BACKGROUND: There is currently significant interest in assessing the role of oxygen in the radiobiological effects at ultra-high dose rates. Oxygen modulation is postulated to play a role in the enhanced sparing effect observed in FLASH radiotherapy, where particles are delivered at 40-1000 Gy/s. Furthermore, the development of laser-driven accelerators now enables radiobiology experiments in extreme regimes where dose rates can exceed 109 Gy/s, and predicted oxygen depletion effects on cellular response can be tested. Access to appropriate experimental enviroments, allowing measurements under controlled oxygenation conditions, is a key requirement for these studies. We report on the development and application of a bespoke portable hypoxia chamber specifically designed for experiments employing laser-driven sources, but also suitable for comparator studies under FLASH and conventional irradiation conditions. MATERIALS AND METHODS: We used oxygen concentration measurements to test the induction of hypoxia and the maintenance capacity of the chambers. Cellular hypoxia induction was verified using hypoxia inducible factor-1α immunostaining. Calibrated radiochromic films and GEANT-4 simulations verified the dosimetry variations inside and outside the chambers. We irradiated hypoxic human skin fibroblasts (AG01522B) cells with laser-driven protons, conventional protons and reference 225 kVp X-rays to quantify DNA DSB damage and repair under hypoxia. We further measured the oxygen enhancement ratio for cell survival after X-ray exposure in normal fibroblast and radioresistant patient- derived GBM stem cells. RESULTS: Oxygen measurements showed that our chambers maintained a radiobiological hypoxic environment for at least 45 min and pathological hypoxia for up to 24 h after disconnecting the chambers from the gas supply. We observed a significant reduction in the 53BP1 foci induced by laser-driven protons, conventional protons and X-rays in the hypoxic cells compared to normoxic cells at 30 min post-irradiation. Under hypoxic irradiations, the Laser-driven protons induced significant residual DNA DSB damage in hypoxic AG01522B cells compared to the conventional dose rate protons suggesting an important impact of these extremely high dose-rate exposures. We obtained an oxygen enhancement ratio (OER) of 2.1 ± 0.1 and 2.5 ± 0.1 respectively for the AG01522B and patient-derived GBM stem cells for X-ray irradiation using our hypoxia chambers. CONCLUSION: We demonstrated the design and application of portable hypoxia chambers for studying cellular radiobiological endpoints after exposure to laser-driven protons at ultra-high dose, conventional protons and X-rays. Suitable levels of reduced oxygen concentration could be maintained in the absence of external gassing to quantify hypoxic effects. The data obtained provided indication of an enhanced residual DNA DSB damage under hypoxic conditions at ultra-high dose rate compared to the conventional protons or X-rays.

Centre for Plasma Physics School of Mathematics and Physics Queen's University Belfast Belfast BT7 1NN Northern Ireland UK

Department of Physics SUPA University of Strathclyde Glasgow G1 1XQ Scotland UK

ELI Beamlines Centre Institute of Physics Czech Academy of Sciences Za Radnicí 835 252 41 Dolní Břežany Czech Republic

Experimental Science Group Central Laser Facility Rutherford Appleton Laboratory Didcot Oxford OX11 0QX England UK

Extreme Light Infrastructure Str Reactorului No 30 077125 Bucharest Magurele Romania

Laboratoire LULI École Polytechnique Route de Saclay 91128 Palaiseau Paris France

Laboratori Nazionali del Sud Istituto Nazionale di Fisica Nucleare Via S Sofia 62 95123 Catania Italy

The Patrick G Johnston Centre for Cancer Research Queen's University Belfast Lisburn Road Belfast BT9 7AE Northern Ireland UK

Zobrazit více v PubMed

Gray LH, Conger AD, Ebert M, Hornsey S, Scott OC. The concentration of oxygen dissolved in tissues at the time of irradiation as a factor in radiotherapy. Br J Radiol. 1953;26(312):638–648. doi: 10.1259/0007-1285-26-312-638. PubMed DOI

Barendsen GW. Responses of cultured cells, tumours and normal tissues to radiations of different linear energy transfer. Curr Top Radiat Res Vol IV Ebert, Michael Howard, Alma (eds) New York, John Wiley Sons, Inc; 1968. pp. 293–356. Available from: https://www.osti.gov/biblio/4500126.

