Reduced corneal toxicity following whole brain irradiation in mice

The emergence of focal techniques such as stereotactic radiosurgery has led to a more selective use of whole brain radiotherapy (WBRT), yet it remains an important modality for treating diffuse or multiple intracranial malignancies. During WBRT, conventional dose rate radiotherapy (CONV-RT) inadvertently exposes ocular structures to radiation, potentially leading to vision-threatening complications. FLASH radiotherapy (FLASH-RT) delivers ultra-high dose rates in milliseconds, compared to CONV-RT where the dose delivery is over several minutes. This study employs 9 MeV electron beams to investigate whether FLASH-RT reduces radiation-induced corneal collagen damage relative to CONV-RT, using second-harmonic generation (SHG) imaging in a murine WBRT model. We hypothesized that FLASH-RT preferentially protects anterior ocular structures from incidental radiation damage compared to CONV-RT, potentially through preservation of extracellular matrix integrity and reduced damage to long-lived structural proteins.

C57BL/6 J wild type mice were divided into three experimental cohorts: untreated controls, FLASH-RT and CONV-RT. Within the radiotherapy cohorts, subgroups received a single whole-brain fraction of 10 Gy, 15 Gy or 20 Gy delivered at an average dose rate of 240 Gy/s for FLASH-RT or 0.1 Gy/s for CONV-RT. Corneal alterations were evaluated at acute (4 days) and chronic (40 days) post-irradiation timepoints. SHG microscopy quantified collagen organization using forward-to-backward (F/B) signal ratios and corneal thickness as markers of radiation-induced damage.

In the CONV-RT cohorts, we found significantly decreased F/B ratios compared to controls (p < 0.05), indicating substantial collagen disorganization, and increased corneal thickness indicative of radiation-induced tissue edema. Conversely, FLASH-RT was associated with higher F/B ratios and reduced corneal thickening compared with CONV-RT, indicating preservation of corneal extracellular matrix organization across both timepoints. However, the tissue sparing effect on corneal thickness was not sustained at the high single fraction threshold of 20 Gy.

FLASH-RT preserves corneal collagen architecture compared to CONV-RT during whole brain irradiation, demonstrating significant potential for minimizing ocular toxicity in WBRT. These findings support clinical investigation of FLASH-RT as a tissue-sparing modality for brain cancer treatment.