Diagnosis of Occult Bone Metastases: Positron Emission Tomography

The current standard of practice for the detection of osseous metastatic disease is the conventional bone scan of the entire body using technetium-99m methylene diphosphonate (Tc-99m MDP). Although Tc-99m MDP scintigraphy is sensitive for the detection of advanced skeletal metastatic lesions, early involvement may be missed because this technique relies on the identification of the osteoblastic reaction of the involved bone rather than the detection of the tumor itself. Positron emission tomography (PET) has proven to be the gold standard in metabolic imaging. Fluorine -18 deoxyglucose (FDG) provides a means of quantitating the glucose metabolism, with the amount of tracer accumulation reflecting the glucose metabolism: high-grade malignancies tend to have higher rates of glycolysis than do low-grade malignancies and benign lesions; therefore, high-grade malignancies have greater uptake of FDG than that of low-grade or benign lesions. Positron emission tomography has been shown to be superior to scintigraphy in the detection of metastases because it detects the presence of tumor directly by metabolic activity, rather than indirectly by showing tumor involvement due to increased bone mineral turnover. This has allowed the detection of metastatic foci earlier with PET than with bone scintigraphy. Although the spectrum of PET applications is unknown, it now is approved for the diagnosis, staging and restaging of many common malignancies and has shown efficacy for the detection of osseous metastasis from several malignancies including lung carcinoma, breast carcinoma, and lymphoma.