A light flash is thought to be due to Cherenkov radiation and previous studies have suggested that this is triggered by irradiation of the retina [6–8]. The current study suggested that the chiasmatic gland is a significant factor for light sensation, and that retina irradiation tends to be related to light sensation, although the results did not suggest significance. Also, the occipital lobe was irradiated in 2 patients without irradiation of the retina, optic nerve or chiasmatic gland.
The retina contains light-sensitive cells referred to as cones and rods, which detect color and light intensity, respectively. The light and color information caught by these cells is translated into digital signals that are transmitted to the optic nerve, through the lateral lemniscus, and finally to the visual cortex located in the occipital lobe, where it is used to form a combined light and color image. The visual information then splits into ventral and dorsal visual pathways. Thus, our results suggest that irradiation of the visual pathway, including the retina, optic nerve, chiasmatic gland and occipital lobe, leads to a light sensation.
Regarding odor, all patients except one who felt odor received irradiation to the nasal cavity. The odor of irradiation is often described as that of ozone [1–3, 7–9]. Ozone is formed in the vicinity of electron beams by collisions between high energy electrons and oxygen molecules. However, ozone is unstable and would only be detectable at low concentrations if produced adjacent to the olfactory receptor region [6]. An unpleasant odorous sensation may also be due to free radicals produced by radiation that remove electrons from stable molecules in the olfactory membrane or overlying mucous layer in the nasal cavity [6].
Our previous results suggested that patients who received radiotherapy for the body trunk could sense odor [6–9]. Kosugi et al. presented the case of a patient who experienced odor during tomotherapy after resection of the olfactory epithelium [5], and it is also well known that a foul odor is caused by uncinate epilepsy. The olfactory pathway starts from the olfactory epithelium, then axons of the olfactory nerve project through the olfactory bulb and accessory olfactory bulb to the piriform cortex and amygdala. The foul odor during uncinate epilepsy is initiated by a hook located in the limbic cortex, which is involved in olfactory function. Thus, it is possible that odor may be sensed by irradiation of the olfactory pathway, but this occurs less frequently than sensing of light due to irradiation of the visual pathway.
The presence of secondary electrons produced during radiotherapy may provide another explanation for sensing of light and odor in patients whose nasal passages are not irradiated. These electrons are X-rays that have higher energy and a greater spatial distribution than that of proton beams. Therefore, the electrons may act on the nasal cavity and retina. In fact, a previous study suggested that some patients who received radiotherapy to the trunk felt light flash and odor, and most of these patients received photon RT. Secondary electrons have a range of a few centimeters and it is possible that these electrons may hit the retina, olfactory membrane or olfactory bulb. We also note that radiation therapy is essential for treatment of pediatric tumors [10] and our results indicate that children are more likely to sense light flashes and odors. Therefore, it should be kept in mind that inclusion of the visual pathway or nasal cavity in the irradiation range in pediatric cases makes it likely that the patient will sense light and odor.
In conclusion, there are still unknown aspects of the abnormal sensations experienced during irradiation. However, our results indicate that light flash during radiotherapy is caused by irradiation of the visual pathway and that odor is caused by irradiation of the nasal cavity or olfactory bulb.