Lung, thyroid, renal cancer, myeloma and neuroendocrine cancers: Role of planar, SPECT and PET in imaging bone metastases

Abstract

This chapter will consider the role of planar scintigraphy, SPECT and PET in the imaging of skeletal metastatic disease from a miscellaneous group of malignancies, including lung, thyroid and renal carcinomas; multiple myeloma; and neuroendocrine tumours, and will examine how recent technical advances may enhance their effectiveness in this field. Bone scintigraphy using technetium-labelled diphosphonates has for many years been the mainstay of functional imaging of bony metastases, but this technique is of limited value in evaluating myeloma and aggressive osteolytic metastases and also carries the drawback of relatively poor specificity. Single photon emission computed tomography (SPECT), being a tomographic imaging technique, produces three-dimensional images of tracer distribution from multiplanar images. Its application to bone scintigrams in selected cases can greatly improve accurate anatomical localisation 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 tumours, including somatostatin analogues for neuroendocrine tumours. The advent of combined SPECT/CT systems in recent years has further enhanced the accuracy of SPECT in all these malignancies. Positron emission tomography (PET) detects pairs of gamma rays emitted indirectly by a positron-emitting radiotracer to achieve a higher spatial resolution than single photon imaging. This high resolution and the ability to cover the entire body in a single scan have made it a highly effective technique for the evaluation of skeletal metastatic disease, and it is now a routine combination with CT as PET/CT can provide combined functional and anatomical imaging data in unprecedented clarity and detail. 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 commonest PET radiotracer, 18F-FDG is dependent on the increased cellular metabolism of most tumours; it may enable earlier detection of metastatic foci than bone scintigraphy, which relies on detecting osteoblastic activity which may be difficult to visualise in the case of small metastases. Another significant advantage of 18F-FDG PET is that it can detect the soft tissue components of metastases. This is particularly important in aggressive osteolytic metastases, where invasion of adjacent connective tissue and muscle is frequent. The effectiveness of 18F-FDG PET is limited in slow-growing tumour types, but 18F-sodium fluoride, a bone radiotracer which can detect very early osteoblastic changes, already shows promise in this area. Bony metastases from many neuroendocrine tumours can be detected with a high degree of specificity by PET using somatostatin analogues. Other novel and often highly specific radiotracers are under evaluation which will further enhance its diagnostic capability. The true potential of PET in this group of malignancies is gradually unfolding; although studied series of patients remain generally small to date, a more detailed evaluation of its role will require the accrual of considerably more data.