The radiation beam itself is invisible and silent, and during most treatments you feel absolutely nothing while it is on. There is no heat, no sting, no vibration. A typical session lasts only a few minutes of actual beam time, and many patients describe the experience as anticlimactic compared to the anxiety leading up to it. But “nothing during the beam” is not the whole story. Some people see flashes of light with their eyes closed. Others smell something like burning plastic that nobody else in the room can detect. And as sessions accumulate over days and weeks, the body starts responding in ways that range from mild sunburn to deep fatigue to site-specific discomfort that can genuinely reshape daily life.
What Happens in the Treatment Room
A radiation therapy session involves lying on a hard, narrow table while a machine called a linear accelerator rotates around you or aims from a fixed position. The room is cool, often deliberately so. You are alone during the actual treatment because the therapists step behind a shielded wall, but they watch you on a camera and can hear you through an intercom. The machine may hum or click, and in some newer techniques the table or the machine rotates during delivery, which can produce a mild sensation of movement. In a study testing dynamic table rotation in healthy volunteers, only about one in 40 people reported a clinically meaningful sense of spinning, and the feeling resolved within five minutes.
1Physics and Imaging in Radiation Oncology. A randomized safety study of tolerance to table rotation in dynamic trajectory radiotherapy in healthy volunteersFor head and neck treatments, you are fitted with a thermoplastic mask that is molded to your face and bolted to the table so you cannot move. This is, for many patients, the single most distressing part of the entire treatment course. One patient described it this way: the mask going over the face was manageable, but being clamped down so the head could not move was “very confronting.”2PubMed Central. ‘Having the mask on didn’t worry me until … they clamped my head down so I wouldn’t move’: A qualitative study exploring anxiety in patients with head and neck cancer during radiation therapy The fear of not being able to breathe is common, even though the mask has holes and breathing is physically fine. Newer open-face mask designs, which leave the eyes, nose, and mouth uncovered, have been shown to dramatically reduce discomfort and anxiety compared to traditional closed masks.3PubMed. Randomized self-controlled study comparing open-face vs. closed immobilization masks in fractionated cranial radiotherapy Full-head masks also cause more claustrophobia than masks covering only part of the face.4PubMed. Randomized trial on two types of thermoplastic masks for patient immobilization during radiation therapy for head-and-neck cancer
Light Flashes and Phantom Smells
Two of the strangest things about radiation therapy are sensory experiences that sound almost hallucinatory. Some patients see flashes of light during treatment, even when their eyes are closed. The flashes are typically blue or purple in color, though some people report white or yellow. These are not imagined. Researchers have directly confirmed that a type of light called Cherenkov radiation is generated inside the eye when the beam passes near the retina.5PubMed Central. Experimentally Observed Cherenkov Light Generation in the Eye During Radiation Therapy In a multicenter study, the dose reaching the retina was the main factor associated with whether a patient saw flashes.6PubMed Central. Light flashes during proton and photon radiotherapy: A multicenter prospective observational study With proton therapy, the beam energy is below the threshold for Cherenkov light, so those flashes are thought to arise from a different mechanism: the charged particles depositing energy directly onto the retina’s photoreceptors. Either way, the experience is real and harmless, though nobody warns most patients about it beforehand, which can be startling.
The phantom smells are equally unsettling. Patients undergoing cranial or head-area radiation sometimes detect odors that nobody else in the room notices. The most commonly reported smells are chemical-like, metallic, or like burning plastic or rubber.7PubMed Central. Abnormal visual and olfactory sensations during radiation therapy: a prospective study One patient, a former industrial worker, said the smell was identical to ozone from the equipment he had used on the job.8Advances in Radiation Oncology. Unusual Olfactory Perceptions During Radiation Therapy: A Review Early theories suggested that the beam was creating actual ozone in the air near the patient’s nose, but measurements found the ozone levels were far too low to explain the smell. The current thinking is that the beam stimulates olfactory neurons directly, producing a phantom perception.9Advances in Radiation Oncology. Phantosmia Among Pediatric, Adolescent, and Young Adult Patients Receiving Proton Beam Therapy Young patients undergoing proton therapy report these phantom odors too, so the phenomenon is not unique to conventional X-ray beams or to adults.
