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Minimum dose rate estimation for pulsed FLASH radiotherapy: A dimensional analysis
Author(s) -
Zhou Sumin,
Zheng Dandan,
Fan Qiyong,
Yan Ying,
Wang Shuo,
Lei Yu,
Besemer Abigail,
Zhou Christina,
Enke Charles
Publication year - 2020
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.14181
Subject(s) - irradiation , oxygen , flash (photography) , oxygen enhancement ratio , radiation , radiation damage , ionizing radiation , radiation therapy , radiosensitivity , materials science , diffusion , chemistry , pulse (music) , radiochemistry , nuclear medicine , optics , physics , medicine , surgery , organic chemistry , detector , nuclear physics , thermodynamics
Purpose/objectives To provide an order of magnitude estimate of the minimum dose rate ( R min ) required by pulsed ultra‐high dose rate radiotherapy (FLASH RT) using dimensional analysis. Materials/methods In this study, we postulate that radiation‐induced transient hypoxia inside normal tissue cells during FLASH RT results in better normal tissue sparing over conventional dose rate radiotherapy. We divide the process of cell irradiation by an ultra‐short radiation pulse into three sequential phases: (a) The radiation pulse interacts with the normal tissue cells and produces radiation‐induced species. (b) The radiation‐induced species react with oxygen molecules and reduce the cell environmental oxygen concentration ( O 2 ). (c) Oxygen molecules, from nearest capillaries, diffuse slowly back into the resulted lowO 2regions. By balancing the radiation‐induced oxygen depletion in phase II and diffusion‐resultedO 2replenishment in phase III, we can estimate the maximum allowed pulse repetition interval to produce a pulse‐to‐pulse superimposedO 2reduction against the baselineO 2 . If we impose a threshold in radiosensitivity reduction to achieve clinically observable radiotherapy oxygen effect and combine the processes mentioned above, we could estimate the R min required for pulsed FLASH RT through dimensional analysis. Results The estimated R min required for pulsed FLASH RT is proportional to the product of the oxygen diffusion coefficient andO 2inside the cell, and inversely proportional to the product of the square of the oxygen diffusion distance and the drop of intracellularO 2per unit radiation dose. Under typical conditions, our estimation matches the order of magnitude with the dose rates observed in the recent FLASH RT experiments. Conclusions The R min introduced in this paper can be useful when designing a FLASH RT system. Additionally, our analysis of the chemical and physical processes may provide some insights into the FLASH RT mechanism.