Prise KM, Folkard M, Davies S, Michael BD. The irradiation of V79 mammalian cells by protons with energies below 2 MeV. Part II. Measurement of oxygen enhancement ratios and DNA damage. Int J Radiat Biol. 1990;58(2):261–77. doi: 10.1080/09553009014551611. PubMed DOI

Nakano T, Suzuki Y, Ohno T, Kato S, Suzuki M, Morita S, et al. Carbon beam therapy overcomes the radiation resistance of uterine cervical cancer originating from hypoxia. Clin Cancer Res. 2006;12:2185–2190. doi: 10.1158/1078-0432.CCR-05-1907. PubMed DOI

Bassler N, Jäkel O, Søndergaard CS, Petersen JB. Dose- and LET-painting with particle therapy. Acta Oncol. 2010;49(July):1170–1176. doi: 10.3109/0284186X.2010.510640. PubMed DOI

Hirayama R, Furusawa Y, Fukawa T, Ando K. Repair kinetics of DNA-DSB induced by X-rays or carbon ions under oxic and hypoxic conditions. J Radiat Res. 2005;46(3):325–332. doi: 10.1269/jrr.46.325. PubMed DOI

Mohan R, Grosshans D. Proton therapy: present and future. Adv Drug Deliv Rev. 2017;109:26–44. doi: 10.1016/j.addr.2016.11.006. PubMed DOI PMC

Fowler JF. What can we expect from dose escalation using proton beams? Clin Oncol. 2003;15(1):10–15. doi: 10.1053/clon.2002.0182. PubMed DOI

Montay-Gruel P, Acharya MM, Petersson K, Alikhani L, Yakkala C, Allen BD, et al. Long-term neurocognitive benefits of FLASH radiotherapy driven by reduced reactive oxygen species. Proc Natl Acad Sci USA. 2019;166(22):10943–10951. doi: 10.1073/pnas.1901777116. PubMed DOI PMC

Montay-Gruel P, Meziani L, Yakkala C, Vozenin MC. Expanding the therapeutic index of radiation therapy by normal tissue protection. Br J Radiol. 2019 doi: 10.1259/bjr.20180008. PubMed DOI PMC

Diffenderfer ES, Verginadis II, Kim MM, Shoniyozov K, Velalopoulou A, Goia D, et al. Design, implementation, and in vivo validation of a novel proton FLASH radiation therapy system. Int J Radiat Oncol Biol Phys. 2020;106(2):440–448. doi: 10.1016/j.ijrobp.2019.10.049. PubMed DOI PMC

Doria D, Kakolee KF, Kar S, Litt SK, Fiorini F, Ahmed H, et al. Biological effectiveness on live cells of laser driven protons at dose rates exceeding 109 Gy/s. AIP Adv. 2012;2(1):011209. doi: 10.1063/1.3699063. DOI

Yogo A, Maeda T, Hori T, Sakaki H, Ogura K, Nishiuchi M, et al. Measurement of relative biological effectiveness of protons in human cancer cells using a laser-driven quasimonoenergetic proton beamline. Appl Phys Lett. 2011;98(5):053701. doi: 10.1063/1.3551623. DOI

Zeil K, Baumann M, Beyreuther E, Burris-Mog T, Cowan TE, Enghardt W, et al. Dose-controlled irradiation of cancer cells with laser-accelerated proton pulses. Appl Phys B Lasers Opt. 2013;110(4):437–444. doi: 10.1007/s00340-012-5275-3.pdf. DOI

Bin J, Allinger K, Assmann W, Dollinger G, Drexler GA, Friedl AA, et al. A laser-driven nanosecond proton source for radiobiological studies. Appl Phys Lett. 2012;110:437–444. doi: 10.1007/s00340-012-5275-3. DOI

Hanton F, Chaudhary P, Doria D, Gwynne D, Maiorino C, Scullion C, et al. DNA DSB repair dynamics following irradiation with laser- driven protons at ultra-high dose rates. Sci Rep. 2019 doi: 10.1038/s41598-019-40339-6. PubMed DOI PMC

Manti L, Perozziello FM, Borghesi M, Candiano G, Chaudhary P, Cirrone GAP, et al. The radiobiology of laser-driven particle beams: focus on sub-lethal responses of normal human cells. J Instrum. 2017;12(03):C03084. doi: 10.1088/1748-0221/12/03/C03084. DOI

