ESTRO 36 Abstract Book

S532 ESTRO 36 2017 _______________________________________________________________________________________________

dose was 50 Gy in 25 fractions to the PTV. Monte Carlo based dose calculation engine was the preferred choice as it is more accurate at low dose levels which is more relevant for estimating SCR. Appropriate model parameters were taken from the literature for the mechanistic model to calculate excess absolute risk (EAR), lifetime attributable risk (LAR), integral dose and relative risk (RR) for both lungs, contralateral breast, heart and spinal cord. Results The mean MU in IMRT and VMAT plans were 751.1±133.3 and 1004.8±180 respectively for IMRT and VMAT. The mean EAR values per 10,000 person years (PY) estimated for IMRT and VMAT treatments including gender-specific correction with and without age correction factor are shown in figure 3. The mean EAR values with one standard deviation without age correction were 42.4±11.3, 10.6±6.0, 12.3±6.7, 1.9±0.7 and 0.6±0.3 for left lung, right lung, contralateral breast, heart and spinal cord respectively for the IMRT plans. These values were 51.9±19.7, 28.7±11.4, 31.9±13.4, 2.3±0.8 and 1.5±0.8 for the VMAT plans. However the values were reduced with age correction, especially for the contralateral breast. The values obtained with age correction were 44.6±11.9, 11.2±6.4, 5.4±4.0, 1.4±0.5 and 0.3±0.2 for left lung, right lung, contralateral breast, heart and spinal cord respectively for the IMRT treatments and 54.6±20.6, 30.2±12.0, 13.8±8.6, 1.6±0.6 and 0.9±0.5 for the VMAT treatments.

Conclusion Our results show that a prompt increase in γH2AX foci at 24 hours post-implant relative to baseline may be a useful measure to assess elevated risk of late RT related toxicities for PPB patients. A subsequent investigation recruiting a larger cohort of patients is warranted to verify our findings.

Poster: Radiobiology track: Radiobiology of breast cancer

PO-0971 Estimating second malignancy risk in IMRT and VMAT in radiotherapy for carcinoma of left breast J. Selvaraj 1 , V. Sakthivel 2 1 The Canberra Hospital, Medical Physics and Radiation Engineering, Canberra, Australia 2 Advanced Medical Physics, Medical Physics, Houston- Texas, USA Purpose or Objective IMRT and VMAT produce dose distributions with superior target dose uniformity and normal tissue sparing. However, this increases amount of volume receiving very low doses substantially compared to conventional techniques. This increases the risk of radiation-induced second malignancy (SCR) as reported in the literature. The aim of this study is to use a mechanistic radiobiological model which is more accurate in predicting the dose- response at low as well as high dose levels to estimate SCR. Studies have shown patient age at exposure is important in estimating SCR, thus patients’ age is also accounted for in the SCR estimation. Moreover, the mechanistic model also takes cell proliferation and dose fractionation into account. Material and Methods Fifty IMRT and VMAT plans with similar dose-volume objectives were selected for the study. The prescription

Conclusion Results showed VMAT plans had a higher risk of developing second malignancy in lung, contralateral breast, heart and cord compared to IMRT plans. However, the increase in risk was found to be marginal. The increase in risk was greater in both IMRT and VMAT for left lung and contralateral breast compared to other organs included in the study. Incorporating the age correction factor decreased the risk of contralateral breast SCR. No strong correlation was found between EAR and MU. PO-0972 Breast cancer cell survival using flattening filter-free beam compared to a standard flattened beam M. Boccia 1 , L. Manti 2 , S. Clemente 3 , C. Oliviero 3 , F. Perozziello 2 , R. Liuzzi 4 , M. Conson 1 , L. Cella 4 , R. Pacelli 1 1 Federico II University School of Medicine, Department of Advanced Biomedical Sciences, Napoli, Italy