A systematic investigation on the response of EBT-XD gafchromic films to varying dose-per-pulse, average dose-rate and instantaneous dose-rate in electron flash beams

Damiano Del Sarto^1,2Luigi Masturzo^1,3*Andrea Cavalieri^1,2Mariagrazia Celentano^1,3Taiusha Fuentes^1,4Giovanni Gadducci^1,4Noemi Giannini^1,4ALESSANDRA GONNELLI^1,4Giuliana Milluzzo⁵Fabiola Paiar^1,4,6Jake Pensavalle^1,3Francesco Romano⁵Fabio Di Martino^1,3

• ¹Centro Pisano ricerca e implementazione clinica Flash Radiotheraphy (CPFR), Presidio S. Chiara, Pisa, Italy, Pisa, Italy
• ²Center for Instrument Sharing of the University of Pisa (CISUP), University of Pisa, Pisa, Italy, Pisa, Italy
• ³U.O. Fisica sanitaria, Azienda Ospedaliero-Universitaria Pisana, Pisa, Italy, Pisa, Italy
• ⁴Department of Translational Research and New Technologies in Medicine and Surgery, University of Pisa, Pisa, Tuscany, Italy
• ⁵National Institute of Nuclear Physics of Catania, Catania, Sicily, Italy
• ⁶Department of Radiation Oncology, Azienda Ospedaliero-Universitaria Pisana, University of Pisa, Pisa, Italy, Pisa, Italy

The FLASH effect is a radiobiological phenomena characterized by the reduction of the damage to healthy tissues while maintaining iso-effectiveness on the tumour, obtained by delivering the entire radiation dose in less than 100-200 ms and with average dose rate greater than 40-100 Gy/s. Despite the enormous interests of the scientific community, a series of issues must be addressed and overcome to reach its clinical implementation. One of these challenges is related to the dosimetry of ultra-high dose-per-pulse (UHDP) beams, which trigger the effect. The most used dosimeters to date in radiobiological experiments with ultra-high dose rate (UHDR) beams are the radiochromic films. However, a systematic and accurate study of their response by varying doseper-pulse and dose rate over a wide range has never yet been done. Purpose: To systematically investigate the response of EBT-XD radiochromic films under UHDR electron beam irradiations by varying different beam parameters independently and over a wide range and using flashDiamond as reference. Materials and Methods: Thanks to a special research linac, average dose rate (ADR), dose-per-pulse (DP) and instantaneous dose rate (IDR) dependencies on film response have been individually investigated. The reference value of ADR, DP and IDR has been calculated by measuring the dose using a flashDiamond and knowing the temporal beam structure provided by the beam monitoring system of the Linac. Results: Our results normalized to fD showed an overresponse of the radiochromic films, compared to a conventional irradiation, for ADR values higher than 1000 Gy/s and an IDR higher than 1MGy/s which could represent a total dose overestimation of about 10%. Discussion: The gafchromic film EBT-XD have their field of application in UHDP beam dosimetry, considering their excellent spatial resolution; However, particular attention must be paid when using this type of dosimeter for absolute dose measurements at very extreme values of IDR (> 1MGy/s).