Patient Education — Basics of Radiation

Understanding the Basics of Radiation

It is already all around you. Here is what it actually is.

Radiation has a frightening reputation, and some forms genuinely deserve caution. But it is also one of the most precise tools in cancer medicine — and understanding it takes most of the fear out of it.

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You are being exposed to radiation right now

occurs naturally in soil, water, air, rock, and food. The amount you receive varies with where you live, what your home is built from, and what you eat.

None of the sources below is a cause for concern. The point is simply that a radiation dose of zero has never been an option for anyone — it has always been part of daily life.

Altitude

Less atmosphere overhead means less shielding from cosmic rays. Background radiation in Denver runs roughly twice the US average.

Your home

Stone, brick, and concrete carry more natural radioactivity than wood. Granite countertops emit a little; radon gas seeping from soil is the bigger factor in most homes.

Flying

At cruising altitude you are above most of the atmosphere. A transatlantic flight delivers roughly the same dose as a single chest X-ray.

Food

Bananas, potatoes, and beans contain potassium, and a small fraction of all potassium is radioactive. Brazil nuts are the most radioactive common food.

The science

Where radiation comes from

Everything is made of

. Each atom has a dense center — the nucleus — packed with protons and neutrons, surrounded by orbiting electrons. Whether an atom is stable comes down to the balance of protons and neutrons in that nucleus.

An atom with an unstable nucleus is called a

—a radioisotope, unstable atom eventually becomes stable by emitting energy. The energy it throws off is called radiation and the process is called

. Sometimes one decay is enough other times an atom passes through several unstable forms. Like a family the unstable atom is called the parent and the subsequent unstable atom is called the daughter which will have it's own decay and half-life. Some Radioisotopes have a long progeny of daughters until it finally becomes stable. Certain radioisotopes can be used to image or treat a cancer based on the type of radiation it emits.

Uranium-238 is an example: it goes through 14 decay steps, passing through thorium, radium, radon, polonium, and bismuth along the way, before finally landing on lead-206, which is stable.

Some radioisotopes occur naturally. The ones used in medicine are made on purpose — in a reactor, a cyclotron, rhodotron, or a small device called a generator. That is why the supply of certain isotopes is a real constraint on cancer care, and why some treatments are only available at certain centers.

Half-life

Every radioisotope has its own clock

How long it takes for an atom to produce a daughter atom or stable atom is measured in

. Decay is random for any single atom, but remarkably predictable across trillions of them. The measure is half-life: the time it takes for half of the atoms present to decay.

The key idea is that it halves rather than counting down evenly. Start with 100 units of something with a one-day half-life: after one day you have 50, after two days 25, after three about 12. It fades fast at first, then trails off.

Isotope	Used for	Half-life
Ga-68	PET imaging — made on site in a generator	68 minutes
C-11	PET imaging — must be made in the same building	20 minutes
F-18	PET imaging — shipped by road within hours	110 minutes
Tc-99	SPECT imaging — the most used isotope in medicine	6 hours
Lu-177	Treatment — prostate & neuroendocrine cancers	6.6 days
I-131	Treatment & imaging — thyroid cancer	8 days
Ac-225	Treatment — alpha therapy	10 days
U-238	Naturally occurring in rock and soil	4.5 billion years

This is not abstract in nuclear medicine — half-life determines how far an isotope can be shipped, how quickly a scan must happen after delivery, and how long you stay mildly radioactive after treatment.

Two broad categories

Ionizing and non-ionizing

Radiation is sorted by whether it carries enough energy to knock an electron off an atom it passes through. An atom that loses an electron becomes electrically charged — an ion — which is where the name comes from.

Low energy

Not enough energy to strip electrons loose. It can warm things up, but it cannot break the chemical bonds that hold molecules together.

Radio wavesMicrowavesVisible lightInfrared

High energy

Enough energy to strip electrons and break chemical bonds — including bonds in DNA. That capacity is what makes it useful against cancer, and what makes it worth handling carefully.

The question patients ask most

Does radiation make you radioactive?

For scans and X-rays: no.

An X-ray or CT scan passes through you and is gone the instant the machine switches off. Nothing is left behind, nothing about you becomes radioactive, and there is nothing to wash off.

For radioactive medicines: temporarily, yes — and that is expected.

PET scans and theranostic treatments give you a radioactive substance, so for a while you are a source of radiation yourself. It leaves your body two ways at once: through natural decay on that isotope's half-life schedule, and through urine and stool.

This is why your care team gives simple precautions for a few days — extra hydration, flushing twice, sleeping separately, keeping some distance from small children and anyone pregnant. These are sensible steps, not signs that anything has gone wrong.

The distinction has a name. Exposure means radiation passed through you. Contamination means radioactive material is on or in you. Scans involve exposure only; radioactive medicines involve both, by design, for a limited and predictable period.

