Systemic radiation is a form of cancer treatment where a radioactive substance travels through your bloodstream to find and destroy cancer cells throughout your body. Unlike external beam radiation, which aims a focused beam at one spot from outside, systemic radiation works from the inside. You either swallow it as a liquid or receive it through an IV, and the radioactive material circulates to reach cancer cells wherever they are.
How Systemic Radiation Finds Cancer Cells
The key to systemic radiation is targeting. A radioactive isotope is attached to a molecule that cancer cells naturally absorb or that binds to proteins on their surface. Once the compound enters your bloodstream, it acts like a homing device, concentrating in tumors rather than healthy tissue. The radiation then damages the DNA inside cancer cells at close range, killing them or stopping them from dividing.
There are two main approaches. In targeted radionuclide therapy, a radioactive atom is linked to a small molecule that cancer cells take up preferentially. Radioactive iodine for thyroid cancer is the classic example: thyroid cells absorb iodine by nature, so when the iodine is radioactive, it delivers a lethal dose directly to thyroid tissue. In radioimmunotherapy, the radioactive atom is attached to a laboratory-made antibody designed to lock onto a specific protein found on the surface of cancer cells, delivering radiation right to the tumor.
How It Differs From External Beam Radiation
External beam radiation treats a defined area. A machine outside your body directs a focused beam at a tumor, and treatment teams carefully shape the radiation field to spare nearby organs. It works well for localized cancers but has limited reach if disease has spread to multiple sites.
Systemic radiation takes the opposite approach. Because it travels through the blood, it can reach tumors in many locations at once, including tiny deposits that imaging might not detect. The tradeoff is that the radioactive substance passes through your entire body before concentrating at tumor sites, which can expose healthy tissues, particularly blood-forming cells in your bone marrow, to some radiation. However, newer agents that emit short-range alpha particles limit that exposure significantly, damaging cancer cells while largely sparing surrounding tissue.
Cancers Treated With Systemic Radiation
Thyroid Cancer
Radioactive iodine (iodine-131) was the first radioactive substance used to treat cancer and remains a cornerstone of thyroid cancer care. After surgery to remove the thyroid gland, radioactive iodine destroys any remaining thyroid tissue and microscopic cancer cells. For straightforward remnant ablation, the standard dose is about 30 millicuries. When there is known or suspected metastatic disease, doses increase substantially, typically ranging from 100 to 200 millicuries depending on tumor burden and how far the cancer has spread.
Advanced Prostate Cancer
Two systemic radiation treatments are now FDA-approved for prostate cancer that has spread. Radium-223, approved in 2013, is a calcium mimic. Because bone naturally incorporates calcium, radium-223 homes in on areas of high bone turnover where prostate cancer metastases tend to grow. It emits alpha particles with a very short range, so the radiation stays concentrated in the bone microenvironment. Animal studies show that even high doses don’t significantly damage blood-forming marrow cells, which makes it notably gentler than many alternatives.
A newer option targets a protein called PSMA that sits on the surface of most prostate cancer cells. Lutetium-177 PSMA-617, approved in 2022, binds to this protein and delivers beta radiation directly to tumors wherever they are in the body. In studies of patients with metastatic castration-resistant prostate cancer, about 39% saw their PSA levels drop by at least 30% during treatment, and 29% experienced a drop of 50% or more.
Neuroendocrine Tumors
Lutetium-177 also plays a role in treating neuroendocrine tumors, which are slow-growing cancers that often have a specific receptor on their surface. A lutetium-177 compound that targets this receptor was approved in 2018 and has become a standard treatment for patients with advanced neuroendocrine tumors of the gastrointestinal tract and pancreas.
Other Uses
Iodine-131 is also used in a different formulation to treat pheochromocytoma (a rare adrenal gland tumor) in adults and neuroblastoma in children. Strontium-89 and yttrium-90 are additional FDA-approved radioactive substances used in various cancer settings.
What Treatment Feels Like
The actual administration is straightforward. For radioactive iodine, you typically swallow a capsule or liquid. For IV-based treatments like lutetium-177 or radium-223, you sit for an infusion that usually takes less than an hour. Most patients do not need to stay in the hospital for the infusion itself, though the schedule varies. Lutetium-177 PSMA therapy, for instance, is given in multiple cycles spaced several weeks apart.
The experience after treatment is where things get more involved. Your body will be temporarily radioactive, and you’ll need to follow specific precautions to protect the people around you.
Safety Precautions After Treatment
After receiving systemic radiation, particularly radioactive iodine, your body eliminates the radioactive material through urine, sweat, and other fluids, mostly within the first 48 hours. During this period, you may be restricted to your hospital room if you received a high dose. Specific guidelines include flushing the toilet two or three times after each use to clear radioactive urine, using disposable eating utensils that go into a special waste container, and sitting down on the toilet (regardless of sex) to prevent urine splatter.
Visitors need to keep a distance of at least three feet. Once you’re home, you’ll typically be asked to limit close contact with others, especially children and pregnant women, for a period your care team will specify based on your dose. These precautions sound intense but are temporary and well-established.
Side Effects
Side effects depend on which radioactive substance you receive and where it concentrates. With radioactive iodine, the salivary glands can absorb some of the iodine along with the thyroid tissue. This can cause swelling and tenderness after the first dose, progressing to dry mouth in some patients. Persistent dry mouth raises the risk of dental problems over time, so dental care becomes especially important.
For treatments that circulate more broadly, the most common concern is the effect on bone marrow. Radiation can temporarily reduce blood cell production, leading to low white blood cell counts (raising infection risk), low red blood cells (causing fatigue), or low platelets (increasing bruising or bleeding). Your care team will monitor blood counts regularly throughout treatment. With alpha-emitting agents like radium-223, this bone marrow suppression tends to be milder because the radiation range is so short.
There is also a small long-term risk of secondary cancers caused by the radiation exposure itself. The absolute risk is estimated at 0.2% to 1% per year among cancer survivors who received radiotherapy. These secondary cancers follow a two-peak pattern: blood cancers like leukemia can appear within the first three years, while solid tumors tend to emerge more than ten years after treatment.
Next-Generation Alpha Therapies
The newest frontier in systemic radiation involves alpha-emitting particles, which deposit far more energy over a shorter distance than beta emitters. This makes them extremely potent against tumor cells while causing less collateral damage. Radium-223 was the first alpha therapy approved, but several new alpha-emitting agents are now in clinical trials.
Actinium-225 is generating particular interest because its decay chain releases four alpha particles in sequence, delivering roughly 28 million electron volts of cumulative energy to the target area. Trials are testing actinium-225 linked to the same PSMA-targeting molecule used in the approved lutetium-177 therapy for prostate cancer, and another trial has reached phase III for neuroendocrine tumors that stopped responding to lutetium-177 treatment. Additional alpha emitters using astatine-211 and lead-212 are in early-phase trials across multiple cancer types, each with different advantages in terms of half-life and how predictably the radiation stays at the tumor site.