ESTRO 2023 - Abstract Book

S1522

Digital Posters

ESTRO 2023

Alto) and three 'Agility'/VersaHD (Elekta Instrument AB, Stockholm) MLCs. The definitions were repeated with collimator leaf speeds ranging from 0.25 cm/sec to the maximum leaf speed. The actual fluence widths of closing and opening collimation were determined with megavolt imager and ionization chambers. Results An average dynamic backlash for studied collimator leafs was: 0.4 mm +/- 0.1 mm for 'Varian Standard 120M' and 0.8 mm +/- 0.2 mm for Agility MLC. The backlash was found to be almost linearly dependent of the leaf speed with both studied MLC models. The maximum backlash was 2.3 mm recorded with Agility MLC with the maximum leaf speed. Conclusion We have demonstrated that the proposed method can be used to determine the backlash of a dynamic multileaf collimation. The dynamic backlash has been found to be dependent of the MLC model and the collimator leaf speed. References [1] A. Kulmala, A. Rintala, L. Koivula, V. Petäjä and M. Tenhunen, A robust method for determining backlash of a multileaf collimator, ESTRO Annual Meeting, 2022, PO-1510 [2] A. Kulmala, T. Paasonen, L. Koivula, A. Rintala and M. Tenhunen, Backlash of a Multileaf Collimator, AAPM 64th Annual Meeting & Exhibition, 2022, PO-GePV-T-375 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 Assessment of imaging doses resulting from cone beam CT (CBCT) scans during image guided radiation therapy (IGRT) procedures is required in order to estimate doses of organs at risk, and to compare between various clinical protocols or imaging systems. Commonly, a male or a female reference phantom of average is used to estimate patient organ doses. Since the of the patient plays a major role in determining organ doses, values obtained with the average cannot be used for assessing doses to individuals. The aim of this study is to assess CBCT doses to organs at risk for a wide range of patient s. This would allow the influence of the patient on the doses to be investigated. Materials and Methods A library of 193 computational adult phantoms (100 male and 93 female) was obtained from the national cancer institute (NCI) of the US. The library covered a wide range of adult s with heights (150 – 190 cm) and weights (40 – 125 kg). Doses of organs at risk resulting from CBCT were estimated using Monte Carlo (MC) simulations. Two scanning protocols, chest and pelvis, with scan parameters similar to those used in the clinical were studied. A validated MC model of a Varian on-board imager (OBI) system, that was used for IGRT procedures and integrated with a TrueBeam linac, was employed using BEAMnrc and DOSXYZnrc codes. A MATLBA code developed in house was used to analyze output of the MC simulations to obtain organ doses. Results For chest scans, heart dose was affected significantly by the patient , where dose to the largest patient studied was 1.41 mGy / 100 mAs and increased to 4.96 mGy / 100 mAs for the smallest patient (Table 1). Doses of lung, thyroid, and esophagus were also influenced comparably by the patient , but less than that to the heart. The lowest impact was found for stomach dose, where the dose differences between the s studied were lower than 1.0 mGy / 100 mAs. This is because the stomach was not included fully within the scan field. For pelvis scans, doses to the testes for male and the ovaries for female were affected remarkably, where the differences between the small and the large patients were 4.9 and 3.1 mGy / 100 mAs, respectively (Table 1). Moreover, urinary bladder dose for both genders was also influenced by the patient stature, with dose increasing with from 1.26 to 5.26 mGy / 100 mAs between the largest and smallest patients. Since colon and small intestine were partially inside the scan field, a minimal impact was found for these organs. PO-1798 Variations in doses to organs at risk resulting from cone beam CT (CBCT) scans with the patient size A. Abuhaimed 1 , C. Martin 2