Abstract Body:

Background

Stereotactic radiosurgery (SRS) provides focalized delivery of high radiation doses to the target lesion in the brain, though inflammation and injury to the surrounding healthy tissue can occur^1,2. When uncontrolled, neuroinflammatory response can evolve causing cognitive decline and eventually radionecrosis^3,4. We hypothesized that neuroinflammation expands progressively with time after SRS and its degree depends on the absorbed dose⁵. Our aim is to detect and assess the expression of neuroinflammatory biomarker translocator protein (TSPO) at early time points in a rat model of SRS using PET imaging⁶.

Methods

The pilot study was carried out on 2-month-old Fisher #344 male rats: 5 animals underwent SRS and 3 animals remained as controls. SRS was performed using Leksell Gamma-knife with 4-mm collimators⁷ and consisted of 80 Gy at 100% in a single fraction targeted on the right prethalamus (bregma -2.0 mm). PET brain imaging was performed 6 and 12 weeks after SRS using two TSPO-specific radiotracers, [¹⁸F]Fluoromethyl-PBR28-d2⁸ and [¹⁸F]DPA-714⁹, with an injected dose of 30 MBq. Dynamic PET acquisitions were conducted on a LabPET2S small-animal scanner with ultra-high resolution (< 1 mm³)¹⁰. The reconstructed PET images were processed using a MATLAB pipeline and co-registered with irradiation dose maps and an open-source MRI-derived neuroanatomical atlas for segmentation of anatomical structures¹¹. Time-activity curves (TACs) were generated from the intersection of 18 anatomical regions with dose distribution around the target (0-10 Gy, 10-25 Gy, 25-45 Gy, 45-80 Gy) to analyze the data as a function of the absorbed dose and brain region (e.g., thalamus * 45-80 Gy). TACs were processed in PMOD using Logan’s Reference Tissue Model with cerebellar gray matter as a pseudoreference region¹². For all volumes of interest, binding potential (BP_ND) and distribution volume ratio (DVR= BP_ND +1) were estimated. Statistical analysis was performed in R 4.2.1.

Results

Compared to controls, 6 weeks after irradiation the accumulation of [¹⁸F]DPA-714 was significantly higher only in regions that received 45-80 Gy (DVR 0.79±0.18 vs 0.65±0.13, p<0.001), and the uptake of [¹⁸F]fmPBR28-d2 was higher in regions irradiated to >10 Gy (DVR 0.67±0.11 vs 0.52±0.15, p<0.0001). At 12 weeks after SRS there was a significant difference in brain uptake of both radiotracers between control and irradiated animals for all isodose levels ([¹⁸F]DPA-714 DVR 1.08±0.28 vs 0.78±0.17, p<0.0001; [¹⁸F]fmPBR28-d2 DVR 0.92±0.26 vs 0.64±0.18, p<0.0001). This time-dependent impact was confirmed by results of mixed effect models in the irradiated group, which revealed an interaction between dose and time-point for both radiotracers ([¹⁸F]DPA-714 F=24.51, p<0.001; [¹⁸F]fmPBR28-d2 F=23.64, p<0.001) with the main effect of radiation dose on DVR value 12 weeks after SRS. Between the two time-points, there was an increase of radiotracers uptake for each dose level with higher accumulation in regions that received 45-80 Gy ([¹⁸F]DPA-714 DVR 0.79±0.18 vs 1.38±0.28, p<0.0001; [¹⁸F]fmPBR28-d2 DVR 0.67±0.10 vs 1.17±0.30, p<0.0001). The most prominent changes in radiotracer uptake were revealed in the right thalamus as this region was targeted during irradiation. Visual assessment of PET images also revealed unequal uptake of both radiotracers in the irradiated zone which can be explained by their different affinity¹³.

Conclusion

According to our pilot results in a rat model of SRS, neuroinflammation can be detected by TSPO imaging as early as 6 weeks after irradiation. Neuroinflammation evolves over time progressively in a dose-dependent manner. Imaging with [¹⁸F]fmPBR28-d2 tends to detect more silent changes of radiotracer uptake than [¹⁸F]DPA-714, though caution should be paid to known confounding variables as vascular binding¹⁴ and SRS-induced BBB permeability¹⁵. It is the first study to evaluate SRS-induced neuroinflammation in a rat model by PET imaging with assessment of radiotracer uptake as a function of the absorbed dose and brain region.

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