What the Skin Goes Through
If there is one side effect that almost everyone undergoing external beam radiation eventually notices, it is a change in the skin over the treated area. During the first week or two, there may be nothing visible at all. Then the skin begins to pink up, like a mild sunburn. As treatment continues, the pinkness can progress to redness, dryness, peeling, and sometimes a dark discoloration. It can itch, sting, or feel tender to the touch. In breast cancer patients, these skin changes affect more than just the surface. Qualitative research has found they cause physical discomfort, changes in body image, emotional distress, and practical problems with clothing and daily activities.10PubMed Central. A qualitative analysis of acute skin toxicity among breast cancer radiotherapy patients
Skin reactions are also where the choice between proton therapy and conventional photon therapy can make a surprisingly tangible difference, and not always in the direction patients expect. In a study of breast cancer patients, about 70% of those treated with protons developed at least moderate skin redness, compared to roughly 30% of those treated with photons.11PubMed. Quantification of Acute Skin Toxicities in Patients With Breast Cancer Undergoing Adjuvant Proton versus Photon Radiation Therapy: A Single Institutional Experience Another comparison found proton therapy produced higher rates of skin inflammation and elevated inflammatory markers in both patients and lab models.12PubMed Central. Radiation-induced skin and heart toxicity in patients with breast cancer treated with adjuvant proton radiotherapy: a comparison with photon radiotherapy This seems counterintuitive, since proton therapy is marketed for its precision. The explanation is that protons deliver a larger dose to the skin surface in certain beam configurations. For head and neck patients, though, a recent longitudinal comparison found the skin’s biophysical response was similar between modern photon and proton techniques, with the superficial dose to the skin being the main driver regardless of which type of beam was used.13PubMed. Biophysical Skin Response in Head and Neck Cancer: Longitudinal Comparison of Modern Photon and Proton Radiotherapy
Hydrogel dressings have emerged as one of the more promising options for managing skin reactions. They keep the skin moist, reduce water loss, feel cool on contact, and can be removed without tearing healing skin.14PubMed. The Efficacy of Hydrogel Dressings in Management and Prevention of Radiation Dermatitis: A Comparative Review Most radiation oncology departments will also recommend fragrance-free moisturizers, loose clothing over the treated area, and staying out of the sun.
The Fatigue That Rest Does Not Fix
Radiation-related fatigue is one of those side effects that catches people off guard because it does not make intuitive sense. You are lying still on a table for a few minutes a day. Why would that make you exhausted? Yet the fatigue is real and often profound. It is described as a persistent, pervasive sense of tiredness that interferes with daily life and is not relieved by rest or sleep.15PubMed Central. The Etiology and management of radiotherapy-induced fatigue It tends to build gradually over the weeks of treatment, and for some patients it lingers well after the last session. Research on prostate cancer patients found that rising levels of an inflammatory signaling molecule in the blood tracked alongside worsening fatigue scores over the treatment course, suggesting the body is mounting a systemic inflammatory response to localized radiation.16Journal of Pain and Symptom Management. Treatment-related fatigue and serum interleukin-1 levels in patients during external beam irradiation for prostate cancer This is not “all in your head” tiredness. The immune system is genuinely activated.
For patients receiving brain radiation, fatigue can escalate into what is called somnolence syndrome: an overwhelming need to sleep, sometimes for most of the day. In a prospective study of brain tumor patients, 90% experienced at least mild somnolence, with the peak hitting at the end of treatment and then gradually improving over the following six weeks.17PubMed. Somnolence syndrome in patients receiving radical radiotherapy for primary brain tumours: a prospective study Patients and families should know this is expected and temporary, not a sign that the cancer is worsening.
How the Treatment Site Changes What You Feel
The location being treated determines the most noticeable side effects far more than the type of machine being used. Radiation therapy is not one experience. It is many different experiences, depending on where the beam is aimed.
Head and Neck
Patients treated for head and neck cancers often develop mucositis, a painful inflammation of the mouth and throat lining. At its worst, eating, swallowing, and even talking can become difficult. Pain descriptions tend to include burning, aching, soreness, and tenderness.18PubMed Central. Neuropathic and nociceptive pain in head and neck cancer patients receiving radiation therapy Even six months after treatment ends, some patients still report increased sensitivity to touch from teeth, food, or fluids in the mouth.19PubMed Central. Oral Complications at Six Months after Radiation Therapy for Head and Neck Cancer Taste changes are common during treatment and may partially recover over time, though some degree of alteration can persist. Dry mouth from damaged salivary glands is another hallmark of head and neck radiation that affects eating and speaking for months or longer.
Pelvis
Pelvic radiation, often given for prostate, bladder, cervical, or rectal cancers, tends to affect the bowel and bladder. Patients frequently report increased urgency, more frequent trips to the bathroom, loose stools, and sometimes incontinence to gas or liquid stool. A study of prostate and bladder cancer patients found that symptoms of rectal inflammation and incontinence rose significantly within six weeks of treatment and remained elevated at six months.20PubMed. Physiologic changes of the anorectum after pelvic radiotherapy for the treatment of prostate and bladder cancer Patients described alterations in their daily lifestyle to accommodate these changes. Urinary symptoms follow a similar pattern: more frequency, urgency, and sometimes a burning sensation during urination.
