1Institut Curie, University Paris Saclay, PSL Research University, Inserm U 1021-CNRS UMR 3347, Centre Universitaire, 91405 Orsay Cedex, France.
Our team recently discovered that big pulses of relativistic electrons at ultrahigh dose-rate (FLASH) allow sparing mice from radio-induced lung fibrosis . In contrast FLASH was as efficient as conventional dose-rate irradiation with tumor control as an endpoint . The same advantage has been found in mouse brain  and confirmed in large mammals . Therefore, FLASH dramatically increases the differential outcome between normal tissues and tumors, thus evoking a promising potential of the FLASH methodology in anticancer radiotherapy.
Were FLASH to be used in the clinic routine, it would involve major changes in the dosimetric techniques medical physicists are accustomed to. Actually, due to ion recombination, space charge and ion drift effects, ionisation chambers may not provide a proportional response with high radiation doses per pulse, typically 5-20 nC in a single microsecond pulse.
We investigated the feasibility and linearity of radiation dosimetry in a small field using Cerenkov light emission within a home-made detector made of a fused silica cylinder calculated for all incident electrons to be stopped within the target. The Cerenkov light was collected using an optical fiber bundle and sent to a photomultiplier optimized for the detection of nanosecond transients. The signal was recorded at a digital oscilloscope and compared with that received from a Faraday cup. The risetime of the whole mounting was ca. 15 ns. The system was investigated by varying (i) the energy of the electron beam between 3.9 and 5.0 MeV; (ii) the pulse width between 0.01 and 2.20 µs; (iii) the number of pulses and the repeat frequency; (iv) the dose between 8 10-4 and 13 nC in 1 µs. In the low dose range an ionisation chamber was used to mesure the charge received at the level of the probe.
The results  show that the integral of the Cerenkov signal collected at the silica probe is strictly proportional to the absorbed charge throughout the dynamic range explored, estimated at ≈ 5 orders of magnitude. The system allowed time-resolved analysis of the electron beam under any conditions, including in the lower range of dose-rate explored. Cerenkov light thus appears ideally suited to dosimetry of electrons though a wide range of dose and dose-rate, in particular in small fields, mini- and micro-beams. The technique is appealing in the prospect of future machines for FLASH radiotherapy in the clinic. Interestingly, the Cerenkov light has recently been considered for the dosimetry of X-ray, electron and proton beams [5-7] and used as a variable-delay probe beam with picosecond time resolution .
1. Favaudon V, Caplier L, Monceau V, Pouzoulet F, Sayarath M, Fouillade C, Poupon MF, Brito I, Hupé P, Bourhis J, Hall J, Fontaine JJ, Vozenin MC (2014). Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Science translational medicine 6, 245ra293.
2. Montay-Gruel P, Petersson K, Jaccard M, Boivin G, Germond JF, Petit B, Doenlen R, Favaudon V, Bochud F, Bailat C, Bourhis J, Vozenin MC (2017). Irradiation in a FLASH: unique sparing of memory in mice after whole brain irradiation with dose rates above 100Gy/s. Radiotherapy & Oncology 124, 365-369.
3. Vozenin MC, de Fornel P, Petersson K, Favaudon V, Jaccard M, Germond JF, Petit B, Burki M, Ferrand G, Patin D, Bouchaab H, Ozsahin M, Bochud F, Bailat C, Devauchelle P, Bourhis J (2019). The advantage of Flash radiotherapy confirmed in mini-pig and cat-cancer patients. Clinical Cancer Research 25, 35-42.
4. Favaudon V, Lentz J-M, Heinrich S, Patriarca A, de Marzi L, Fouillade C, Dutreix M (2019). Time-resolved dosimetry of pulsed electron beams in very high dose-rate, FLASH irradiation for preclinical studies. Nucl Instrum Meth Phys Res A 944, 162537.
5. Teymurazian A, Pang G (2012). Megavoltage X-ray imaging based on Cerenkov effect: a new application of optical fibres to radiation therapy. In International Journal of Optics, Volume 2012.
6. Glaser AK, Andreozzi JM, Davis SC, Zhang R, Pogue BW, Fox CJ, Gladstone DJ (2014). Video-rate optical dosimetry and dynamic visualization of IMRT and VMAT treatment plans in water using Cherenkov radiation. Medical Physics 41, https://doi.org/10.1118/1111.4875704.
7. Zlateva Y, El Naqa I (2015). Cherenkov emission dosimetry for electron beam radiotherapy: a Monte Carlo feasibility study of absolute dose prediction. In World Congress on Medical Physics and Biomedical Engineering, Volume 51, Jaffray DA, ed. (Toronto: IFMBE Proceedings), pp. DOI: 10.1007/1978-1003-1319-19387-19388_19203.
8. Grigoryants VM, Lozovoy VV, Chernousov YD, Shebolaev IV, Arutyunov VA, Anisimov OA, Molin YN (1989). Pulse radiolysis system with picosecond time resolution referred to Cherenkov radiation. Radiat Phys Chem 34, 349-352.