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

In prostate cancer, bone is the second most common site of metastatic disease after lymph nodes. This is related to a poor prognosis and is one of the major causes of morbidity and mortality in such patients. Early detection of metastatic bone disease and the definition of its extent, pattern, and aggressiveness are crucial for proper staging and restaging; it is particularly important in high-risk primary disease before initiating radical prostatectomy or radiation therapy. Different patterns of bone metastases, such as early marrow-based involvement, osteoblastic, osteolytic, and mixed changes can be seen. These types of metastases differ in their effect on bone, and consequently, the choice of imaging modalities that best depict the lesions may vary. During the last decades, bone scintigraphy has been used routinely in the evaluation of prostate cancer patients. However, it shows limited sensitivity and specificity. Single-photon emission computed tomography increases the sensitivity and specificity of planar bone scanning, especially for the evaluation of the spine. Positron emission tomography is increasing in popularity for staging newly diagnosed prostate cancer and for assessing response to therapy. Many positron emission tomography tracers have been tested for use in the evaluation of prostate cancer patients based on increased glycolysis (¹⁸F-FDG), cell membrane proliferation by radiolabeled phospholipids (¹¹C and ¹⁸F choline), fatty acid synthesis (¹¹C acetate), amino acid transport and protein synthesis (¹¹C methionine), androgen receptor expression (¹⁸F-FDHT), and osteoblastic activity (¹⁸F-fluoride). However, there are presently no accurate imaging modalities to directly, reproducibly, and effectively delineate bone metastases in prostate cancer.