The effects of ionizing radiation on the nervous system

Extraretinal induced visual sensations during IMRT of the brain

Background

We observed visual sensations (VSs) in patients undergoing intensity modulated radiotherapy (IMRT) of the brain without the beam passing through ocular structures. We analyzed this phenomenon especially with regards to reproducibility, and origin.

Methods and Findings

Analyzed were ten consecutive patients (aged 41-71 years) with glioblastoma multiforme who received pulsed IMRT (total dose 60Gy) with helical tomotherapy (TT). A megavolt—CT (MVCT) was performed daily before treatment. VSs were reported and recorded using a triggered event recorder. The frequency of VSs was calculated and VSs were correlated with beam direction and couch position. Subjective patient perception was plotted on an 8x8 visual field (VF) matrix. Distance to the orbital roof (OR) from the first beam causing a VS was calculated from the Dicom radiation therapy data and MVCT data. During 175 treatment sessions (average 17.5 per patient) 5959 VSs were recorded and analyzed. VSs occurred only during the treatment session not during the MVCTs. Plotting events over time revealed patient-specific patterns. The average cranio-caudad extension of VS-inducing area was 63.4mm (range 43.24-92.1mm). The maximum distance between the first VS and the OR was 56.1mm so that direct interaction with the retina is unlikely. Data on subjective visual perception showed that VSs occurred mainly in the upper right and left quadrants of the VF. Within the visual pathways the highest probability for origin of VSs was seen in the optic chiasm and the optic tract (22%).

Conclusions

There is clear evidence that interaction of photon irradiation with neuronal structures distant from the eye can lead to VSs.

The radiation phosphene

A low flux of X-rays below the Cerenkov energy threshold generates a phosphene by direct action on the retina without a fluorescence in the ocular media. X-rays above the Cerenkov threshold can generate only a faint luminescence in the lens and cornea. From experimental work on humans in 1905 with unencapsulated radium, it is known that approximately 80% of the intensity of the radium phosphene is from the beta-ray component and approximately 20% from the gamma-ray. From calculations of the photon yield due to Cerenkov radiation in the eye from radium, one finds intensities of approximately 90% and approximately 10% for beta and gamma-rays, respectively, if only Cerenkov radiation is considered. Thus, one may conclude that the dominant mechanism of the radium phosphene is Cerenkov radiation, primarily from electrons and not fluorescence as previously speculated. The term "radium phosphene" is a misnomer and should be subsumed along with the X-ray phosphene and particle induced visual sensations under the name "radiation phosphene".

Observations of visual sensations produced by Cerenkov radiation from high-energy electrons

Ten cancer patients whose eyes were therapeutically irradiated with 6-18 MeV electrons reported visual light sensations. Nine reported seeing blue light and one reported seeing white light. Controls reported seeing no light. Additionally, tests with patients ruled out the x-ray contamination of the electron beam as being important. The photon yield due to Cerenkov radiation produced by radium and its daughters for both electrons and gamma rays was calculated; it was found to account for a turn-of-the-century human observation of the "radium" phosphene. We conclude that the dominant mechanism of this phosphene is Cerenkov radiation, primarily from betas. From our own patient data, based on the color seen and the Cerenkov production rates, we conclude that the dominant mechanism is Cerenkov radiation and that high-energy electrons are an example of particle induced visual sensations.

Radiation-induced cerebral lesions in childhood

Post-irradiation cerebral pathologies may appear in various forms from localized radiation necrosis to a plurifocal type or from local to diffuse vasculopathies. Contrary to the current prevalent opinion, these lesions are not rare in children since young nerve tissue is particularly sensitive to ionizing radiation. Given the seriousness of some of these lesions, the authors recommend careful evaluation of the risk involved in relation to the real necessity of administering irradiation therapy in childhood.

