Breast Cancer: Role of SPECT and PET in Imaging Bone Metastases

Breast cancer is the most common cause of bone metastases in women. Imaging studies are useful to identify bone involvement and associated complications, for follow-up of disease spread and for the assessment of response to therapy. Bone scintigraphy with ^99mtechnetium-labeled diphosphonates is most widely used, due to its availability, high sensitivity, and low cost, despite the relatively low specificity. The addition of single-photon emission computed tomography and recently single-photon emission computed tomography/computed tomography improves the diagnostic accuracy of this modality. Serial follow-up scans can demonstrate disease progression, but this method is less accurate in determining response to treatment. Positron emission tomography (PET), a tomographic modality with improved resolution shows improved sensitivity and specificity. ¹⁸F-fluorodeoxyglucose (FDG)-PET is the most common clinically used procedure. FDG is taken up by the tumor cells and has therefore the advantage of demonstrating the presence of disease in both bone and soft tissues. FDG-PET is highly sensitive mainly in diagnosis of early metastatic disease, which may still be confined to the bone marrow, as well as for the detection of lytic bone metastases and can be also reliably used to monitor response to therapy. For the detection of sclerotic lesions, however, imaging with a bone-seeking tracer such as ¹⁸F-fluoride, may have a complementary role. As a nonspecific skeletal imaging tracer, ¹⁸F-fluoride has great potential, being more sensitive than bone scintigraphy and when PET/computed tomography is performed it is highly accurate for detection of both lytic and sclerotic lesions and to distinguish benign from malignant skeletal findings.