Weiss H, Epp ER, Heslin JM, Ling CC, Santomasso A. Oxygen depletion in cells irradiated at ultra-high dose-rates and at conventional dose-rates. Int J Radiat Biol Relat Stud Physics, Chem Med. 1974;26(1):17–29. doi: 10.1080/09553007414550901. PubMed DOI

Durante M, Brauer-Krisch E, Hill M. Faster and safer? FLASH ultra-high dose rate in radiotherapy. Br J Radiol. 2017 doi: 10.1259/bjr.20170628. PubMed DOI PMC

Favaudon V, Caplier L, Monceau V, Pouzoulet F, Sayarath M, Fouillade C, et al. Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Sci Transl Med. 2014;6(245):1–10. doi: 10.1126/scitranslmed.3008973. PubMed DOI

Montay-Gruel P, Petersson K, Jaccard M, Boivin G, Germond JF, Petit B, et al. Irradiation in a flash: unique sparing of memory in mice after whole brain irradiation with dose rates above 100 Gy/s. Radiother Oncol. 2016;124(3):365–369. doi: 10.1016/j.radonc.2017.05.003. PubMed DOI

Adrian G, Konradsson E, Lempart M, Bäck S, Ceberg C, Petersson K. The FLASH effect depends on oxygen concentration. Br J Radiol. 2020;93(1106):20190702. doi: 10.1259/bjr.20190702. PubMed DOI PMC

Petersson K, Adrian G, Butterworth K, McMahon SJ. A quantitative analysis of the role of oxygen tension in FLASH radiation therapy. Int J Radiat Oncol Biol Phys. 2020;107(3):539–547. doi: 10.1016/j.ijrobp.2020.02.634. PubMed DOI

Khan S, Bassenne M, Wang J, Manjappa R, Melemenidis S, Breitkreutz DY, et al. Multicellular spheroids as in vitro models of oxygen depletion during FLASH irradiation. Int J Radiat Oncol. 2021;110:833–844. doi: 10.1016/j.ijrobp.2021.01.050. PubMed DOI

Lee KB, Kim K-R, Huh T-L, Lee YM. Proton induces apoptosis of hypoxic tumor cells by the p53-dependent and p38/JNK MAPK signalling pathways. Int J Oncol. 2008;33:1247–56. doi: 10.3892/ijo_00000115. PubMed DOI

Ma NY, Tinganelli W, Maier A, Durante M, Kraft-Weyrather W. Influence of chronic hypoxia and radiation quality on cell survival. J Radiat Res. 2013;54:13–22. doi: 10.1093/jrr/rrs135. PubMed DOI PMC

Tinganelli W, Durante M, Hirayama R, Krämer M, Maier A, Kraft-Weyrather W, et al. Kill-painting of hypoxic tumours in charged particle therapy. Sci Rep. 2015;5:1–13. doi: 10.1038/srep17016. PubMed DOI PMC

Kanemoto A, Hirayama R, Moritake T, Furusawa Y, Sun L, Sakae T, et al. RBE and OER within the spread-out Bragg peak for proton beam therapy: in vitro study at the Proton Medical Research Center at the University of Tsukuba. J Radiat Res. 2014;55(5):1028–32. doi: 10.1093/jrr/rru043. PubMed DOI PMC

Kumareswaran R, Ludkovski O, Meng A, Sykes J, Pintilie M, Bristow RG. Chronic hypoxia compromises repair of DNA double-strand breaks to drive genetic instability. J Cell Sci. 2012;125:189–99. doi: 10.1242/jcs.092262. PubMed DOI

Vordermark D, Menke DR, Brown JM, Ma N-YN-Y, Tinganelli W, Maier A, et al. Similar radiation sensitivities of acutely and chronically hypoxic cells in HT 1080 Fibrosarcoma Xenografts. Nat Rev Cancer. 2013;4(1):443–7. doi: 10.1016/j.jscs.2010.02.005. PubMed DOI

Metsälä O, Kreutzer J, Högel H, Miikkulainen P, Kallio P, Jaakkola PM. Transportable system enabling multiple irradiation studies under simultaneous hypoxia in vitro. Radiat Oncol. 2018;13(1):220. doi: 10.1186/s13014-018-1169-9. PubMed DOI PMC

Tinganelli W, Ma NY, Von Neubeck C, Maier A, Schicker C, Kraft-Weyrather W, et al. Influence of acute hypoxia and radiation quality on cell survival. J Radiat Res. 2013;54(Suppl 1):23–30. doi: 10.1093/jrr/rrt065. PubMed DOI PMC

