and bladder. Results The results of MOSFETs characteristic were shown in Figure 1. Detectors correspond linearly depending on a given number of monitor units and do not show significant angular, dose rate, or energy dependence for MV beams. The average calibration factor for the detectors is 2.969 mV/cGy. The results of the measurements carried out in the anthropomorphic phantom for the 2D-MV imaging are shown in Figure 2. Based on these data - the femoral heads are exposed to the highest doses regardless of the selected protocol, respectively – for 6 MV HQ up to 5.58 cGy, for 6 MV LD till 2.85 cGy, for 2.5 MV HQ below 2.00 cGy. The lowest doses were measured in the area of the bladder - for 6 MV HQ up to 2.45 cGy, for 6 MV LD till 1.13 cGy, for 2.5 MV HQ below 0.75 cGy. In the rectum, the doses are average - for 6 MV HQ up to 4.70 cGy, for 6 MV LD till 2.31 cGy, for 2.5 MV HQ below 1.65 cGy.