A bone scan is a nuclear imaging test used to see if cancer has spread, or metastasized, to the bones. Detecting this spread early can inform treatment decisions. The scan provides a detailed picture of the entire skeleton, allowing doctors to check for abnormalities that might indicate cancer. It is used when a patient with cancer experiences unexplained bone pain or has a cancer type known to spread to bone.
The Bone Scan Procedure
The procedure begins with an injection of a small, safe amount of a radioactive substance, called a radiotracer, into a vein in the arm. This substance travels through the bloodstream and accumulates in the bones over two to four hours. During this waiting period, patients are encouraged to drink water to help flush any unabsorbed tracer from their system to ensure the clearest possible images.
After the waiting period, the patient lies still on a table while a gamma camera moves slowly over their body. The camera detects radiation from the tracer that has collected in the skeleton, creating images of the bones. The painless scan takes 30 to 60 minutes, and the patient must remain still to avoid blurring the images.
Interpreting Normal Scan Images
When a radiologist examines the results, they first look for signs of a healthy skeleton. A normal or “negative” scan is characterized by a uniform distribution of the radiotracer, which makes the skeleton appear as a consistent, light gray color. This uniformity indicates that bone metabolism is proceeding at a normal rate.
Slight variations in darkness can be seen in a normal scan. For example, areas around joints may appear slightly darker, reflecting a higher rate of bone turnover in these locations. The bladder also appears dark as it collects excess radiotracer being flushed from the body. A scan with these characteristics suggests the absence of widespread bone abnormalities.
Identifying Abnormalities on Scan Images
Abnormalities on a bone scan appear as “hot spots,” which are dark areas where the tracer has accumulated in high concentrations. This buildup occurs in locations with increased metabolic activity or rapid bone repair. When cancer cells invade bone, they disrupt the normal structure, causing the bone to try and rebuild itself, which attracts the tracer.
The intensity of the hot spot can correlate with the level of metabolic activity in that part of the bone. While less common, a scan may also reveal “cold spots,” which are lighter areas where the tracer has not been taken up. These spots can indicate a lack of blood supply to the bone or be caused by certain cancers like multiple myeloma.
Differentiating Cancer from Other Conditions
The presence of hot spots on a bone scan does not automatically mean cancer has spread to the bones, as they can be caused by non-cancerous conditions. The scan highlights areas of bone stress or repair, which can have multiple causes. For this reason, an abnormal scan is often the starting point for further investigation.
Common conditions that produce hot spots include arthritis, healing bone fractures, and bone infections (osteomyelitis). Each of these can lead to increased metabolic activity that appears as a dark spot. Other metabolic bone diseases can also mimic the appearance of cancer, requiring additional tests for a definitive diagnosis.