ESTRO 2023 - Abstract Book

S1538

Digital Posters

ESTRO 2023

Conclusion The results in the validation tests of the algorithms are within the expected range, with the largest discrepancies in the simulation of asymmetric fields, where the modelling of the secondary source becomes more important. For the periodic quality control, the differences found for the 3 algorithms with respect to the reference are larger than those usually found before the algorithm version change. Varian has reported that the results achieved with the new secondary source modelling are closer to reality.

PO-1812 Influence of beam width of cone beam CT (CBCT) scan on effective dose

A. Abuhaimed 1 , C. Martin 2

1 King Abdulaziz City for Science and Technology (KACST), Department of Applied Physics, Riyadh, Saudi Arabia; 2 University of Glasgow, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom Purpose or Objective The use of cone beam CT (CBCT) scans is becoming essential for ensuring accurate delivery of radiotherapy treatment at the majority of cancer centers around the global. The aim of this work is to investigate the influence of beam width along the patient body, i.e. of the scan field, on effective dose of CBCT scans. Materials and Methods A previously validated Monte Carlo (MC) model of a Varian kV imaging system, namely on-board imager (OBI), integrated into a TrueBeam linear accelerator was used to assess effective doses resulting from various clinical protocols. MC simulations were conducted with BEAMnrc and DOSXYZnrc codes, which are based on ESGnrc MC system. Two computational reference male and female phantoms with lengths of 175 and 165 cm and weights of 75 and 60 kg, respectively, were involved. Sizes of these phantoms were similar to the reference s recommended by ICRP to represent average adult patients. Two scan areas, chest and pelvis, were studied using scan parameters similar to those used in the clinic. The beam width was varied in the range of 18 – 26 cm, with intervals of 2 cm, while the rest of the scan parameters were kept constant. Results The effective doses were averaged between the male and female phantoms, and doses of chest and pelvis scans were 2.03 and 1.14 mSv/100 mAs, respectively, when a beam width of 18 cm was used (Table 1). These values were increased by 49% and 47% to reach 3.03 and 1.68 mSv/100 mAs, respectively, when the beam width was extended to 26 cm (Table 1). The effective dose of a chest scan was found to increase by about 10% with each 2 cm increment of the beam width. However, the first increment of the beam width, from 18 to 20 cm, led to an increase in effective dose for the pelvis scan by 12%, but was only 7% for the last increment and 10% on average. This was a result of the contribution from the gonads, which became fully inside the scan field when the beam width was extended.