Bone spot found by chance: follow-up scans

The main guidance

Society of Skeletal Radiology 2022 (Bone-RADS): a reporting system for a single bone spot found by chance on CT or MRI in adults, with four steps from leave alone to specialist referral. (SSR 2022) It does not cover children or people with several spots, who usually need a different work-up. (SSR 2022)
Canadian Association of Radiologists 2023: recommendations for bone and other muscle and joint findings on chest and belly CT or MRI, including focal bone spots and marrow changes, and when they should lead to more tests. (CAR 2023)
Chest CT review 2022: a step-by-step approach for bone findings seen on chest CT, from "do not touch" spots to those that need more imaging. (Mansoori 2022)

MedlinePlus notes that a bone tumor can be benign or cancerous, that some benign bone tumors have no symptoms, and that a benign one may be watched with regular imaging such as X-rays. (MedlinePlus)

The four Bone-RADS steps

Bone-RADS categories for a single bone spot found by chance in an adult (simplified)
Category	Meaning	Usual next step
Bone-RADS 1	Likely benign, such as a bone island or a non-ossifying fibroma	Leave alone
Bone-RADS 2	Not fully assessed on this scan	A different kind of scan
Bone-RADS 3	Unclear (indeterminate)	Repeat the same scan at 6, 6 and 12 months, about 2 years in total
Bone-RADS 4	Suspicious, or may need treatment	Further tests or referral to a bone tumor specialist team

Source for the table: Table 1 and the management section of the white paper. (SSR 2022) The authors weighted the system towards finding spots that can be left alone, and say it is a framework that does not override expert opinion or the clinical picture. (SSR 2022)

Spots usually left alone

Bone island (enostosis): a dense spot; on CT a mean reading above 885 Hounsfield units is a strong sign of a bone island, though the number counts for less in older people. (SSR 2022)
Spots that contain fat: on CT or MRI these are often benign, for example a fatty spot (intraosseous lipoma) or a blood vessel spot (hemangioma). (SSR 2022)
Other classic benign spots with typical features, such as a non-ossifying fibroma. (SSR 2022)

These steps apply when there is no pain from the spot, no concerning features on the scan and no history of a cancer that often spreads to bone. (SSR 2022)

Cartilage spots (enchondroma)

Small cartilage spots are often seen on knee and shoulder MRI. (Danish guideline 2020)(Stomp 2015) Because an enchondroma can be hard to tell apart from a low-grade cartilage cancer, it is often followed over time. (Danish guideline 2020) Bone-RADS looks for signs of active growth, such as deep thinning of the bone wall from inside, bone expansion, a break through the bone wall or a soft tissue mass, and says size alone should not decide. (SSR 2022) A cartilage spot in a flat bone such as the pelvis, ribs or breastbone is a concerning feature. (SSR 2022)

What changes the plan

Pain that comes from the spot moves it to the suspicious group. (SSR 2022)
Concerning scan features: involvement of the bone wall, spread into nearby soft tissue, a break in the bone through the spot (pathologic fracture), an aggressive reaction of the bone lining or swelling of the marrow around it. (SSR 2022)
A history of cancer that often spreads to bone (kidney, prostate, breast, lung or thyroid): another kind of scan or repeat scans rather than leave alone. (SSR 2022)
Growth or new worrying features on a repeat scan are what the repeat scans look for. (SSR 2022)
More than one spot falls outside Bone-RADS and usually needs a different work-up. (SSR 2022)

MedlinePlus, when to contact a medical professional: "Contact your provider if you have symptoms of a bone tumor." (MedlinePlus) MedlinePlus lists a bone that breaks after a slight injury, bone pain that may be worse at night, and sometimes a lump or swelling at the spot. (MedlinePlus)

Routine age-based screening

A bone spot found by chance does not replace the routine screenings for your age. A bone spot is not a bone density result; the Canadian recommendations also discuss using CT scans to flag possible thin bones (osteoporosis). (CAR 2023) For bone density, see bone density test results and how often to have a bone density test. For other findings on a scan, see lung nodule found by chance and pituitary spot found by chance.

Where they differ:

How to repeat scans for an unclear spot. Bone-RADS lists the same CT or MRI at 6, 6 and 12 months, about 2 years in total; a clinic series on long bone enchondromas proposed X-rays every 3 to 6 months for the first year, then yearly for at least 3 years in total, and limiting MRI when there is no concern. (SSR 2022)(Akoh 2019)
Whether evidence settles the schedule. The Danish guideline says there is no agreed evidence on when and how to follow an enchondroma. (Danish guideline 2020)
The role of size. Bone-RADS says size alone should not decide whether a cartilage spot is aggressive; the Dutch knee MRI study referred spots larger than 20 mm for more checks. (SSR 2022)(Stomp 2015)

Your doctor decides what your result means for you.

Questions people ask

How common is a bone spot found by chance?

Common. In a Dutch study of adults aged 45 to 65 who had a knee MRI, about 2.8% had a small cartilage spot (enchondroma type) in the bone, most of them small and without worrying features. (Stomp 2015)

What is a bone island?

A bone island (enostosis) is a small patch of dense, normal bone inside a bone. The Society of Skeletal Radiology lists it among the clearly benign spots that are left alone, and notes that a very dense reading on CT (a mean above 885 Hounsfield units) points strongly to a bone island, with less weight on that number in older people. (SSR 2022)

Can an enchondroma turn into cancer?

Rarely. The Danish guideline notes that an enchondroma can change into a low-grade cartilage cancer and is hard to tell apart on scans, which is why it is often followed. In one clinic series, 3 of 55 long bone enchondromas grew, at about 2 years. (Danish guideline 2020) (Akoh 2019)

Why does a history of cancer matter?

Some cancers often spread to bone: kidney, prostate, breast, lung and thyroid. For a person with one of these, Bone-RADS does not simply leave an unclear spot alone; it suggests another kind of scan or repeat scans. (SSR 2022) (MedlinePlus)

How long do repeat scans go on?

For an unclear spot, Bone-RADS lists repeat scans at 6, 6 and 12 months, about 2 years in total. A clinic series on enchondromas proposed X-rays for at least 3 years in total. (SSR 2022) (Akoh 2019)