Flashes of light-radiation therapy to the brain

Light flashes and other sensory illusions perceived in space travel and on ground, including proton and heavy ion therapies

Most of the astronauts experience visual illusions, apparent flashes of light (LF) in absence of light. The first reported observation of this phenomenon was in July 1969 by Buzz Aldrin, in the debriefing following the Apollo 11 mission. Several ground-based experiments in the 1970s tried to clarify the mechanisms behind these light flashes and to evaluate possible related risks. These works were supported by dedicated experiments in space on the following Apollo flights and in Low Earth Orbit (LEO). It was soon demonstrated that the LF could be caused by charged particles (present in the space radiation) traveling through the eye, and, possibly, some other visual cortical areas. In the 1990s the interest in these phenomena increased again and additional experiments in Low Earth Orbit and others ground-based were started. Recently patients undergoing proton and heavy ion therapy for eye or head and neck tumors have reported the perception of light flashes, opening a new channel to investigate these phenomena. In this paper the many LF studies will be reviewed, presenting an historical and scientific perspective consistent with the combined set of observations, offering a single comprehensive summary aimed to provide further insights on these phenomena. While the light flashes appear not to be a risk by themselves, they might provide information on the amount of radiation induced radicals in the astronauts' eyes. Understanding their generation mechanisms might also support radiation countermeasures development. However, even given the substantial progress outlined in this paper, many questions related to their generation are still under debate, so additional studies are suggested. Finally, it is also conceivable that further LF investigations could provide evidence about the possible interaction of single particles in space with brain function, impacting with the crew ability to optimally perform a mission.

Olfactory Sensations During Radiation Sessions: A Review

PURPOSE

During radiation therapy sessions, certain patients may perceive smells, particularly in younger populations. The sense of smell generally does not interrupt treatment; hence, it has attracted limited attention from oncologists worldwide. However, in certain cases, the prevention of smell perception may be necessary when treating pediatric patients. In this regard, this study reviewed previous studies pertaining to this unusual phenomenon of smell perception during radiation sessions.

METHODS AND MATERIALS

A PubMed search using the terms "radi*" AND ("olfaction" OR "odor" OR "smell" OR "phantosmia") was performed, and 9 articles related to this sensation were identified. Furthermore, through the references in these 9 articles, 3 additional related studies were identified.

RESULTS

Perception of smell during radiation treatment sessions was first reported in 1989. This phenomenon entails the perception of a pungent, repulsive, and unpleasant smell and is generally only observed in the presence of radiation beams. Occasionally, these smell perceptions are also accompanied by visual and gustatory sensations. Patients complain about this sensation more frequently when their olfactory regions are irradiated. Nevertheless, some patients have also reported experiencing this sensation when these areas are not included in the radiation field. Although the intensity of this sensation is low, intervention is required to prevent it. The cause of this perception remains unclear and requires further investigation.

CONCLUSIONS

Conclusions: During radiotherapy sessions, certain patients may perceive smells. The cause of this perception remains unclear and requires further investigation.

Abnormal sensation during total body irradiation: a prospective observational study

Light flash and odor during radiotherapy are well-known phenomena. Two prospective observational studies have indicated that 55% of patients observed a light flash during irradiation of the retina and 27% of patients sensed an odor during radiotherapy for the nasal cavity. A prospective observational study was performed in all patients at our hospital who received total body irradiation (TBI) between January 2019 to October 2021. Light flash and odor during TBI were examined using the same method as that used in previous studies. A total of 32 patients received TBI during the study period. The patients had a median age of 41 (18-60) years, and included 20 males and 12 females. A survey checklist showed that 14 patients (44%) sensed light and 14 patients (44%) sensed odor during TBI,. The color of the light during irradiation was yellow in six cases, white in four cases, and blue in four cases. The intensity of the light was 2-5 (median 3, 1 is very weak, 5 is very strong) and the time over which the light flash was felt was 4-60 s (median 10 s). Two patients each sensed smells of plastic, ozone and bleach, and others sensed one smell each. The intensity of the odor was 1-4 (median 3, 1 is very weak, 5 is very strong) and the time over which the odor was sensed was 1-25 s (median 3 s). We conclude that light flashes and odors are each sensed by 44% of patients during TBI. Various types of light flashes and odors were reported in this study.

Light flash and odor during proton beam therapy for pediatric patients: a prospective observational study

Light flash and odor during radiation therapy are well-known phenomena, but the details are poorly understood, particularly in pediatric patients. Therefore, we conducted a prospective observational study of these events in pediatric patients (age ≤20 years old) who received radiotherapy at our center from January 2019 to November 2021. Light flash and odor were evaluated using a patient-reported checklist including the presence, strength, and duration of the phenomenon, and color of light or type of odor. 53 patients who received proton therapy (n=47) and photon radiotherapy (n=6) were enrolled in this study. The median age of the patients was 10, ranged from 5 to 20. The patients who was able to see the light flash was 4, and all of them received retina irradiation. This was equivalent to 57% of the patients who received radiotherapy to retina (n=7). The light was bright and colored mainly blue and purple, which seemed to be consistent with Cherenkov light. Odor was sensed by 9 (17%) patients, and seven patients of the 9 received nasal cavity irradiation. This was equivalent to 41% of the patients who received nasal cavity irradiation (n=17). Other 2 patients received proton therapy to brain tumor. The odors were mainly described as plastic, burnt and disinfectant, which may be caused by ozone generated during irradiation. These data suggest that pediatric patients with retinal and nasal cavity irradiation frequently sense light flashes or odor. So adequate care is necessary so that these patients are not worried about this phenomenon.