Snavely RA, Key MH, Hatchett SP, Cowan TE, Roth M, Phillips TW, et al. Intense high-energy proton beams from Petawatt-laser irradiation of solids. Phys Rev Lett. 2000;85(14):2945–8. doi: 10.1103/PhysRevLett.85.2945. PubMed DOI

Macchi A, Borghesi M, Passoni M. Ion acceleration by superintense laser-plasma interaction. Rev Mod Phys. 2013;85(2):751–793. doi: 10.1103/RevModPhys.85.751. DOI

Karsch L, Beyreuther E, Burris-Mog T, Kraft S, Richter C, Zeil K, et al. Dose rate dependence for different dosimeters and detectors: TLD, OSL, EBT films, and diamond detectors. Med Phys. 2012;39(5):2447–2455. doi: 10.1118/1.3700400. PubMed DOI

Jaccard M, Petersson K, Buchillier T, Germond JF, Durán MT, Vozenin MC, et al. High dose-per-pulse electron beam dosimetry: Usability and dose-rate independence of EBT3 Gafchromic films: Usability. Med Phys. 2017;44(2):725–735. doi: 10.1002/mp.12066. PubMed DOI

Chaudhary P, Marshall TI, Perozziello FM, Manti L, Currell FJ, Hanton F, et al. Relative biological effectiveness variation along monoenergetic and modulated Bragg peaks of a 62-MeV therapeutic proton beam: a preclinical assessment. Int J Radiat Oncol Biol Phys. 2014;90(1):27–35. doi: 10.1016/j.ijrobp.2014.05.010. PubMed DOI

McKeown SR. Defining normoxia, physoxia and hypoxia in tumours-implications for treatment response. Br J Radiol. 2013;2014(87):20130676. doi: 10.1259/bjr.20130676. PubMed DOI PMC

Neumaier T, Swenson J, Pham C, Polyzos A, Lo AT, Yang P, et al. From the cover: evidence for formation of DNA repair centers and dose-response nonlinearity in human cells. Proc Natl Acad Sci. 2012;109(2):443–8. doi: 10.1073/pnas.1117849108. PubMed DOI PMC

Asaithamby A, Chen DJ. Cellular responses to DNA double-strand breaks after low-dose gamma-irradiation. Nucleic Acids Res. 2009;37(12):3912–3923. doi: 10.1093/nar/gkp237. PubMed DOI PMC

Panier S, Boulton SJ. Double-strand break repair: 53BP1 comes into focus. Nat Rev Mol Cell Biol. 2014;15(1):7–18. doi: 10.1038/nrm3719. PubMed DOI

Zimmermann M, de Lange T. 53BP1: pro choice in DNA repair. Trends Cell Biol. 2014;24(2):108–117. doi: 10.1016/j.tcb.2013.09.003. PubMed DOI PMC

Freyer JP, Jarrett K, Carpenter S, Raju MR. Oxygen enhancement ratio as a function of dose and cell cycle phase for radiation-resistant and sensitive CHO cells. Radiat Res. 1991;127(3):297–307. doi: 10.2307/3577945. PubMed DOI

Wenzl T, Wilkens JJ. Theoretical analysis of the dose dependence of the oxygen enhancement ratio and its relevance for clinical applications. Radiat Oncol. 2011;6(1):1–9. doi: 10.1186/1748-717X-6-171. PubMed DOI PMC

Burroughs SK, Kaluz S, Wang D, Wang K, Meir EG Van, Wang B. Therapeutics. 2014; 5(5):1–31. 10.4155/fmc.13.17

Busk M, Overgaard J, Horsman MR. Imaging of tumor hypoxia for radiotherapy: current status and future directions. Semin Nucl Med. 2020;50(6):562–583. doi: 10.1053/j.semnuclmed.2020.05.003. PubMed DOI

Malinen E, Søvik Å. Dose or LET painting: What is optimal in particle therapy of hypoxic tumors? Acta Oncol (Madr) 2015;54(9):1614–1622. doi: 10.3109/0284186X.2015.1062540. PubMed DOI

