# Impact of hypoxic versus oxic conditions on local tumor control after proton irradiation in a rat prostate carcinoma

## TL;DR

This study compares proton and photon radiation effectiveness in hypoxic versus normal oxygen conditions in rat prostate tumors.

## Contribution

The study reveals that protons are more effective than photons in hypoxic tumor conditions.

## Key findings

- Protons showed a 5% lower oxygen enhancement ratio compared to photons.
- Protons had a 6% higher relative biological effectiveness under hypoxia.
- Local tumor control was significantly lower under hypoxic conditions.

## Abstract

•Clamping reversibly inhibits tumor perfusion in rat prostate tumors.•Under hypoxic conditions, protons were slightly more effective than photons.•The oxygen enhancement ratio of protons was 5 % lower than for photons.•The relative biological effectiveness of protons was 6 % higher under hypoxia.

Clamping reversibly inhibits tumor perfusion in rat prostate tumors.

Under hypoxic conditions, protons were slightly more effective than photons.

The oxygen enhancement ratio of protons was 5 % lower than for photons.

The relative biological effectiveness of protons was 6 % higher under hypoxia.

Hypoxia in tumors significantly contributes to radiation resistance, often leading to poor treatment outcomes. In vitro studies demonstrated that proton irradiation not only exhibits an increased relative biological effectiveness (RBE), but also a lower oxygen enhancement ratio (OER). This study explored the impact of hypoxic and oxic conditions on local tumor control after proton irradiation in a rat prostate carcinoma.

Subcutaneously transplanted Dunning R3327-HI rat prostate carcinomas were irradiated with varying single doses of protons under oxic or hypoxic conditions. Hypoxia was induced by clamping the tumor-supplying vessels. The biological endpoint was local tumor control assessed 300 days after irradiation. Dose-response curves were determined and based on the doses required for 50 % tumor control probability (TCD50), the RBE and OER were calculated.

The TCD50 value was significantly higher under hypoxic than under oxic conditions (73.4 ± 1.9 Gy vs. 50.5 ± 1.6 Gy), resulting in an OER of 1.45 ± 0.06 for proton irradiation. Compared to photon irradiation, the RBE for protons was of 1.23 ± 0.07 under oxic and 1.30 ± 0.04 under hypoxic conditions.

Proton irradiations showed a 5 % reduction in OER compared to the previously measured photon value of 1.53 ± 0.08, suggesting a slightly higher effectiveness of protons in hypoxic tumors as compared to photons.