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The symptoms nobody warned you about
Palpitations and Rapid Heartbeat During Radiation — What It Usually Is, and When It Is Not
A heart that thumps, flutters or races partway through a course of radiation is common, and it is usually not the beam. Low haemoglobin, dehydration, fever, a thyroid change after neck treatment and plain anxiety account for most of it. A few patterns are not safe to watch at home, and those are listed first.
Medically reviewed by Dr. Venkata Sushma P, Radiation Oncologist, MBBS · MD (Radiation Oncology) · Last reviewed August 2026
- A straight answer on the beam — radiation to the breast, chest or neck does not usually race the heart during the course itself — the cause is almost always somewhere else, and it is testable.
- Four common causes, told apart — anaemia, a thyroid change, anxiety and a rhythm problem each have their own pattern. One table separates them so you know what to ask for.
- A written escalation threshold — the pulse numbers, the accompanying symptoms and the timings that mean call today — and the ones that mean go to an emergency department now.
- Nothing brushed aside as nerves — a racing heart during cancer treatment is not called anxiety until the blood count, the salts and the thyroid have actually been looked at.
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Read this part before the explanations. A fast heartbeat during cancer treatment is not automatically harmless, and it is never dismissed as nerves before it has been checked. Some patterns need an emergency department today, not a mention at the next review.
Call 1800 202 8726 now — or go straight to the nearest emergency department — if the palpitations come with chest pain or pressure, breathlessness while sitting still, fainting or near-fainting, fever or shaking chills, or a resting pulse above 120 that will not settle.
Everything below is for a heartbeat that is fast or thumping but does not carry any of those. If you are reading this at 2am and unsure which group you are in, treat it as the urgent group and call.
Is the Fast Heartbeat Coming From the Radiation Itself?
Usually not. Radiation treats a targeted area, and a racing heart during a course is far more often caused by anaemia, dehydration, fever, a thyroid change after neck treatment, or anxiety. Heart effects tied to radiation dose are a long-term consideration your team plans for in advance, not a day-three one. Report it anyway — every common cause is testable.
Low haemoglobin
Red cells fall when chemotherapy runs alongside radiation. The heart beats faster to move the oxygen that is left, so tiredness and breathlessness on stairs arrive with it.
Not enough fluid
Nausea, a sore mouth, loose motions or a long fasting wait leaves you dry. The pulse rises to hold blood pressure up, and it settles once the fluid is corrected.
Fever or infection
A fast pulse with a temperature is the pattern that matters most. It is checked the same hour, not watched, because blood counts can be low during a course of treatment.
A thyroid change
Fields that pass near the lower neck can leave the thyroid overactive or underactive later. An overactive thyroid races the pulse and adds tremor, heat intolerance and weight loss.
Anxiety and the mask
Panic under an immobilisation mask, or in the minutes before a session, produces a genuinely racing heart. It is real, it is common, and it is not nothing.
The heart’s own rhythm
Some patients already have, or newly develop, an irregular rhythm. A newly irregular pulse is not a radiation side effect and needs its own heart tracing.
These are the causes seen most often alongside treatment. The list follows general supportive-care guidance from NCCN and ASTRO and is not exhaustive. If what you are noticing is not described here, still report it.
Did you know?
The World Health Organization estimates that anaemia affects roughly a quarter of the world’s population, which makes it one of the most common reasons any heart beats faster than it should. During cancer treatment it is also one of the easiest causes to confirm — a single blood count, taken the same day, usually settles the question before anyone starts calling it stress.
Question 2 — is it anxiety or anaemia?
Is It Anxiety, or Is It Anaemia?
Both race the heart, and they can be told apart. Anxiety comes in waves tied to the mask, the machine or a scan result, and settles within minutes. Anaemia is steady, worse on stairs, and arrives with pallor and tiredness. A blood count separates them the same day.
