Immunohistochemical changes of the blood-brain barrier in rat spinal cord after heavy-ion irradiation

DCE-MRI detected vascular permeability changes in the rat spinal cord do not explain shorter latency times for paresis after carbon ions relative to photons

BACKGROUND AND PURPOSE

Radiation-induced myelopathy, an irreversible complication occurring after a long symptom-free latency time, is preceded by a fixed sequence of magnetic resonance- (MR-) visible morphological alterations. Vascular degradation is assumed the main reason for radiation-induced myelopathy. We used dynamic contrast-enhanced (DCE-) MRI to identify different vascular changes after photon and carbon ion irradiation, which precede or coincide with morphological changes.

MATERIALS AND METHODS

The cervical spinal cord of rats was irradiated with iso-effective photon or carbon (¹²C-)ion doses. Afterwards, animals underwent frequent DCE-MR imaging until they developed symptomatic radiation-induced myelopathy (paresis II). Measurements were performed at certain time points: 1 month, 2 months, 3 months, 4 months, and 6 months after irradiation, and when animals showed morphological (such as edema/syrinx/contrast agent (CA) accumulation) or neurological alterations (such as, paresis I, and paresis II). DCE-MRI data was analyzed using the extended Toft's model.

RESULTS

Fit quality improved with gradual disintegration of the blood spinal cord barrier (BSCB) towards paresis II. Vascular permeability increased three months after photon irradiation, and rapidly escalated after animals showed MR-visible morphological changes until paresis II. After ¹²C-ion irradiation, vascular permeability increased when animals showed morphological alterations and increased further until animals had paresis II. The volume transfer constant and the plasma volume showed no significant changes.

CONCLUSION

Only after photon irradiation, DCE-MRI provides a temporal advantage in detecting early physiological signs in radiation-induced myelopathy compared to morphological MRI. As a generally lower level of vascular permeability after ¹²C-ions led to an earlier development of paresis as compared to photons, we conclude that other mechanisms dominate the development of paresis II.

Longitudinal MRI study after carbon ion and photon irradiation: shorter latency time for myelopathy is not associated with differential morphological changes

BACKGROUND

Radiation-induced myelopathy is a severe and irreversible complication that occurs after a long symptom-free latency time if the spinal cord was exposed to a significant irradiation dose during tumor treatment. As carbon ions are increasingly investigated for tumor treatment in clinical trials, their effect on normal tissue needs further investigation to assure safety of patient treatments. Magnetic resonance imaging (MRI)-visible morphological alterations could serve as predictive markers for medicinal interventions to avoid severe side effects. Thus, MRI-visible morphological alterations in the rat spinal cord after high dose photon and carbon ion irradiation and their latency times were investigated.

METHODS

Rats whose spinal cords were irradiated with iso-effective high photon (n = 8) or carbon ion (n = 8) doses as well as sham-treated control animals (n = 6) underwent frequent MRI measurements until they developed radiation-induced myelopathy (paresis II). MR images were analyzed for morphological alterations and animals were regularly tested for neurological deficits. In addition, histological analysis was performed of animals suffering from paresis II compared to controls.

RESULTS

For both beam modalities, first morphological alterations occurred outside the spinal cord (bone marrow conversion, contrast agent accumulation in the musculature ventral and dorsal to the spinal cord) followed by morphological alterations inside the spinal cord (edema, syrinx, contrast agent accumulation) and eventually neurological alterations (paresis I and II). Latency times were significantly shorter after carbon ions as compared to photon irradiation.

CONCLUSIONS

Irradiation of the rat spinal cord with photon or carbon ion doses that lead to 100% myelopathy induced a comparable fixed sequence of MRI-visible morphological alterations and neurological distortions. However, at least in the animal model used in this study, the observed MRI-visible morphological alterations in the spinal cord are not suited as predictive markers to identify animals that will develop myelopathy as the time between MRI-visible alterations and the occurrence of myelopathy is too short to intervene with protective or mitigative drugs.