Chang JH, Wada M, Anderson NJ, Lim Joon D, Lee ST, Gong SJ, et al. Hypoxia-targeted radiotherapy dose painting for head and neck cancer using (18)F-FMISO PET: a biological modeling study. Acta Oncol. 2012;2013(52):1723–1729. PubMed

Romano F, Subiel A, McManus M, Lee ND, Palmans H, Thomas R, et al. Challenges in dosimetry of particle beams with ultra-high pulse dose rates. J Phys Conf Ser. 2020;1662(1):5. doi: 10.1088/1742-6596/1662/1/012028. DOI

Bravatà V, Tinganelli W, Cammarata FP, Minafra L, Calvaruso M, Sokol O, et al. Hypoxia transcriptomic modifications induced by proton irradiation in u87 glioblastoma multiforme cell line. J Pers Med. 2021;11(4):308. doi: 10.3390/jpm11040308. PubMed DOI PMC

Chaudhary P, Marshall TI, Currell FJ, Kacperek A, Schettino G, Prise KM. Variations in the processing of DNA double-strand breaks along 60-MeV therapeutic proton beams. Int J Radiat Oncol Biol Phys. 2016;95(1):86–94. doi: 10.1016/j.ijrobp.2015.07.2279. PubMed DOI PMC

Wang R, Jin F, Zhong H. A novel experimental hypoxia chamber for cell culture. Am J Cancer Res. 2014;4(1):53–60. PubMed PMC

Kaida A, Miura M. Differential dependence on oxygen tension during the maturation process between monomeric Kusabira Orange 2 and monomeric Azami Green expressed in HeLa cells. Biochem Biophys Res Commun. 2012;421(4):855–859. doi: 10.1016/j.bbrc.2012.04.102. PubMed DOI

Marshall TI, Chaudhary P, Michaelidesová A, Vachelová J, Davídková M, Vondráček V, et al. Investigating the implications of a variable RBE on proton dose fractionation across a clinical pencil beam scanned spread-out Bragg peak. Int J Radiat Oncol Biol Phys. 2016;95(1):70–77. doi: 10.1016/j.ijrobp.2016.02.029. PubMed DOI PMC

Gomez-Roman N, Stevenson K, Gilmour L, Hamilton G, Chalmers AJ. A novel 3D human glioblastoma cell culture system for modeling drug and radiation responses. Neuro Oncol. 2017;19(2):229–241. doi: 10.1093/neuonc/now164. PubMed DOI PMC

Ahmed U, Carruthers R, Gilmour L, Yildirim S, Watts C, Chalmers AJ. Selective inhibition of parallel DNA damage response pathways optimizes radiosensitization of glioblastoma stem-like cells. Cancer Res. 2015;75(21):4416–4428. doi: 10.1158/0008-5472.CAN-14-3790. PubMed DOI

Carruthers R, Ahmed SU, Strathdee K, Gomez-Roman N, Amoah-Buahin E, Watts C, et al. Abrogation of radioresistance in glioblastoma stem-like cells by inhibition of ATM kinase. Mol Oncol. 2015;9(1):192–203. doi: 10.1016/j.molonc.2014.08.003. PubMed DOI PMC

Asaithamby A, Hu B, Chen DJ. Unrepaired clustered DNA lesions induce chromosome breakage in human cells. Proc Natl Acad Sci USA. 2011;108(20):8293–8298. doi: 10.1073/pnas.1016045108. PubMed DOI PMC

Raschke S, Spickermann S, Toncian T, Swantusch M, Boeker J, Giesen U, et al. Ultra-short laser-accelerated proton pulses have similar DNA-damaging effectiveness but produce less immediate nitroxidative stress than conventional proton beams. Sci Rep. 2016;6(1):32441. doi: 10.1038/srep32441. PubMed DOI PMC

Vozenin MC, De Fornel P, Petersson K, Favaudon V, Jaccard M, Germond JF, et al. The advantage of FLASH radiotherapy confirmed in mini-pig and cat-cancer patients. Clin Cancer Res. 2019;25(1):35–42. doi: 10.1158/1078-0432.CCR-17-3375. PubMed DOI

Alaghband Y, Cheeks SN, Allen BD, Montay-Gruel P, Doan NL, Petit B, et al. Neuroprotection of radiosensitive juvenile mice by ultra-high dose rate flash irradiation. Cancers (Basel) 2020;12(6):1–21. doi: 10.3390/cancers12061671. PubMed DOI PMC