Prediction of phosphenes occurrence in patients undergoing photon irradiation to ocular structures due to brain tumor

BACKGROUND AND OBJECTIVE

Phosphenes are visual light phenomena that are experienced when there is no apparent light that stimulates the eye. In oncology, phosphenes are also present during radiation therapy for patients with tumors of the central nervous system, eyes, head and neck. Due to the discomfort of patients treated with irradiation to the head regions, research is conducted to determine whether the dose to the ocular structures is predictive for the occurrence of phosphenes. The objective was to demonstrate the relationship between the doses of the retina and vitreous humour with the appearance of phosphenes.

MATERIAL AND METHOD

A descriptive study was carried out in a prospective cohort in 25 patients older than 18 years, with malignant tumours located at the level of the brain, both of primary and secondary origin, subjected to irradiation in 6 MV linear accelerators, during February 2020 to January 2021. As independent variables: Retinal dose and vitreous humour dose, and as dependent variables: Light flashes and stable light. Logistic regression analysis was used for prediction, using the SPSS statistical program (version 26.0).

RESULT

A final date of 380 external radiotherapy treatments. The presence of any of the events in a prevalence of 58.7% of the total of fractions. The distribution for the presence of both events, flash of light and stable light, was 69.1%, 20.6% and 10.3% respectively. In the logistic regression analysis, for the light flare, only the dose factor in vitreous was significant (OR: 1.74, IC [1.059-2.419] p: 0.001). For stable light, the dose in the retina (OR: 1.73, IC [1.121-2.341] p: 0.005), and dose in the vitreous humor (OR: 1.82, IC [1.335-2.315] p: 0.003).

CONCLUSIONS

There is a predictive relationship between the doses of irradiation of the retina and vitreous humour, for the generation of phosphenes. These results help radiotherapy centres take these anatomical structures into account to reduce the presence of phosphenes in patients. Likewise, it would help to reduce phosphenes, keeping the bunker area illuminated during the treatment.

Light flashes during proton and photon radiotherapy: A multicenter prospective observational study

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A total of 88 patients experienced light flashes among 416 who received photon radiotherapy and 205 who received proton beam therapy in our prospective study.

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The frequency of light flashes during radiotherapy was associated with the retina dose and younger age, and were commonly blue or white light for brain or head and neck irradiation.

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Light flashes were seen by only 35/524 patients (6.7%) without retinal irradiation, but by 13/33 (39.4%) and 41/64 (64.1%) with retinal maximum isodose lines of 10–50% and 60–100%, respectively.

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Totals of 52, 15, 15, 9, 16 and 8 patients sensed light of blue, purple, yellow, red, white and other colors, respectively.

Background

Patients who receive radiation therapy sometimes complain of a light flash during irradiation. The details of the characteristics of this light have not been described.

Purpose

To evaluate light flashes during photon and proton radiotherapy.

Methods and Materials

A prospective observational study was performed in all adult patients (≥20 years old) who received photon and proton therapy at two centers between January 2019 and August 2020, except for patients who could not communicate and those with visual abnormality. Evaluations were obtained for the presence or absence of light flashes, light darkness (7 levels), light intensity (5 levels), frequency, light movement, light flashing, and time seeing the light, using a weekly checklist.

Results

A total of 650 courses were examined for 621 patients, of whom 416 received photon radiotherapy and 205 received proton beam therapy. The checklist indicated that 88 patients (16.1%) sensed light during photon or proton radiotherapy. In multivariate logistic regression analysis, the factors that were significantly associated with a light flash were a higher retina dose and younger age (p < 0.001). Light flashes were seen by only 35/524 patients (6.7%) for whom the retina was not irradiated, but by 13/33 (39.4%) and 41/64 (64.1%) with maximum isodose lines for the retina of 10–50% and 60–100%, respectively. The numbers of patients who sensed blue, purple, yellow, red, white and other colors were 52, 15, 15, 9, 16 and 8, respectively (multiple selections possible). Light movement was observed by 52 patients (59%). The location of the light was defined as near, far, and middle by 70, 13, and 5 patients, respectively. The median time the light was seen was 10 s.

Conclusions

Many patients sense light flashes during radiotherapy. The retina dose and a younger age were significantly associated with the frequency of light flashes.

Initial Clinical Experience of Cherenkov Imaging in External Beam Radiation Therapy Identifies Opportunities to Improve Treatment Delivery

PURPOSE

The value of Cherenkov imaging as an on-patient, real-time, treatment delivery verification system was examined in a 64-patient cohort during routine radiation treatments in a single-center study.

