The FLASH enigma

TL;DR

This paper explores the physical mechanisms behind FLASH radiation therapy, highlighting how tissue structural differences influence responses to ultra high dose rates, leading to effective tumor targeting while sparing normal tissues.

Contribution

It introduces a physical model explaining the differential responses of normal and tumor tissues based on their structural morphologies and charge densities during FLASH radiation therapy.

Findings

Normal tissues form electron-hole liquids reducing free radical generation.

Tumor tissues facilitate free radical formation due to disordered structures.

The paper describes thresholds for doses and dose rates in FLASH therapy.

Abstract

We consider physics behind the FLASH modality of cancer radiation treatment where extremely short treatment times are achieved with ultra high dose rates maintaining the conventional antitumor effectiveness and yet substantially reducing damage to normal tissues (sparing effect). The difference in responses between normal and tumor tissues is attributed here to different recombination rates related to their structure morphologies: ordered in normal vs disordered in the tumor tissues. Correspondingly different are their charge densities under ionizing radiation. In normal tissues it is high enough to form electron-hole liquid (EHL). Because of low EHL diffusivities, the chemical reaction and generation of free radicals are suppressed; hence, sparing effect. To the contrary, a disordered tumor tissue renders efficient energy relaxation channels forming antitumor free radicals. We describe the FLASH thresholds for doses and dose rates.