Breast and Chest
Beyond the skin reactions discussed earlier, breast radiation can cause edema, meaning the breast swells and feels heavier. After breast-conserving surgery combined with radiation, the breast can increase by more than one cup size due to fluid retention and tissue reaction. Other signs include skin thickening, visible pore enlargement, and a pitting texture when the skin is pressed.21PubMed Central. Breast edema, from diagnosis to treatment: state of the art Most of this edema resolves within six months, but when it persists beyond that window, additional monitoring and treatment may be needed. Some patients also report a sharp, fleeting pain in the chest wall that can come and go for months after treatment.
Nerve Pain and Long-Term Tissue Changes
Radiation does not just affect the immediate target. Over time, it can damage nerves in the treated area, leading to what is called radiation-induced neuropathy. The underlying process involves fibrosis, meaning scar-like tissue forms around and within nerves, compressing them and cutting off their blood supply. This leads to breakdown of the nerve’s insulating layer and eventually to damage to the nerve fibers themselves.22PubMed. Radiation-induced neuropathy in cancer survivors The result can be numbness, tingling, burning sensations, or shooting pains in the affected region. One well-known form is brachial plexopathy after breast or chest radiation, where patients develop pain, weakness, or loss of sensation in the arm and shoulder on the treated side.23PubMed Central. Radiation-induced neuropathic pain successfully treated with systemic lidocaine administration
Chronic fibrosis is the broader late effect that underlies many of these long-term changes. The treated area can become stiff, shrunken, and less elastic over months and years. Skin may thin and lose hair permanently.24PubMed Central. Radiation-Induced Skin Fibrosis: Pathogenesis, Current Treatment Options, and Emerging Therapeutics These changes develop slowly, sometimes appearing a year or more after treatment ends, which means some patients have already moved on mentally from their cancer treatment when the late effects begin.
Hyperbaric Oxygen for Chronic Radiation Damage
For patients dealing with persistent pain, fibrosis, or tissue breakdown months or years after radiation, hyperbaric oxygen therapy is sometimes offered. The idea is that breathing pure oxygen under pressure stimulates the growth of new blood vessels and activates tissue repair in areas starved of oxygen by radiation damage.25PubMed Central. Hyperbaric oxygen therapy for chronic radiotherapy-related adverse effects: A clinically focused review In a series of 67 breast cancer patients treated with hyperbaric oxygen for late radiation effects, 85% reported at least some improvement in their symptom scores, and both pain and fibrosis showed meaningful decreases that held steady at one year.26PubMed Central. The effect of hyperbaric oxygen treatment on late radiation tissue injury after breast cancer: A case-series of 67 patients The picture is less clear-cut for all patient groups, however. A study of gynecological patients found the treatment had limited effect on pain, daily function, and tissue damage visible on imaging, though half of the patients still reported feeling some benefit.27PubMed. Hyperbaric oxygen therapy for late radiation tissue injury in gynaecological patients The treatment involves multiple sessions inside a pressurized chamber, which is its own sensory experience: ear pressure changes, warmth, and the confined space can be uncomfortable for some people.
What Children Go Through
Radiation therapy for children involves the same beams and machines as for adults, but the experience is profoundly different because young children cannot understand instructions to lie still. The typical solution is general anesthesia. According to a Children’s Oncology Group survey, the median age at which children no longer need anesthesia is around six years old, though some centers manage to treat children as young as three or four without it.28PubMed Central. Improving the Pediatric Patient Experience During Radiation Therapy – A Children’s Oncology Group Study For the child, this means daily sedation for the entire treatment course, which adds its own layer of discomfort and time. Anesthesia adds more than 45 minutes to the daily visit in over half of reporting centers.
Newer preparation strategies aim to reduce the need for sedation. One approach uses immersive virtual reality to familiarize children with the treatment room and the sounds of the machine before their first session. In a pilot study, children tolerated the VR experience well, with only mild eyestrain or dizziness and no significant nausea.29Technical Innovations & Patient Support in Radiation Oncology. Effects of immersive virtual reality exposure in preparing pediatric oncology patients for radiation therapy Play-based desensitization, where children practice lying on the table and wearing a mask in non-threatening settings, has also helped some centers lower the age at which they can treat without anesthesia. For families facing this, asking the radiation team about their specific approach to pediatric preparation is worth doing early.