Bourhis J, Sozzi WJ, Jorge PG, Gaide O, Bailat C, Duclos F, et al. Treatment of a first patient with FLASH-radiotherapy. Radiother Oncol. 2019;139:18–22. doi: 10.1016/j.radonc.2019.06.019. PubMed DOI

Favaudon V, Caplier L, Monceau V, Pouzoulet F, Sayarath M, Fouillade C, et al. Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Sci Transl Med. 2014;6(245):245ra93. doi: 10.1126/scitranslmed.3008973. PubMed DOI

Buonanno M, Grilj V, Brenner DJ. Biological effects in normal cells exposed to FLASH dose rate protons. Radiother Oncol. 2019;139:51–55. doi: 10.1016/j.radonc.2019.02.009. PubMed DOI PMC

Adrian G, Konradsson E, Beyer S, Wittrup A, Butterworth KT, McMahon SJ, et al. Cancer cells can exhibit a sparing FLASH effect at low doses under normoxic in vitro-conditions. Front Oncol. 2021;11(July):1–9. doi: 10.3389/fonc.2021.686142. PubMed DOI PMC

Boscolo D, Scifoni E, Durante M, Krämer M, Fuss MC. May oxygen depletion explain the FLASH effect? A chemical track structure analysis. Radiother Oncol. 2021;162:68–75. doi: 10.1016/j.radonc.2021.06.031. PubMed DOI

Jansen J, Knoll J, Beyreuther E, Pawelke J, Skuza R, Hanley R, et al. Does FLASH deplete oxygen? Experimental evaluation for photons, protons, and carbon ions. Med Phys. 2021;48(7):3982–3990. doi: 10.1002/mp.14917. PubMed DOI

Abolfath R, Grosshans D, Mohan R. Oxygen depletion in FLASH ultra-high-dose-rate radiotherapy: a molecular dynamics simulation. Med Phys. 2020;47(12):6551–6561. doi: 10.1002/mp.14548. PubMed DOI

Favaudon V, Labarbe R, Limoli CL. Model studies of the role of oxygen in the FLASH effect. Med Phys. 2021 doi: 10.1002/mp.15129. PubMed DOI PMC

Bourton EC, Plowman PN, Smith D, Arlett CF, Parris CN. Prolonged expression of the γ-H2AX DNA repair biomarker correlates with excess acute and chronic toxicity from radiotherapy treatment. Int J Cancer. 2011;129(12):2928–2934. doi: 10.1002/ijc.25953. PubMed DOI PMC

Najafi M, Farhood B, Mortezaee K, Kharazinejad E, Majidpoor J, Ahadi R. Hypoxia in solid tumors: a key promoter of cancer stem cell (CSC) resistance. J Cancer Res Clin Oncol. 2020;146(1):19–31. doi: 10.1007/s00432-019-03080-1. PubMed DOI PMC

Antonovic L, Lindblom E, Dasu A, Bassler N, Furusawa Y, Toma-Dasu I. Clinical oxygen enhancement ratio of tumors in carbon ion radiotherapy: the influence of local oxygenation changes. J Radiat Res. 2014;55:902–911. doi: 10.1093/jrr/rru020. PubMed DOI PMC

Thompson HF, Butterworth KT, McMahon SJ, Ghita M, Hounsell AR, Prise KM. The impact of hypoxia on out-of-field cell survival after exposure to modulated radiation fields. Radiat Res. 2017;188(6):716–724. doi: 10.1667/RR14836.1. PubMed DOI

Butterworth KT, McGarry CK, Clasie B, Carabe-Fernandez A, Schuemann J, Depauw N, et al. Relative biological effectiveness (RBE) and out-of-field cell survival responses to passive scattering and pencil beam scanning proton beam deliveries. Phys Med Biol. 2012;57(20):6671–6680. doi: 10.1088/0031-9155/57/20/6671. PubMed DOI

Wenzl T, Wilkens JJ. Modelling of the oxygen enhancement ratio for ion beam radiation therapy. Phys Med Biol. 2011;56(11):3251–3268. doi: 10.1088/0031-9155/56/11/006. PubMed DOI

Chan CC, Chen FH, Hsiao YY. Impact of hypoxia on relative biological effectiveness and oxygen enhancement ratio for a 62-mev therapeutic proton beam. Cancers (Basel) 2021;13(12):1–18. doi: 10.3390/cancers13122997. PubMed DOI PMC