A bone density test does not diagnose cancer, but it may show evidence that cancer has spread to the bones

A bone density test (also called a DXA scan or DEXA scan) measures how much mineral is packed into your bones. It is designed to detect osteoporosis—a condition where bones become brittle and break more easily. The test cannot identify cancer cells or tell you whether you have cancer.

However, if you already have a cancer diagnosis, a bone density test might incidentally show changes that suggest cancer has spread to your bones. That said, a bone density test is not the tool doctors use to look for bone metastases (cancer that has spread). They use different imaging—CT scans, MRI, or PET scans—which are designed to spot tumors and abnormal tissue.

The confusion often arises because both tests involve imaging your skeleton, but they answer different questions. A bone density test answers: "Are my bones weak?" A cancer staging scan answers: "Has the cancer spread, and where?"

Key Takeaways

  • A bone density test measures bone strength and screens for osteoporosis; it is not a cancer screening or diagnostic tool.
  • If cancer has already spread to bone, a bone density test might show abnormal density, but doctors would not rely on it to detect metastases.
  • Doctors use CT, MRI, or PET scans—not bone density tests—when they are looking for cancer that may have spread to bone.
  • Bone density tests are routine for people over 65 and for younger people at risk of osteoporosis, regardless of cancer status.

How a bone density test works and what it measures

A bone density test uses low-dose X-rays to measure the amount of calcium and other minerals in a small section of bone, usually the hip, spine, or forearm. The machine compares your bone density to that of a healthy young adult and produces a score called a T-score. A normal T-score is -1.0 or higher. A score between -1.0 and -2.5 indicates low bone mass (sometimes called osteopenia). A score below -2.5 indicates osteoporosis.

The test takes about 10 to 30 minutes and involves no injection or contrast dye. You lie still on a table while the scanner passes over your bones. The radiation exposure is minimal—much less than a chest X-ray.

The test is painless and non-invasive, which is why it is used so widely for routine screening. But because it only measures density, not the structure or composition of bone tissue, it cannot detect tumors or cancer cells.

Why bone density tests cannot show cancer

Cancer cells in bone do not necessarily change the mineral density that a bone density test measures. Some cancers that spread to bone make bones denser (osteoblastic metastases), while others make them weaker and more porous (osteolytic metastases). A bone density test might pick up one type but miss the other, and it would not tell you whether the change was caused by cancer, osteoporosis, Paget's disease, or another condition entirely.

More importantly, a bone density test shows only the bones it scans—usually just the hip and spine. If cancer has spread to other bones (ribs, pelvis, femur, skull), the test would not detect it. A cancer staging scan, by contrast, images the entire body or large sections of it, allowing doctors to see where tumors are located.

Bone density tests also lack the resolution to show individual lesions or tumors. They produce a single number (the T-score) that reflects overall mineral content in the scanned area, not a detailed picture of bone structure.

What imaging doctors actually use to check for bone cancer or metastases

When a doctor suspects cancer has spread to bone, they order one of three types of imaging:

  • CT scan (computed tomography): Creates detailed cross-sectional images of bone and soft tissue. Good for showing the size and location of lesions and whether they are osteoblastic or osteolytic.
  • MRI (magnetic resonance imaging): Uses magnetic fields and radio waves to create very detailed images of bone marrow and soft tissue. Particularly useful for detecting early metastases and assessing spinal involvement.
  • PET scan (positron emission tomography): Uses a radioactive tracer to show areas of high metabolic activity, which often indicates cancer. Can reveal metastases before they show up on CT or MRI.

A doctor may order more than one type depending on the cancer type, the symptoms, and what information is needed to plan treatment. Bone scans (which use a different radioactive tracer) are also sometimes used, though they are less common now.

When bone density tests are recommended and who should have them

Bone density testing is recommended for all women age 65 and older and all men age 70 and older, regardless of cancer history. Younger people may be screened if they have risk factors such as a family history of osteoporosis, long-term use of corticosteroids, rheumatoid arthritis, or a personal history of fractures.

Some cancer treatments—particularly hormone therapies and chemotherapy—can weaken bones over time. If you are undergoing cancer treatment or have finished treatment, your oncologist may recommend a bone density test to monitor bone health and assess fracture risk. This is separate from imaging to check whether cancer has spread.

The test is also used to monitor the effectiveness of osteoporosis treatment and to track bone loss in people taking medications that affect bone metabolism.

What to do if a bone density test shows abnormal results

If your bone density test shows low bone mass or osteoporosis, your doctor will discuss treatment options, which may include calcium and vitamin D supplementation, weight-bearing exercise, and medications such as bisphosphonates. An abnormal result does not mean you have cancer.

If you have a cancer diagnosis and your bone density test shows unexpected changes, tell your oncologist. They can determine whether the change is related to your cancer, your cancer treatment, or an unrelated condition like osteoporosis. They may order additional imaging if they have concerns about metastases, but they would not rely on the bone density test alone to make that information.

If you are concerned about whether cancer has spread to your bones, the conversation to have with your doctor is about staging scans (CT, MRI, or PET), not bone density testing.

Frequently Asked Questions

Can a bone density test detect cancer in the bones?

No. A bone density test measures mineral content in bone, not the presence of cancer cells. It cannot diagnose cancer or detect tumors. If cancer has spread to bone, a bone density test might show abnormal density, but doctors would use CT, MRI, or PET scans to actually locate and assess metastases.

If I have cancer, do I still need a bone density test?

That depends on your age, your cancer type, and your treatment. Some cancer treatments weaken bones, so your oncologist may recommend bone density screening to assess fracture risk. This is different from imaging to check whether cancer has spread. Ask your oncologist whether a bone density test is appropriate for you.

What should I ask my doctor if I'm worried cancer has spread to my bones?

Ask whether you need staging imaging such as a CT, MRI, or PET scan. Describe any symptoms you have—bone pain, swelling, or fractures—because these may prompt imaging. Do not assume a bone density test will answer this question; it is designed for osteoporosis screening, not cancer detection.

Are bone density tests safe if I have cancer?

Yes. Bone density tests use very low-dose radiation and are safe for people with cancer. The radiation exposure is minimal and much lower than a chest X-ray. If you are pregnant or think you might be, tell your doctor before the test.