Late normal tissue response in the rat spinal cord after carbon ion irradiation

Background

The present work summarizes the research activities on radiation-induced late effects in the rat spinal cord carried out within the “clinical research group ion beam therapy” funded by the German Research Foundation (DFG, KFO 214).

Methods and materials

Dose–response curves for the endpoint radiation-induced myelopathy were determined at 6 different positions (LET 16–99 keV/μm) within a 6 cm spread-out Bragg peak using either 1, 2 or 6 fractions of carbon ions. Based on the tolerance dose TD₅₀ of carbon ions and photons, the relative biological effectiveness (RBE) was determined and compared with predictions of the local effect model (LEM I and IV). Within a longitudinal magnetic resonance imaging (MRI)-based study the temporal development of radiation-induced changes in the spinal cord was characterized. To test the protective potential of the ACE (angiotensin converting enzyme)-inhibitor ramipril™, an additional dose–response experiment was performed.

Results

The RBE-values increased with LET and the increase was found to be larger for smaller fractional doses. Benchmarking the RBE-values as predicted by LEM I and LEM IV with the measured data revealed that LEM IV is more accurate in the high-LET, while LEM I is more accurate in the low-LET region. Characterization of the temporal development of radiation-induced changes with MRI demonstrated a shorter latency time for carbon ions, reflected on the histological level by an increased vessel perforation after carbon ion as compared to photon irradiations. For the ACE-inhibitor ramipril™, a mitigative rather than protective effect was found.

Conclusions

This comprehensive study established a large and consistent RBE data base for late effects in the rat spinal cord after carbon ion irradiation which will be further extended in ongoing studies. Using MRI, an extensive characterization of the temporal development of radiation-induced alterations was obtained. The reduced latency time for carbon ions is expected to originate from a dynamic interaction of various complex pathological processes. A dominant observation after carbon ion irradiation was an increase in vessel perforation preferentially in the white matter. To enable a targeted pharmacological intervention more details of the molecular pathways, responsible for the development of radiation-induced myelopathy are required.

Neuropathology of delayed encephalopathy in cats induced by heavy-ion irradiation

AIM

The pathogenesis of delayed encephalopathy induced by heavy-ion irradiation was investigated experimentally in cats. The left cerebral hemispheres were irradiated with 15-40 Gy of heavy ions (carbon), and histologically and morphometrically examined 12 months later.

RESULTS

In the irradiated cerebral white matter the following occurred as the dose increased: astrocytic swelling, then the dilatation of small blood vessels with a fibrous thickening of the wall, and then loosening of the white matter with cavity formation and diffuse albumin deposition. Pathological features of these cavities suggested that they are induced by long-standing edema. Although the dilated vessels were arteries, veins, and capillaries, arteriovenous shunt and damage of the smooth muscle cells of the arterial media were absent. Changes of the cerebral cortex were scarce. Morphometrically, the irradiated cerebral white matter was swollen, and the capillary density tended to be reduced in the deep cortex and subcortical white matter, but this effect was not dose dependent.

CONCLUSION

Heavy-ion irradiation induces delayed encephalopathy in cats, preferentially involving the white matter. The cardinal pathogenesis was long-standing edema of the white matter due to vascular hyperpermeability, and the vascular dilatation seemed to be caused by a reduction in the vascular bed and/or hemoconcentration due to hyperpermeability.

Pathology of radiation myelopathy

Radiation myelopathy is principally a white matter injury of the spinal cord induced by ionizing radiation after a certain latent period. It involves myelinated fibers and blood vessels, and the lateral funiculi is most preferentially affected. Several factors, such as radiation dose, fractionation or linear energy transfer, modify its occurrence and severity. Although glial cells and vascular endothelium are proposed to be the main targets, and to play a role in the pathogenesis of radiation myelopathy, experimental researches support that radiation-induced vascular damage resulting in vascular hyperpermeability and venous exudation is a basic process. Effect of ionizing radiation on each cellular component of the central nervous system, their contribution to radiation myelopathy, mechanisms of selective permeability and remaining problems are discussed.