| Likely cause | What the heartbeat feels like | What comes with it | What settles the question |
|---|---|---|---|
| Low haemoglobin | Fast and steady all day, worse after walking or stairs | Tiredness, pallor, breathlessness on effort, sometimes giddiness on standing | A full blood count — often the same visit |
| Dehydration or low salts | Fast on standing, easier once lying down | Dry mouth, less urine, loose motions or vomiting, cramps | Salts and kidney function on a blood test, plus a lying and standing blood pressure |
| Fever or infection | Fast and will not settle with rest | Temperature, shivering, feeling suddenly unwell | Not a waiting question — this is checked urgently, the same hour |
| Thyroid change after neck or upper chest treatment | Fast even at complete rest, including in bed | Tremor, heat intolerance, weight loss without dieting, loose motions | A thyroid function blood test |
| Anxiety or panic | Sudden, in waves, tied to a trigger, settling in minutes | Tight chest, tingling hands or lips, a sense of dread, fast breathing | A normal blood count and a normal heart tracing, then support for the anxiety itself |
| A rhythm problem in the heart | Irregular, skipping or thudding rather than simply fast | May come with breathlessness, chest discomfort or near-fainting | A heart tracing (ECG) — and urgently if it comes with any of those |
Why the order matters. Anxiety is the easiest label to reach for and the one most often reached for too early. On this cluster we do it the other way round: the blood count, the salts and the thyroid are looked at first, and anxiety is named once those are clear. That way nobody spends a fortnight being reassured while a correctable cause goes unchecked.
Not sure which row you are in? Call 1800 202 8726 and describe it in your own words.
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A Racing Heart Deserves a Blood Test, Not a Guess
Our radiation oncology team can tell you which checks your palpitations need, and arrange them around your treatment schedule rather than after it.
Question 3 — when is it urgent?
When Is a Fast Heartbeat Urgent?
Go now if palpitations come with chest pain, breathlessness at rest, fainting or near-fainting. Go now if a fast pulse arrives with fever or shaking chills. Call the same day if the resting pulse stays above 100, if the beat has become newly irregular, or if the racing wakes you from sleep.
| What you are noticing | Common during treatment | Call the team the same day | Emergency — go now |
|---|---|---|---|
| A brief thump or flutter | A few skipped or heavy beats, settling on its own | Happening many times a day, or new this week | With chest pain, or with near-fainting |
| Racing before or during a session | Nerves before the machine, easing once you are off the couch | Bad enough that you dread the session or cannot stay still | With chest pain or breathlessness that does not ease after the session |
| Your resting pulse | 60 to 100 after sitting quietly for five minutes | Staying above 100 across several readings on different days | Above 120 at rest and not settling |
| Palpitations with breathlessness | Slightly puffed on stairs, recovering within a minute | New breathlessness on your usual walk | Breathless while sitting still, or unable to finish a sentence |
| Palpitations with fever | Nothing in this row is expected | Feeling off-colour with no temperature | Fast pulse with fever or shaking chills — treat as a same-hour emergency |
| An irregular beat | An irregular rhythm already known to your doctors and stable | Any new irregularity you can feel at the wrist | New irregularity with dizziness, chest pain or breathlessness — call 108 |
| Palpitations with dizziness | A second or two of swimming on standing, then clear | Giddiness at every stand, or worsening daily | Blacking out, or a fall — go to the nearest emergency department |
What not to do while you decide. Do not stop, halve or add to anything you have been prescribed on your own. Do not add anything bought over the counter, and do not add a supplement or a tonic without telling the treating team. Cut out coffee, strong tea, energy drinks and tobacco while you are working this out — they push the pulse up and muddy the picture. Do not skip a radiation session without telling anyone.
If the answer is genuinely unclear, call 1800 202 8726. Being told it can wait costs one phone call.
What happens next
What Will the Team Actually Check?
Expect a short, cheap set of tests rather than a scan. Pulse and blood pressure lying and standing, a full blood count, salts and kidney function, thyroid function if the neck or upper chest was treated, and a heart tracing. Most causes show up in that set.
Your pulse, lying and standing
Counted properly, in both positions, with blood pressure alongside. A pulse that jumps on standing points at fluid rather than at the heart.
A full blood count
This is the test that either finds anaemia or rules it out. It is quick, it is inexpensive, and it comes before anyone calls the symptom anxiety.
Salts and kidney function
Especially after vomiting, loose motions or several days of poor drinking. Low salts race the heart and cause cramps at the same time.
Thyroid function, if the neck was in the field
ASTRO and NCCN survivorship guidance both include thyroid testing in follow-up after head, neck and upper chest radiation. Ask for it rather than waiting.
A heart tracing
An ECG takes minutes and answers the one question a pulse at the wrist cannot: whether the rhythm itself is the problem. A cardiology opinion is arranged if it is.
A review of everything you are taking
Including anything from another system of medicine, and anything bought without a prescription. Bring the actual packets, not a description of them.