METHODS AND MATERIALS

Cherenkov cameras were mounted in treatment rooms and used to image patients during their standard radiation therapy regimen for various sites, predominantly for whole breast and total skin electron therapy. For most patients, multiple fractions were imaged, with some involving bolus or scintillators on the skin. Measures of repeatability were calculated with a mean distance to conformity (MDC) for breast irradiation images.

RESULTS

In breast treatments, Cherenkov images identified fractions when treatment delivery resulted in dose on the contralateral breast, the arm, or the chin and found nonideal bolus positioning. In sarcoma treatments, safe positioning of the contralateral leg was monitored. For all 199 imaged breast treatment fields, the interfraction MDC was within 7 mm compared with the first day of treatment (with only 7.5% of treatments exceeding 3 mm), and all but 1 fell within 7 mm relative to the treatment plan. The value of imaging dose through clear bolus or quantifying surface dose with scintillator dots was examined. Cherenkov imaging also was able to assess field match lines in cerebral-spinal and breast irradiation with nodes. Treatment imaging of other anatomic sites confirmed the value of surface dose imaging more broadly.

CONCLUSIONS

Daily radiation therapy can be imaged routinely via Cherenkov emissions. Both the real-time images and the posttreatment, cumulative images provide surrogate maps of surface dose delivery that can be used for incident discovery and/or continuous improvement in many delivery techniques. In this initial 64-patient cohort, we discovered 6 minor incidents using Cherenkov imaging; these otherwise would have gone undetected. In addition, imaging provides automated, quantitative metrics useful for determining the quality of radiation therapy delivery.

Visual light perceptions caused by medical linear accelerator: Findings of machine-learning algorithms in a prospective questionnaire-based case-control study

This study aimed to investigate the possible incidence of visual light perceptions (VLPs) during radiation therapy (RT). We analyzed whether VLPs could be affected by differences in the radiation energy, prescription doses, age, sex, or RT locations, and whether all VLPs were caused by radiation. From November 2016 to August 2018, a total of 101 patients who underwent head-and-neck or brain RT were screened. After receiving RT, questionnaires were completed, and the subjects were interviewed. Random forests (RF), a tree-based machine learning algorithm, and logistic regression (LR) analyses were compared by the area under the curve (AUC), and the algorithm that achieved the highest AUC was selected. The dataset sample was based on treatment with non-human units, and a total of 293 treatment fields from 78 patients were analyzed. VLPs were detected only in 122 of the 293 exposure portals (40.16%). The dataset was randomly divided into 80% and 20% as the training set and test set, respectively. In the test set, RF achieved an AUC of 0.888, whereas LR achieved an AUC of 0.773. In this study, the retina fraction dose was the most important continuous variable and had a positive effect on VLP. Age was the most important categorical variable. In conclusion, the visual light perception phenomenon by the human body during RT is induced by radiation rather than being a self-suggested hallucination or induced by phosphenes.

Multiple sensory illusions are evoked during the course of proton therapy

Visual illusions from astronauts in space have been reported to be associated with the passage of high energy charged particles through visual structures (retina, optic nerve, brain). Similar effects have also been reported by patients under proton and heavy ion therapies. This prompted us to investigate whether protons at the Loma Linda University Proton Therapy and Research Center (PTRC) may also affect other sensory systems beside evoking similar perceptions on the visual system. A retrospective review of proton radiotherapy patient records at PTRC identified 29 sensory reports from 19 patients who spontaneously reported visual, olfactory, auditory and gustatory illusions during treatment. Our results suggest that protons can evoke neuronal responses sufficient to elicit conscious sensory illusion experiences, in four senses (auditory, taste, smell, and visual) analogous to those from normal sensory inputs. The regions of the brain receiving the highest doses corresponded with the anatomical structures associated with each type of illusion. Our findings suggest that more detailed queries about sensory illusions during proton therapy are warranted, possibly integrated with quantitative effect descriptions (such as electroencephalography) and can provide additional physiological basis for understanding the effects of protons on central nervous system tissues, needed for radiation risk assessment in advance of deep space human exploration.

Occurrence and mechanism of visual phosphenes in external photon beam radiation therapy and how to influence them

BACKGROUND AND PURPOSE

Two plausible mechanisms to explain the appearance of visual phosphenes are: direct activation of the photochemicals in the retina and the generation of Cherenkov radiation in the vitreous humour. In this clinical trial we investigated the occurrence of visual phosphenes in external photon beam radiation therapy.

MATERIAL AND METHODS

Logistic regression analysis is used to examine whether seeing light flashes and seeing steady light depended on the ambient light intensity and the dose.

RESULTS

In total, 465 treatments of 25 patients were analysed. The odds of seeing light flashes multiply by 0,926 as the ambient light intensity increases by 10 lux. Similarly, the odds multiply by 1,604 as the dose to the retina increases by 10 cGy. The odds of seeing steady light multiply by 1,540 as the dose to the vitreous humour increases by 10 cGy.

CONCLUSIONS

We postulate that one should reduce the dose rate, instruct patients to keep the eyes open and increase the illuminance in the treatment room to reduce the probability of experiencing visual phosphenes. We hypothesize that melanopsin is involved in the visual phosphenes and that fatigue of patients might be correlated with the observation of visual phosphenes.