Dual-Energy X-ray Absorptiometry (DEXA) is a specialized imaging technique designed to measure Bone Mineral Density (BMD). It is the standard method for assessing a person’s risk for osteoporosis and bone fractures. While DEXA focuses on skeletal health, it is not a routine cancer screening tool. However, since cancer and its treatments can profoundly affect the skeleton, the scan may provide information that prompts further evaluation for malignancy.
How DEXA Scans Measure Bone Health
The DEXA machine uses two distinct low-dose X-ray energy beams aimed at the patient’s bones. By measuring how much of each beam is absorbed by the tissue, the machine can subtract the absorption by soft tissue, leaving a calculated value for the bone mineral content. This technique generates a measurement called areal Bone Mineral Density (aBMD), which is the most widely used metric for bone strength. The scan most commonly focuses on the hip and the lumbar spine, which are areas highly susceptible to fracture.
The results of the scan are expressed using two scores: the T-score and the Z-score. The T-score compares the patient’s BMD to that of a healthy young adult of the same sex, and is used to diagnose osteoporosis in postmenopausal women and men over 50. The Z-score compares the patient’s bone density to that of people the same age, sex, and ethnicity, and is often used for younger patients or those with underlying conditions.
A low T-score indicates low bone mass, signaling an increased risk of fracture. The process is quick and non-invasive, taking 10 to 20 minutes. DEXA measures density, but it does not provide the detailed internal structure or soft-tissue visualization that a CT or MRI scan offers.
Incidental Findings: When Cancer Affects Bone Density
While the DEXA scan is not designed for tumor detection, cancer can alter bone density in ways that are visible as abnormal patterns on the resulting image. When cancer metastasizes, or spreads, to the bone, it disrupts the natural balance of bone breakdown and formation. These changes in bone metabolism can manifest as localized areas of abnormal density, which a radiologist may flag for further, more specific imaging.
The two main ways that cancer affects bone tissue are through lytic and blastic lesions. Lytic lesions occur when cancer cells stimulate osteoclasts, the cells that break down bone, leading to the destruction of bone material and a localized area of very low density. Cancers such as breast, lung, kidney, and multiple myeloma often cause this type of bone destruction, appearing on the DEXA scan as an unexpected dip in BMD.
Conversely, blastic lesions occur when cancer cells stimulate osteoblasts, the cells responsible for building bone, resulting in abnormal, disorganized bone formation. This process creates localized areas of unusually high density that stand out against the surrounding bone. Prostate cancer is a common example of a malignancy that produces blastic lesions. The presence of an irregular Z-score or noticeable asymmetry in bone density can prompt a recommendation for a nuclear bone scan or CT to characterize the abnormality.
Monitoring Bone Health During Cancer Therapy
Once a patient has a cancer diagnosis, the DEXA scan shifts its role to monitoring the skeletal side effects of treatment. Many effective cancer therapies can accelerate bone loss, a condition known as treatment-induced bone loss. Monitoring bone density is an important part of long-term cancer care to prevent fractures.
Hormone ablation therapy, common for hormone-sensitive malignancies like breast and prostate cancer, is a concern. By suppressing hormones like estrogen or testosterone, these treatments rapidly compromise bone health, as these hormones normally help maintain density. For example, aromatase inhibitors used for breast cancer can quickly accelerate bone thinning.
DEXA scans establish a baseline BMD before starting such treatments and are used for periodic follow-up, typically every one to two years. Tracking these scores allows oncologists to determine if bone loss is severe enough to warrant intervention. This intervention often involves starting bone-strengthening medications like bisphosphonates or denosumab.