ESTRO 2024 - Abstract Book

S5276 ESTRO 2024 Adam Galdi 1,2 , Gyöngyvér Farkas 1 , Szilvia Gazdag Hegyesi 1,3 , Enikő Koszta 1 , Csilla Pesznyák 1,4 , Zoltán Takácsi Nagy 1,5 , Tibor Major 1,5 1 National Institute of Oncology, Centre of Radiotherapy, Budapest, Hungary. 2 Semmelweis University, Doctoral School, Budapest, Hungary. 3 University of Technology and Economics, Doctoral School of Physical Sciences, Budapest, Hungary. 4 University of Technology and Economics, Institute of Nuclear Techniques, Budapest, Hungary. 5 Semmelweis University, Department of Oncology, Budapest, Hungary Radiobiology - Normal tissue radiobiology

Purpose/Objective:

At our institute, a Varian Ethos linear accelerator with online kV-CBCT imaging is used for online adaptive radiotherapy (oART). High frequency CBCT imaging is essential for this technique. Investigation of the biological effect of CBCT alone is a well investigated topic in the international literature, and the cost-benefit of this imaging method is accepted. The aim of our study is to determine the combined biological effect of the therapeutic beam and different numbers of CBCT imaging used in our clinical oART routine.

Material/Methods:

In vitro chromosome aberrations (CAs) were investigated after irradiation of blood samples in phantoms. Two types of phantom were used. First, an in house-made phantom was used which was filled with clear water (approximately volume of 380 cm3) with one central hole to accommodate the blood sample. In the second part of the investigation, a CTDI PMMA phantom with pelvis cylinder with 9 holes (approximately volume of 11 965 cm3) was used. IMRT plans are routinely used for online adaptation in our department, therefore 12 equidistant beams were applied for the investigation. The plans were created in Ethos TPS. Six dose steps (0.5, 1, 2, 4, 6, 8 Gy) were used to create the dose/chromosome aberration calibration curve for therapeutic beam with the two phantoms. We performed measurements for different numbers of CBCT (1-5 CBCT) to evaluate the aberrations from CBCT’s only. Afterwards, clinically relevant oART irradiation were performed with increasing number of CBCT’s, combined with a 2 Gy therapeutic beam. Based on the calibration curve of the therapeutic beam, the estimated biological dose was calculated. We analyzed the radiobiological variation due to possible changes in the CBCT profile in 9 readout points of the CTDI phantom.

Results:

Both types of phantom could be used for chromosome aberration measurements. No clinically relevant CAs were found if the samples were irradiated with CBCT, only. When we combined therapeutic and CBCT radiations, and compared the summarized physical and biological doses, we found differences depending on the number of the CBCTs + 2 Gy. The estimated biological dose was higher than physical dose with 14% (1-2 CBCT), 24.8% (3 CBCT), 46.4% (4 CBCT) and 59.8 % (5 CBCT) (Figure 1).