Internal radiation therapy is a cancer treatment where a radioactive source is placed inside your body, either directly next to a tumor or circulated through your bloodstream. Unlike external radiation, which aims beams at you from a machine outside the body, internal radiation works from the inside out. It comes in two main forms: brachytherapy, which uses small implants placed in or near the tumor, and systemic radiation therapy, which uses liquid radioactive drugs that travel through your blood to find cancer cells.
How Brachytherapy Works
Brachytherapy delivers radiation through tiny physical objects, including seeds, capsules, ribbons, wires, needles, or small balloons, placed directly into or beside a tumor. Because the radiation source sits so close to the cancer, it can deliver a strong, focused dose while limiting exposure to nearby healthy tissue. The radiation travels only a short distance from the implant, which is what makes this approach effective for tumors that are well-defined and accessible.
There are several ways these implants are positioned, depending on where the cancer is:
- Interstitial brachytherapy places the radiation source directly inside the tumor itself. This is commonly used for prostate cancer.
- Intracavity brachytherapy places the source inside a natural body cavity or one created by surgery. Cervical and endometrial cancers are often treated this way, with the source positioned inside the vagina.
- Episcleral brachytherapy attaches the radiation source to the surface of the eye, used for eye melanoma.
- Radioembolization uses tiny radioactive beads placed into the blood vessel that feeds the liver. This treats liver cancer or cancers that have spread to the liver.
Cancers Commonly Treated With Brachytherapy
Brachytherapy is particularly well suited for cancers that form localized tumors. It’s most often used for prostate cancer, cervical cancer, endometrial (uterine) cancer, breast cancer, head and neck cancers, and eye cancer. For cancers like prostate and cervical, brachytherapy is considered especially effective because the tumors tend to stay in one area, making them ideal targets for a nearby radiation source.
Permanent vs. Temporary Implants
Not all brachytherapy implants stay in your body the same length of time. Some are permanent, and some are temporary. The distinction matters for what the treatment experience feels like.
Permanent implants, often called seed implants, are tiny radioactive seeds placed directly into the tumor (most commonly the prostate). These seeds stay in place permanently, but the radiation they emit fades over weeks to months until it’s essentially gone. The seeds themselves are so small they don’t cause discomfort and don’t need to be removed.
Temporary implants deliver a higher dose of radiation over a shorter period. They’re inserted, left in place for minutes to days depending on the treatment plan, and then removed. Some temporary treatments are given in multiple sessions. During treatment with temporary implants, you may need to stay in the hospital and limit movement to keep the implant in the correct position.
How Systemic Radiation Therapy Works
Systemic radiation therapy takes a completely different approach. Instead of placing a physical implant near the tumor, you receive a liquid radioactive drug, called a radiopharmaceutical, either by swallowing it or through an injection into a vein. Once in your bloodstream, the drug circulates throughout your body, seeking out and attaching to cancer cells based on specific markers on their surface. This makes systemic therapy useful when cancer has spread to multiple locations, since it doesn’t rely on targeting just one spot.
One well-known example is radioactive iodine therapy for thyroid cancer. Thyroid cells naturally absorb iodine, so a radioactive form of iodine concentrates in thyroid tissue and destroys it. A newer approach uses a radioactive drug that targets a protein found on prostate cancer cells, delivering radiation directly to those cells wherever they are in the body. This type of targeted treatment is an active area of clinical development, with recent trials showing it can improve outcomes for certain prostate cancer patients when combined with other radiation techniques.
How It Differs From External Radiation
External beam radiation sends focused beams from a machine outside your body through your skin to reach the tumor. You lie on a table, the machine rotates around you, and each session typically lasts 15 to 30 minutes. You go home afterward and aren’t radioactive at any point.
Internal radiation flips that setup. The radiation comes from inside you, which means higher doses can reach the tumor with less damage to surrounding tissue. With brachytherapy, the radiation drops off sharply just a short distance from the implant, sparing organs and structures nearby. The tradeoff is that internal radiation requires a procedure to place the implant, and in some cases, a hospital stay. With systemic therapy, you temporarily carry radioactive material in your body, which requires safety precautions around other people.
Many treatment plans combine both. A patient with cervical cancer, for instance, might receive external beam radiation to shrink the tumor followed by brachytherapy to deliver a concentrated finishing dose.
Side Effects by Treatment Area
Fatigue is the most universal side effect. Nearly everyone who undergoes radiation therapy experiences some degree of exhaustion. Beyond that, side effects depend heavily on where the radiation is focused.
Radiation to the pelvis, common with cervical, prostate, and rectal cancers, can cause diarrhea, urinary and bladder problems, sexual side effects, and changes to fertility. Breast brachytherapy may lead to skin changes, swelling, and tenderness. Head and neck treatments can affect swallowing, cause mouth problems, alter taste, and reduce thyroid function. Radiation to the abdomen or stomach area often brings nausea and digestive issues.
Because brachytherapy targets a small area, its side effects tend to be more localized and sometimes less severe than those from external radiation covering a wider field. But they still occur, and they can take days to weeks to fully develop after treatment.
Safety Precautions After Treatment
One of the biggest practical differences between internal and external radiation is what happens after your session. With external radiation, you walk out the door with zero radiation risk to anyone. With internal radiation, your body is temporarily radioactive, and that requires precautions to protect the people around you.
During brachytherapy with temporary implants, you may need to stay in the hospital with restricted visitors. Pregnant people and children may not be allowed to visit at all. You’ll need to maintain a certain distance from anyone who does come to see you.
After systemic radiation therapy, precautions are typically needed for the first few days. Your treatment team will give you specific instructions, which commonly include: sleeping in a separate bed and room, using separate utensils and towels, sitting on the toilet to avoid splashing and flushing twice, washing your clothes and sheets separately, drinking extra fluids to flush the radioactive material from your body faster, and avoiding kissing, sexual contact, and close physical contact for at least a week. Avoiding infants, children, pregnant people, and even pets is standard guidance. Some people are told to stay home from work and avoid public transportation during this period.
After radioembolization for liver cancer, contact restrictions can last up to one week, with special emphasis on avoiding children and pregnant people.
These precautions sound intense, but they’re temporary. The radioactive material in your body decays or gets flushed out relatively quickly, and your treatment team will tell you exactly when it’s safe to resume normal contact.