ESTRO 2021 Abstract Book

S1631

ESTRO 2021

Using summary statistics for CTV and OARs at the distal edge of beam (LAD and rib), the differences in RBE-weighted dose (using fixed RBE 1.1) and LET d distributions between different beam arrangements were analysed by Friedman’s Test. Results All plans were clinically acceptable in terms of CTV coverage and OAR doses: mean heart dose ≤2.2 Gy, mean left lung dose ≤12 Gy. There were no statistically significant differences in dose to mean CTV, dose to maximum LAD and dose to rib (0.5cc) between different beam arrangements. For the corresponding LET d calculations there were statistically significant differences between the beam arrangements (Figure 1). Single beam plans demonstrated higher LET d in these distal OARs. Figure 1 : Differences between beam arrangements .

Conclusion Results demonstrate that the distribution of LET d

varies between beam arrangements. Single beam plans showed the

highest LET d contributing to an increased risk of rib fracture, our data support consideration of additional beams and varied beam angles. These considerations should be weighed against any disadvantages in dosimetry or plan robustness. References [1] Offersen B V. et al Radiother Oncol 2015;114:3–10. doi:10.1016/j.radonc.2014.11.030 [2] Bertolet A et al Med Phys 2020;47:2495–505. doi:10.1002/mp.14108 [3] Bertolet A et al Phys Medica 2021;81:69–76. doi:10.1016/j.ejmp.2020.11.024 in rib and LAD. In conclusion, to reduce the potential for high LET d

Digital Poster: Radiobiology of normal tissues

PO-1911 Low-dose irradiation could mitigate osteoarthritis progression by modulating mitochondrial function B.H. Kim 1 , H.C. Bae 2 , J. Kwon 3 , H. Han 2 , H.J. Kim 4 1 Seoul Metropolitan Government Seoul National University Boramae Medical Center, Radiation Oncology, Seoul, Korea Republic of; 2 Seoul National University College of Medicine, Orthopaedic Surgery, Seoul, Korea Republic of; 3 Chungnam National University School of Medicine, Radiation Oncology, Daejeon, Korea Republic of; 4 Seoul National University College of Medicine, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective The use of low-dose radiation therapy (LDRT) for osteoarthritis (OA) are rarely implemented except some European regions. There is a controversy over its clinical effects whereas little is known about how LDRT affects the actual disease progression. Materials and Methods Using primary cultured human chondrocytes and synovium-derived cells from OA patients, the effects of LDRT were measured by cell viability assay, quantitative real-time PCR, western blotting, and RNA sequencing. For in vivo validation, a surgically induced OA model was used. Results In vitro experiments indicated that LDRT have no significant impact on cell death, but slightly decreased expressions of pro- inflammatory factors including MMP13, without change in chondrogenesis markers. LDRT induces large transcriptomic changes in these cells, especially in mitochondrial activities. Growth differentiation factor 15 (GDF15) which is a mitohormetic signaling factor increased after LDRT and seemed to mediate anti-inflammatory effects of LDRT. We next found that rats treated with LDRT after anterior cruciate ligament transection or surgical destabilization of the medial meniscus exhibited decreased severity of OA compared to no-irradiation group at 10 weeks after surgery (mean OARSI score