Where your treatment happens. Your radiotherapy is delivered at an NABH-accredited partner centre; CION Cancer Clinics coordinates your treatment plan, your oncology team and your care throughout. That coordination is what turns a symptom you noticed at home into a blood test done alongside tomorrow’s session instead of a separate appointment three weeks away.
This page explains what is usually checked. It is not a diagnosis and it is not a treatment plan — what is actually advised depends on the examination, the test results and everything you are already taking, and only your treating team can decide it.
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How Do You Count Your Own Pulse, and What Should You Write Down?
Sit quietly for five minutes. Feel the pulse at the thumb side of the wrist, count the beats for a full sixty seconds, and note whether it is regular. One number means little; the same number written down at the same time each day means a great deal.
- The number and the time — beats a minute, counted after five minutes of sitting, at roughly the same hour each day.
- Regular or irregular — a steady drum, or beats that skip and stumble. This single detail changes what is tested first.
- How long an episode lasts — seconds, minutes or hours, and whether it stops suddenly or fades away.
- What was happening at the time — standing up, walking, lying in bed, waiting for a session, or after being sick.
- What came with it — breathlessness, chest discomfort, giddiness, sweating, fever or nothing at all.
Keep it in a phone note rather than only in your head. Three dated lines are worth more at the review than a fortnight of remembering.
Related reading on the side effects nobody warns you about
Palpitations rarely arrive alone. If your body has started behaving oddly partway through a course, these cover the other symptoms that patients are most often told to just put up with:
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Common questions
Palpitations During Radiation: Your Questions Answered
Does radiation therapy cause palpitations?
Rarely on its own. Radiation treats a targeted area, and a racing or thumping heartbeat during a course of treatment is far more often caused by something around the treatment than by the beam. Low haemoglobin, not enough fluid, fever, a thyroid change after neck or upper chest treatment, and anxiety account for most cases. Heart effects related to radiation dose are a long term consideration that your team plans for in advance, not something that appears on day three. Report every episode anyway, because every one of the common causes can be confirmed with a simple test.
Is a fast heartbeat during radiation caused by anxiety or anaemia?
It can be either, and the two feel different. Anxiety arrives in waves that are tied to something, such as the immobilisation mask, the machine, a scan result or the waiting room, and it usually settles within minutes of the trigger passing. Anaemia is steadier. The pulse stays fast through the day, it is worse on stairs and after walking, and it comes with tiredness, pallor and breathlessness on effort. A blood count taken the same day separates the two. Anxiety is never assumed until the blood count has been seen.
When are palpitations during radiation an emergency?
Go to the nearest emergency department now if the palpitations come with chest pain or pressure, with breathlessness while you are sitting still, or with fainting or near fainting. Go now if a fast pulse arrives with fever or shaking chills, because blood counts can be low during treatment and that combination is treated urgently. Call the team the same day if your resting pulse stays above 100 once you have been sitting quietly for five minutes, if the beat has become newly irregular, or if the racing wakes you from sleep.
Can radiation to the neck cause a fast heartbeat?
It can, but usually months or years later rather than during the course. The thyroid gland sits in the lower neck, and treatment fields that pass close to it can leave it working too fast or too slowly afterwards. An overactive thyroid causes a fast resting pulse, heat intolerance, tremor and weight loss without dieting. ASTRO and NCCN survivorship guidance both list thyroid function testing as part of follow up after head, neck and upper chest radiation, so ask for the test rather than waiting for it to be offered.
What resting pulse is too high during cancer treatment?
A normal adult resting pulse sits between 60 and 100 beats a minute. Count it after sitting quietly for five minutes, not straight after climbing stairs or after a session. A resting pulse that stays above 100 across several readings on different days should be reported the same day. A resting pulse above 120, or any fast pulse with fever, chest pain, breathlessness at rest or fainting, is an emergency. Write down the number, the time and what you were doing, because that record is what the team acts on.
Should I stop radiation sessions if my heart keeps racing?
Do not stop or skip sessions on your own. Tell the radiation team the same day and let them decide. Most patients continue treatment unchanged once the cause is found and corrected, which may mean fluids, a blood count, a thyroid test or a review of everything you are already taking. Sometimes a session is postponed while the cause is investigated, and that is a decision for your radiation oncologist. Missed sessions that nobody is told about can affect how well a course of treatment works, so the phone call matters more than the pause.