Room: Exhibit Hall | Forum 3
Purpose: To perform a dimensional analysis of the minimum dose rate required by ultra-high dose rate radiotherapy (FLASH RT).
Methods: We assume radiation-induced hypoxia inside normal tissue cells causes FLASH RT to spare normal tissues much better than conventional dose rate radiotherapy. We divide cell irradiation by a radiation pulse into three roughly sequential phases: 1) First radiation interacts with the tissue and produce species whose concentration is proportional to the radiation dose per pulse. 2) Then radiation-induced species react with intra-cellular oxygen molecules and reduce oxygen concentration ([Oâ‚‚]) inside cells. 3) Finally, surrounding oxygen molecules diffuse slowly into the lower [Oâ‚‚] regions hit by radiation ionization tracks. By balancing radiation-induced oxygen depletion in phase two and diffusion-resulted [Oâ‚‚] enhancement in phase three, we can estimate the maximum time interval between radiation pulses. We further require that at least a 10% drop in intracellular [Oâ‚‚] is needed to cause an observable cell radiosensitivity change, which ultimately leads to a minimum dose per pulse requirement. Combining all factors mentioned above, we estimate the minimum dose rate required by FLASH RT through dimensional analysis.
Results: The estimated minimum dose rate required by FLASH RT is directly proportional to the product of the oxygen diffusion coefficient and [Oâ‚‚] inside the cell and inversely proportional to the product of the square of the width of the radiation-induced track and the drop in a cell [Oâ‚‚] per unit radiation dose. Under typical conditions, our minimum dose rate estimation (58 Gy/s) is close to the lowest dose rate (>40 Gy/s) used in recent FLASH RT experiments.
Conclusion: Our dimensional analysis gives us insights into the FLASH RT process. The estimation of the required minimum dose rate can be useful when designing a FLASH RT system.