ESTRO 2023 - Abstract Book

S1509

Digital Posters

ESTRO 2023

magnitude of activation-concentrations of radio-isotopes in concrete for a 30 year old, clinically used radiotherapy bunker and confirm this estimate with gamma spectroscopy measurements. Materials and Methods The bunker under investigation has been used for treating patients successively on a dual (6 MV-18MV) energy LINACs between 1992 and 2022. The shielding is made of ordinary concrete. The number of radio-isotopes after 30 years for several radio isotopes has been determined by calculating the netto result after historic activations and subsequent decay. The algorithm is validated with published data [1] for Mn54 , using neutron flux data from NCRP 151/ literature [2][3]. A maximum irradiation time for 18 MV beams during this period has been estimated and for this purpose, distributed equally over time during operational hours. Calculations are performed for concrete in the floor, which is nearest to the neutron-source in the head of the LINAC. The conversion to activity concentration for each radio-isotope is based on measurements from Ž agar et al [l]. Finally 6 concrete samples have been analyzed [5] using gamma-ray spectroscopy, 20 days after a final 1 Gy 18 MV irradiation at dmax. Results A total irradiation time of 939 hours has been estimated or daily irradiations of 7 minutes at 0.1 Gy/sec, 5 days a week. An example of a calculation is shown in figure 1.

Calculated activity concentration for several radio-isotopes present, are shown in table 1. After 20 days the remaining activity of the 1 Gy irradiation is predicted to be less than 0.1 Bq/gram. Gamma-ray spectroscopy measurements show no detectable activity of the radioisotopes in table 1.

Conclusion After 30 years of normal clinical use no radioactive waste due to 18 MV irradiations is present in the concrete shielding of this radiotherapy bunker.