Long-lasting cerebral functional changes following moderate dose x-radiation treatment to the scalp in childhood: an electroencephalographic power spectral study

EEG tracings were compared in 44 young adults who received scalp x-radiation treatment for tinea capitis during childhood and 59 non-irradiated control subjects. The irradiated subjects were exposed, over 20 years previously, to a mean dose of 130 rads to the brain. Visual analysis of the EEG revealed an insignificant excess of abnormalities among the irradiated subjects compared to the controls. Power spectral density function analysis showed increased power values among the irradiated subjects, particularly in the beta wave frequencies. This finding provides further evidence for suspecting that x-irradiation during brain maturation may cause long-lasting damage to the brain tissue.

The effects of single dose X-irradiation on the guinea-pig spinal cord

Lumbar or cervical regions of the guinea-pig spinal cord were irradiated with a single dose of 250kV X-rays. The latency for paralysis, whether of hind- or forelimbs, and the histopathology of the radiation-induced cord lesions depended critically on the radiation dose. There were definite but only minor differences between the reactions of lumbar and cervical cord to the same radiation dose. After 30 or 40 Gy there was white matter necrosis but after 20 Gy widespread demyelination associated with vacuolar spaces occurred. After irradiation of the lumbar cord with 30-40 Gy, the lesions in the guinea-pig differed from those reported in the rat. White-matter necrosis in the guinea-pig cord was only occasionally associated with spinal nerve root necrosis, whereas in the rat, nerve-root necrosis with sparing of the white matter was the main lesion. After 20 Gy to the cervical or lumbar cord the guinea-pig showed widespread demyelination and vacuolation whereas in rats vascular lesions were the main result.

Late functional changes in the vasculature of the rat brain after local X-irradiation

The brains of young adult rats were irradiated with doses of 500-4000 rad. At intervals of 3-15 months after irradiation regional changes in the functional vasculature were investigated using the iodoantipyrine extraction technique. Modifications in vascular function were restricted to animals locally irradiated with doses of 2000 and 3000 rad. The first change was observed three months after irradiation and was characterized by a reduction in antipyrine extraction in the mid-brain and brain stem of animals irradiated with 2000 rad. At six and nine months after exposure to both 2000 and 3000 rad significant increases in antipyrine extraction were found in the four brain regions examined, although the effect was greatest in the mid-brain. These results are compared and contrasted with functional changes reported in other normal tissues; the link between these functional changes in the brain vasculature and the appearance of gross vascular lesions after a latent period of one year is discussed. It is suggested that the increase in iodoantipyrine extraction represents a regulatory reaction by the vasculature of the brain to tissue hypoxia and that focal vascular lesions in the brain occur as a consequence of the failure of this reaction to hypoxia.

Delayed myeloradiculopathy produced by spinal X-irradiation in the rat

Rats were subjected to 3,500 r of X-irradiation in a single dose while breathing oxygen at 1 ATM pressure. Comparison was made between the delayed effects of irradiating thoracic, lumbar, and the cauda equina fields. The lumbar field involved the alpha-motoneurons and spinal roots supplying the sciatic nerve, while the cauda equina field involved these spinal roots but spared the alpha-motoneurons in the spinal cord. Thoracic irradiation produced paraplegia after an interval of 127-150 days. In the irradiated zone, the spinal cord was severely damaged, but the thoracic spinal roots were spared. Lumbar irradiation produced paraplegia after an interval of 83-211 days. In the irradiated zone, the alpha-motoneurons were largely spared, the spinal cord showed mild to moderate white matter damage, but the most severe damage was of the lumbosacral spinal roots. The posterior roots were more affected than the anterior. In longer interval cases the degeneration of the roots appeared to be due to focal devitalization. Evidence is advanced that root degeneration had been progressing for at least 4 weeks before the onset of paraplegia. In the cauda equina series the lumbosacral spinal root changes were similar to those in the lumbar series. This study indicates that different levels of the neuraxis have different degrees of susceptibility to X-irradiation. The thoracic cord appears more susceptible than the lumbosacral; the lumbosacral roots appear more susceptible than the thoracic; the posterior roots are more susceptible than the anterior. These findings may have relevance to the study of radiation damage in man, even though the dose schedule used in this experimental study differs greatly from that used for radiotherapy.