Biology Contribution
Better Efficacy of Synchrotron Spatially Microfractionated Radiation Therapy Than Uniform Radiation Therapy on Glioma

https://doi.org/10.1016/j.ijrobp.2016.03.040Get rights and content

Purpose

Synchrotron microbeam radiation therapy (MRT) is based on the spatial fractionation of the incident, highly focused synchrotron beam into arrays of parallel microbeams, typically a few tens of microns wide and depositing several hundred grays. This irradiation modality was shown to have a high therapeutic impact on tumors, especially in intracranial locations. However, mechanisms responsible for such a property are not fully understood.

Methods and Materials

Thanks to recent progress in dosimetry, we compared the effect of MRT and synchrotron broad beam (BB) radiation therapy delivered at comparable doses (equivalent to MRT valley dose) on tumor growth control and on classical radiobiological functions by histologic evaluation and/or transcriptomic analysis.

Results

MRT significantly improved survival of rats bearing 9L intracranial glioma compared with BB radiation therapy delivered at a comparable dose (P<.001); the efficacy of MRT and BB radiation therapy was similar when the MRT dose was half that of BB. The greater efficacy of MRT was not correlated with a difference in cell proliferation (Mki67 and proliferating cell nuclear antigen) or in transcriptomic stimulation of angiogenesis (vascular endothelial growth factor A or tyrosine kinase with immunoglobulin-like and epidermal growth factor-like domains 2) but was correlated with a higher cell death rate (factor for apoptosis signals) and higher recruitment of macrophages (tyrosine kinase with immunoglobulin-like and epidermal growth factor-like domains 1 and CD68 transcripts) a few days after MRT.

Conclusions

These results show the superiority of MRT over BB radiation therapy when applied at comparable doses, suggesting that spatial fractionation is responsible for a specific and particularly efficient tissue response. The higher induction of cell death and immune cell activation in brain tumors treated by MRT may be involved in such responses.

Introduction

Although many cancers are currently treated by radiation therapy, scientists and clinicians continually seek more effective, safer means to deliver radiation therapy to the tumor targets without damaging surrounding healthy tissue. Synchrotron microbeam radiation therapy (MRT), a preclinical alternative radiation therapy, is based on the spatial fractionation of the incident synchrotron beam into arrays of parallel microbeams, which are typically a few tens of microns wide, separated by a few hundreds of microns [which has been reviewed previously (1)]. This unique use of synchrotron-generated x-ray beams is characterized by kilovoltage energy, high flux, and minimal divergence. It allows the targeted deposition of a very high dose, known as the peak dose (several hectograys), in tumors by the microbeams. In contrast, the valley area, that is, the tissue area between the microbeam paths, receives a lower dose (a few grays) known as the valley dose.

Although MRT has been shown to slow or even stop the growth of several tumor types 2, 3, 4, 5, 6, 7, 8, a surprisingly high tolerance has been observed for normal tissues 4, 5, 9, 10, 11, 12, 13, 14, 15. Several mechanisms have been suggested, including cytotoxic effects on tumor cells (16) and endothelial cells; a decrease in blood vessel number 4, 17, leading to decreased perfusion (4) and tumor hypoxia (9) or, on the contrary, normalization of the vasculature (17); modulation of the immune system 18, 19, 20; and communication between lethally irradiated cells in the microbeam path and slightly damaged cells located between microbeams (21). However, the specific contribution of microbeam components has never been clearly demonstrated in vivo.

In this article, we present an assessment of the efficacy of MRT by comparing it with synchrotron broad beam (BB) radiation therapy at a dose equivalent to the dose deposited in the MRT valley area. The complex geometry of MRT, with its alternating high and low doses deposited in micrometric volumes, makes this comparison very difficult. Recent advances in theoretical and experimental dosimetry have made it possible to accurately determine the valley dose for this study. In the protocol used in this study, rats bearing intracranial 9L brain tumors were treated by 3 irradiation modalities: synchrotron x-ray irradiation in the BB mode or MRT with a valley dose corresponding to the full dose deposited by the BB or to half of that dose. The efficacy of those treatments was determined based on animal survival and tumor growth. The impact of BB irradiation and MRT at equivalent valley doses on tumor cell proliferation, the mitotic process, and inflammatory responses in the tumor bed were also studied by histology and tracking of transcriptomic modulation over a period of 15 days.

Section snippets

Methods

Procedures related to animal care conformed to the French government guidelines, under license numbers 380325 and B3818510002. Rats were anesthetized with isoflurane (5% in air) before intraperitoneal injection of a combination of xylazine (64.5 mg/kg) and ketamine (5.4 mg/kg) for tumor implantation or isoflurane, 2.5%, for magnetic resonance imaging examination and irradiation.

Monte Carlo simulations and dose calculations

The dose rate measured in reference conditions amounted to 72 Gy ⋅ s−1 ⋅ mA−1. According to Monte Carlo simulations, the relationship between the peak dose (Dpeak) and the reference dose (Dreference) can be expressed as follows: the peak dose at 7MM of depth in rat brain, that is, Dpeak Rat(7 mm) = Dreference (water at 2 cm) × 0.88. The valley dose (Dvalley) can be calculated using the PVDR, that is, Dvalley Rat(7 mm) = Dpeak Rat(7 mm)/PVDR. Thus, for a 400-Gy peak dose, the Dreference was

Discussion

The choice of the dose applied with spatially nonfractionated radiation therapy remains critical for reliable comparison with MRT. The complex irradiation geometry is very challenging for the accurate determination of the dose received by the tissue between the microbeams (valley dose; detailed in Appendix E1; available online at www.redjournal.org). Thanks to the latest advances, we can present a comparison of spatially fractionated radiation therapy (400 Gy) to BB radiation therapy deposing

Conclusion

Thanks to recent advances in dosimetry, MRT reveals clearly better efficacy in tumor control and animal lifespan than comparable uniform irradiation, showing that spatial fractionation significantly contributes to the therapeutic properties of MRT. This efficacy is not correlated with a large difference in cell proliferation or in stimulation of angiogenesis. However, the higher induction of cell death and recruitment of macrophages a few days after MRT pave the way for more extensive

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    Laurent Pelletier and Géraldine Le Duc contributed equally.

    This project was partially funded by the French National Cancer Institute (Institut National du Cancer, Bioresys Project No. 200187) and by La Ligue Nationale Contre le Cancer–Comité de Haute Savoie. The Grenoble magnetic resonance imaging facility IRMaGe was partly funded by the French program Investissement d'Avenir run by the Agence Nationale pour la Recherche (Infrastructure d'Avenir en Biologie Santé grant No. ANR-11-INBS-0006).

    Conflict of interest: none.

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