Miscellaneous Cancers (Lung, Thyroid, Renal Cancer, Myeloma, and Neuroendocrine Tumors): Role of SPECT and PET in Imaging Bone Metastases

In this review, we assess the current role of single-photon emission computed tomography (SPECT) and positron emission tomography (PET) in the imaging of skeletal metastatic disease from a miscellaneous group of malignancies, including lung, thyroid, and renal carcinomas; multiple myeloma; and neuroendocrine tumors, and consider how recent advances may enhance their effectiveness in this area. Bone scintigraphy using technetium-labeled diphosphonates has long been the mainstay of functional imaging of bony metastases, but is of limited value in myeloma and aggressive osteolytic metastases, and has the limitation of relatively poor specificity. SPECT, as a tomographic imaging technique, produces three-dimensional images of tracer distribution from multiplanar images. Its application to bone scintigrams greatly aids accurate anatomic localization and sensitivity in detection of foci of tracer uptake. SPECT can equally be applied to scintigrams using radiotracers, which are specific for particular groups of tumors, such as somatostatin analogs for neuroendocrine tumors. The advent of combined SPECT/computed tomography (CT) systems has further enhanced the accuracy of SPECT in all these malignancies. PET uses positron-emitting radiotracers and achieves a higher spatial resolution than single-photon imaging. Its high resolution and coverage of the entire body have made it a highly effective technique for the evaluation of skeletal metastatic disease, particularly when combined with CT. ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG)-PET/CT now forms part of routine staging for many carcinomas, such as non–small-cell lung carcinomas, and may obviate the need for routine staging scintigraphy in these patients. As uptake of the most common PET radiotracer, ¹⁸F-FDG, is dependent on the increased cellular metabolism of most tumors, it may enable earlier detection of metastatic foci than bone scintigraphy, which relies on detecting an osteoblastic response. Another significant advantage of ¹⁸F-FDG-PET is that it can detect soft-tissue components of metastases, which is particularly important in aggressive osteolytic metastases. The effectiveness of ¹⁸F-FDG-PET is limited in slow-growing tumor types, but ¹⁸F-sodium fluoride, a bone radiotracer that can detect early osteoblastic changes, shows promise in this area. Bony metastases from many neuroendocrine tumors can be detected with a high degree of specificity by PET using somatostatin analogs. Other novel and often highly specific radiotracers are under evaluation, which will further enhance the diagnostic capability of PET. The true potential of PET in this group of malignancies is gradually unfolding, although studied series of patients remain generally small and much further evaluation of its role is required.