# Optimal fractionation scheme for lymphocyte infiltration in glioblastoma multiforme radiotherapy

## TL;DR

This study finds that a single high radiation dose is needed to boost immune cell infiltration in glioblastoma, but it may be too toxic with current methods.

## Contribution

The study identifies the optimal high-dose radiation scheme to enhance immune infiltration in glioblastoma.

## Key findings

- A single high dose (25 Gy) or extreme hypofractionation significantly increases immune cell infiltration in tumors.
- Conventional RT methods require very high and potentially toxic doses to prime the immune system in glioblastoma.
- FLASH-RT or minibeam RT could offer safer alternatives to achieve the required high doses.

## Abstract

Radioresistant and immunosuppressive tumors, such as glioblastoma multiforme (GBM), remain a challenge, as current clinical approaches—surgical resection and chemoradiation—do not yet provide effective treatment. Immunotherapy (IT) has emerged as a powerful tool in cancer; however, phase III clinical trials in GBM have yielded unsuccessful results, likely due to its critical dependence on preexisting antitumor immunity. Given its immunomodulatory potential, radiotherapy (RT) could serve as a tool to induce tumor inflammation and enhance responsiveness to IT. However, the optimal radiation configuration required to achieve the critical level of tumor inflammation for IT success remains elusive. This study assessed the most effective dose fractionation scheme for maximizing immune cell infiltration into tumors.

Two orthotopic rat glioma models with differing vascularization and immunogenicity were irradiated with three dose fractionation schemes. Tumor immune cell populations were analyzed by flow cytometry.

A single high dose (25 Gy) or extreme hypofractionation is required to elicit a significant immune infiltration in tumors.

Using RT as an immune primer in GBM would require very high and toxic doses with conventional RT methods. While 25 Gy is used in conventional stereotactic radiosurgery, such a high dose is typically limited to small brain volumes. Novel approaches, such as FLASH-RT or minibeam RT, offer alternatives to mitigate toxicity while achieving the required doses.