Medical uses

Which types do which jobs

Ionizing radiation has been saving and extending lives for over a century — through imaging (X-ray, CT, PET, SPECT) and treatment (external beam, brachytherapy, and now

). In cancer care, the split comes down to one thing: whether the radiation leaves your body or stays inside it.

Radiation that leaves the body lets us see. Gamma rays and positrons pass out through tissue to a detector, producing a picture — gamma rays via a gamma camera or SPECT scanner, positrons via PET.

Radiation that stays inside can treat. Alpha and beta particles stop within tissue, depositing their energy exactly where they land. Delivered to a tumor, that energy breaks the DNA inside cancer cells.

Alpha

Powerful & local

About 7,300× heavier than a beta particle. Travels under 1 mm — the width of 2 to 10 cells — and hits very hard, spending all its energy within a few cell widths. It takes just 2–10 hits to kill a cancer cell.

Barely touches neighborsMust be delivered very close

Beta

Gentle & far-reaching

Very light — a single high-speed electron. Travels 1–10 mm, hundreds of cell widths, spreading its energy thinly along a longer path. It takes roughly 2,000 hits to kill a cancer cell.

Reaches cells the drug missedCan affect nearby healthy tissue

Auger

Tiny & precise

A shower of very low-energy electrons, pronounced OH-zhay. Their range is measured in billionths of a meter — smaller than a single cell — so they must be delivered right up against the DNA to work. When they are, the damage is intense and highly localised.

Sub-cellular rangeMust reach inside the cell

None is better than the others. What matters is not how much energy a particle carries but how tightly it concentrates it. An alpha spends everything within a few cell widths; a beta spreads a similar amount across a thousand times the distance; an Auger electron unloads at point-blank range to the DNA. Alpha suits scattered single cells, beta suits bulkier tumors, and Auger suits single cells when the drug can get inside them.

A closer look

Auger electrons, up close

Auger radiation (say it OH-zhay) is the newest of the three therapy types to draw serious interest, and it works on a completely different scale from the others.

When certain radioisotopes decay, they release a burst of many very low-energy electrons all at once. Because each electron carries so little energy, it travels only a few billionths of a meter — a distance far smaller than a single cell. That sounds like a weakness, but it's the whole point: if the radioisotope is sitting inside the cell, right next to the DNA, that burst delivers extremely dense, hard-to-repair damage exactly where it counts, and almost nothing lands on neighboring healthy cells.

The catch is delivery. An Auger emitter only works if the targeting molecule can carry it not just to the cancer cell, but into it — ideally close to the nucleus. That makes Auger therapy especially promising for tiny deposits of disease and single scattered cancer cells, the kind that are hardest to treat any other way. Terbium-161 is one example of an isotope that adds Auger electrons to its beta radiation.

Measurement

Three questions, three different units

These are easy to mix up because they sound interchangeable. They are not — each answers a separate question.

Bq / Ci

Becquerel / Curie

How much radioactive material is here?

Counts how many atoms are decaying each second — the strength of the source itself, before it reaches anyone. This is what appears on the vial when a dose is prepared. Curies in the US, becquerels elsewhere.

Gray

How much energy did my tissue absorb?

Measures energy actually deposited in a kilogram of tissue. This is the unit used to plan treatment doses and to estimate what organs like the kidneys receive.

Sv / rem

Sievert / rem

How much biological harm does that represent?

Takes the absorbed dose and adjusts for how damaging that type of radiation is. One gray of alpha does far more biological damage than one gray of gamma — about twenty times as much. Sieverts internationally, rem in the US.

A sense of scale

The banana radiation comparison

Believe it or not, bananas are known to be radioactive — but not at harmful amounts, and you should absolutely not stop eating them. The radioactivity of a banana is so tiny and so harmless that it has instead become a handy way to compare many of the other radiation exposures we encounter in everyday life.

One banana ≈ 0.1 microsieverts.

Bananas are rich in potassium, and a small fraction of all potassium in nature is radioactive potassium-40. Eating one delivers about a ten-thousandth of the dose of a chest X-ray. It has become a common way to put radiation numbers into everyday terms.

One honest caveat: your body holds potassium at a steady level and sheds any excess, so eating more bananas does not actually accumulate. The comparison is a useful sense of scale rather than a precise measurement — and it makes the underlying point well. Please keep eating bananas.

Radiation is not one thing. It ranges from the radio waves carrying music to your car to the alpha particles that can destroy a cancer cell from within. What separates them is energy, and how tightly it is concentrated. The kind used in cancer care is powerful — and it is measured, planned, and targeted with a precision unimaginable a generation ago. That's it for the basics of understanding radiation — thank you for reading.

Keep going

Where to next

Now that radiation makes sense, see how it is aimed at cancer — and look up anything that is still unfamiliar.

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