Optimal fractionation scheme for lymphocyte infiltration in glioblastoma multiforme radiotherapy

Abstract

Purpose: 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.

Materials and methods: 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.

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

Conclusions: 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.

Keywords: fractionation; glioma; infiltration; lymphocytes; radiotherapy.

Figure 1

Flow cytometry analysis of immune cells in glioblastoma (RG2 model) across different temporal schemes. See Supplementary Figure S1 for the gating strategy. * p< 0.05, ** p<0.005.

Figure 2

Flow cytometry analysis of immune cells in glioblastoma (F98 model) under different temporal schemes. See Supplementary Figure S1 for the gating strategy. * p<0.05, ** p<0.005, *** p<0.001 and **** p<0.0001.

Figure 3

Pie charts depicting the average proportions of immune cells in the RG2 model. Top: Proportions among all CD45-expressing immune cells. Bottom: Proportions among all CD3-expressing T cells. CD4+ T_conv, CD4+ T cells negative for CD25 (Treg); CD8+ Tconv, CD8+ T cells negative for CD103 (TRM).

Figure 4

Pie charts showing the average proportions of different immune cells in the F98 model. Top: Proportions among all CD45-expressing immune cells. Bottom: Proportions among all CD3-expressing T cells. CD4+ T_conv, CD4+ T cells negative for CD25 (Treg); CD8+ Tconv, CD8+ T cells negative for CD103 (TRM).