ESTRO 2021 Abstract Book

S1510

ESTRO 2021

for the reference phantoms, and this value was considered as a reference value. Moreover, lung dose for the same scan protocols and phantoms were assessed using DCCs based on the ED and D w methods, and these values were compared to the reference ones. Results Figure 1 shows comparisons between lung doses of the reference phantoms for the scan protocols studied. By comparing DCCs doses to the reference doses assessed by the NCICT software, doses estimated by DCCs based on the ED method were in the range of -19.2 to 7.6%, -23.7 to 5.2%, and -14.7 to -2.4%, for chest, chest- abdomen, and CAP protocols, respectively. These were reduced by DCCs based on the D w method to be in the range of -11.7 to 9.9 %, -11.6 to 4.9% and 13.6 to 3.7, respectively.

Conclusion The results showed that using DCCs based on the D w method would give a better estimation for lung dose. Doses of other organs in the trunk may be comparable for both methods, but an attention should be paid for organs in the chest region, particularly for paediatric patients, where large differences were found. PO-1784 On the Risk of Secondary Cancer from Thymoma Radiotherapy B. Youssef 1 , W. Jalbout 1 , R. Jbara 1 , C. Rizk 2 1 American University of Beirut Medical Center, Radiation Oncology, Beirut, Lebanon; 2 Lebanese Atomic Energy Commission, Dosimetry, Beirut, Lebanon Purpose or Objective This study aims at quantifying the lifetime attributable risk of secondary fatal cancer (LARFAC) to patients receiving adjuvant radiotherapy treatment for thymoma, a neoplasm where cure rates and life expectancy are relatively high, patient age at presentation relatively low and indications for radiotherapy controversial depending on the disease stage. Materials and Methods An anthropomorphic phantom was scanned, organs were contoured and a standard 6 MV 3DCRT treatment plan was produced for thymoma treatment. The phantom was loaded with thermoluminescent dosimeters (TLDs) and treated by linear accelerator per plan. The TLDs were subsequently read for out-of-field dose distribution while in-field dose distribution was obtained from the planning system. Sex and age-specific lifetime radiogenic cancer risk was calculated as the sum of in-field and out-of-field induced risks. The latter was estimated using a hybrid ICRP 103- BEIR VII tables of organ-specific risks based on the LNT model and applicable at low doses, while the former using mathematical risk models applicable at high doses.