A bone scan is an imaging test used to determine if prostate cancer has spread to the skeletal system, a common site for metastasis. This test involves injecting a small amount of radioactive material, called a tracer, into a vein to create a detailed picture of the entire skeleton. A bone scan is not routinely performed for all newly diagnosed prostate cancer cases. Instead, it is reserved for situations where the risk of the cancer having already spread is considered elevated.
When Bone Scans Are Necessary for Prostate Cancer
The decision to order a bone scan is based on a patient’s risk stratification, which uses clinical factors to predict the likelihood of the cancer having spread outside the prostate. Medical organizations like the American Urological Association (AUA) and the National Comprehensive Cancer Network (NCCN) provide clear guidelines on which patients require this imaging. For asymptomatic patients with low-risk or favorable intermediate-risk prostate cancer, a bone scan is typically not recommended because the chance of finding bone metastasis is extremely low.
The scan becomes necessary for patients with higher-risk disease, as prostate cancer frequently metastasizes to the bones. Indicators that suggest a higher risk include a Prostate-Specific Antigen (PSA) level above 20 nanograms per milliliter (ng/mL) at the time of diagnosis. High-grade disease, indicated by a Grade Group of 4 or 5 (Gleason score 8 to 10), also prompts the use of a bone scan regardless of the PSA level. Patients who present with symptoms consistent with bone pain, such as new or localized back, hip, or rib discomfort, should also receive a bone scan to investigate a possible spread.
How the Bone Scan Procedure Works
The procedure, technically known as bone scintigraphy, begins with an intravenous injection of a small dose of a radiotracer, commonly Technetium-99m labeled methylene diphosphonate (Tc-99m MDP). This tracer travels through the bloodstream and is selectively absorbed by areas of the bone that are undergoing rapid turnover or repair. The patient then waits for a period, usually between two and four hours, allowing the radiotracer time to clear from the soft tissues and accumulate within the bone structure.
Following the waiting period, the patient lies on a table while a specialized device called a gamma camera passes over the body. This camera detects the gamma rays released by the tracer that has settled in the bones. The collected data is used to create a two-dimensional image of the entire skeleton, revealing areas where the radiotracer uptake is concentrated. The process is painless and generally takes about 30 to 60 minutes to complete.
Understanding What the Scan Reveals
The resulting bone scan image shows the distribution of the radiotracer throughout the skeleton, with normal bone appearing as a uniform, light gray background. Areas where the tracer has accumulated in higher-than-normal amounts are described as “hot spots” because of the intense signal they produce. These hot spots indicate increased osteoblastic activity, which is the bone’s reaction of forming new cells. Prostate cancer cells that have metastasized typically provoke this blastic reaction, causing the tracer to concentrate in those sites.
A hot spot is not definitive proof of cancer, as the bone’s repair process can also be triggered by benign conditions.
Common Causes of Non-Cancerous Hot Spots
- Arthritis
- Recent or old fractures
- Infections
- Other degenerative joint diseases
If a suspicious lesion is found, a radiologist may recommend correlation with other imaging, such as an X-ray or Magnetic Resonance Imaging (MRI), to help distinguish between a benign injury and a malignant skeletal metastasis. The presence of skeletal metastasis confirms the cancer is at an advanced stage (Stage IV), shifting the treatment focus toward systemic treatments that address disease throughout the body.
Alternative Imaging Methods for Staging
While the traditional bone scan is an established tool, it is often used alongside or replaced by other imaging methods for a comprehensive staging evaluation. A Computed Tomography (CT) scan is commonly used to visualize soft tissues and lymph nodes in the abdomen and pelvis, which are common sites for prostate cancer to spread. Magnetic Resonance Imaging (MRI) provides detailed images of the prostate and surrounding pelvic structures, aiding in the assessment of local tumor extension.
Newer technologies, particularly Prostate-Specific Membrane Antigen Positron Emission Tomography (PSMA PET) scans, are becoming increasingly common for high-risk staging. The PSMA PET scan uses a tracer that specifically targets a protein highly expressed on the surface of prostate cancer cells, offering superior sensitivity for detecting small metastases in both soft tissue and bone compared to conventional imaging. This newer modality can often detect metastatic disease at lower PSA levels than a traditional bone scan, and it provides a comprehensive, one-visit assessment of local, nodal, and distant disease spread.