ESTRO 2025 - Abstract Book

S4022

Radiobiology - Tumour radiobiology

ESTRO 2025

microalgae, which produces oxygen under illumination. This study investigates the use of the photosynthetic microalgae Chlamydomonas reinhardtii in co-culture with MCF-7 tumor cells in in vitro radiotherapy experiments. The study contemplate both the oxygen production of microalgae under illuminated conditions and their oxygen consumption in darkness. This requires precise control over oxygenation, irradiation, and illumination timing to maintain consistent experimental variables. To apply different radiation doses within the same timeframe, different dose rates were used. Material/Methods: Irradiation of 2D cell cultures was conducted with the XRAD 320 preclinical irradiator (Precision X Ray Inc.) using 125 kVp X-rays (HVL=5,02 mm Al). Experimental conditions included normoxia and hypoxia, with or without light, and co culture with microalgae. Hypoxia was induced using a hypoxia chamber inside the irradiator (0.1-1% O 2 , 5% CO 2 , 94 94.9% N 2 ). An absorbed dose to water at the base of the culture wells of 2, 4, and 6 Gy (AAPM TG-61 protocol) was delivered. Illumination was provided by red LEDs (1.60 mW/cm2). Cell survival was assessed through clonogenic assays, while microalgae viability and oxygen production were measured with a hemocytometer and an oxygraph, respectively. Results: Preliminary results indicate that, under the conditions tested, ionizing radiation does not affect microal gae viability or their oxygen production capability. No significant cell death was observed in the co culture after 2 and 24 hours of incubation. Cell survival fractions following a 6 Gy dose, applied at the same dose rate but over different times, were similar under normoxia (without microalgae) and under hypoxic co-culture with illumination, both being lower than under hypoxia without microalgae. Conclusion: Photosynthetic microalgae could be an effective strategy for radiosensitizing hypoxic tumor cells in radiotherapy, due their biocompatibility, radiation resistance, and ability to produce sufficient oxygen to mitigate tumor hypoxia. These findings open new possibilities for clinical optimization of microalgae use in the future. References: H.E.Barker, J. T. E. Paget, A. A. Khan, and K. J. Harrington, The Tumour Microenvironment after Radiotherapy: Mechanisms of Resistance and Recurrence, Nat Rev Cancer , 15, 409–425 (2015). Y. Qiao, F. Yang, T. Xie, Z. Du, D. Zhong, Y. Qi, Y. Li, W. Li, Z. Lu, J. Rao, Y. Sun, M. Zhou, Engineered algae: A novel oxygen-generating system for effective treatment of hypoxic cancer. Sci. Adv. 6, eaba5996 (2020). C. M.Ma,C.W.Coffey, L. A. DeWerd, C. Liu, R. Nath, S. M. Seltzer, and J. P. Seuntjens, AAPM protocol for 40-300 kV x ray beam dosimetry in radiotherapy and radiobiology, Med Phys 28, 868 (2001). N. A. P. Franken, H. M. Rodermond, J. Stap, J. Haveman, and C. van Bree, Clonogenic assay of cells in vitro, Nat Protoc 1, 2315 (2006). Keywords: hypoxia, microalgae, radioresistance

4138

Digital Poster Impact of Treatment Duration on Biologically Effective Dose in Stereotactic Radiosurgery - A Comparison Between Gamma Knife, Cyberknife and Truebeam Matthew G Skinner 1 , Lauren Weinstein 1 , Ian Paddick 2 1 Radiation Oncology, Kaiser Permanente, South San Francisco, USA. 2 Radiation Oncology, Cromwell Hospital, London, United Kingdom