A bone density test, formally known as a Dual-Energy X-ray Absorptiometry (DEXA or DXA) scan, is a non-invasive imaging tool designed to measure bone mineral density (BMD). This measurement represents the amount of calcium and other minerals packed into a segment of bone. The test uses low-dose X-rays to generate images, primarily of the hip and spine. The primary utility of this technology is to evaluate the risk for fractures and to diagnose conditions associated with generalized bone loss, particularly osteoporosis.
The Primary Purpose of Bone Density Testing
The intended function of a DEXA scan is to diagnose osteoporosis, a condition characterized by low bone mass and the structural deterioration of bone tissue. The results are presented as standardized scores that compare a patient’s bone density to established norms, determining whether intervention is necessary to reduce the likelihood of future bone fractures.
The primary measurement is the T-score, which compares the patient’s BMD to that of a healthy young adult at peak bone mass. A T-score of -2.5 or lower is the diagnostic threshold for osteoporosis in postmenopausal women and men aged 50 and older. Scores between -1.0 and -2.5 indicate low bone mass, known as osteopenia.
Another element is the Z-score, which compares the patient’s BMD to that of people of the same age, sex, and ethnic background. This score is particularly useful for premenopausal women, men under 50, and children. A Z-score significantly below the expected range, often less than -2.0, may suggest that an underlying medical condition, other than typical aging, is the cause of the low bone density, prompting a search for secondary causes.
Indicators of Abnormal Bone Health on a DEXA Scan
While the DEXA scan cannot definitively identify cancer cells, it can reveal abnormalities in bone density that warrant further medical investigation. Cancer, whether a primary bone tumor or a metastasis, affects the balance of bone destruction and formation, manifesting as an unexpected change in measured density.
Cancers that spread to the bone can produce two main types of lesions: lytic and blastic. Lytic lesions are bone-destroying, resulting in areas of significantly reduced density. Conversely, blastic lesions are bone-forming, stimulating excessive, disorganized bone growth, leading to areas of abnormally high density.
A localized area of extreme density change, known as a focal lesion, contrasts sharply with the generalized, symmetrical bone loss seen in typical osteoporosis. The detection of a rapid, unexplained decline in BMD, especially in a specific region, or a very low Z-score, often alerts a physician to the possibility of an underlying malignancy. Such findings signal the need to look beyond common age-related bone loss.
Definitive Tests Used to Diagnose Bone Cancer
When a DEXA scan or other clinical symptoms suggest a malignancy, physicians turn to more sophisticated diagnostic tools that can visualize soft tissue and confirm the presence of cancer cells. These tests provide the detailed anatomical and functional information that the density-focused DEXA scan lacks.
Nuclear medicine bone scans are often used to screen the entire skeleton for metastatic activity. This involves injecting a radioactive tracer that collects in areas of rapid bone turnover, which appear as “hot spots” on the scan. These hot spots can indicate cancer, but also other conditions like infection or fracture, requiring additional investigation.
More detailed imaging, such as Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scans, is used to examine specific lesions identified. MRI is effective for visualizing soft tissue components, including bone marrow and the extent of the tumor. CT scans offer high-resolution images of the bone structure, helping to guide the most definitive test for cancer: the biopsy.
A biopsy remains the gold standard for confirming a cancer diagnosis. During this procedure, a surgeon removes a small sample of the suspicious tissue, often guided by CT imaging, for laboratory analysis. A pathologist then examines the cells under a microscope to determine if they are malignant and to identify the type and grade of the cancer.