Skeletal metastases: what is the future role for nuclear medicine?

Abstract

Although planar bone scintigraphy with Tc-labelled bisphosphonates has been the mainstay for imaging skeletal metastases for nearly four decades, it has been limited by poor specificity and has been losing ground to the improving anatomical imaging modalities such as multislice CT and newly developing whole body MRI imaging techniques [1, 2]. The addition of SPECT to planar bone scintigraphy improves sensitivity and specificity to a degree [3] but has not been able to negate the need for anatomical imaging correlation, leading to extra imaging procedures for the patient and a subsequent delay in clinical management decisions. The advent of hybrid SPECT/CT and PET/ CT would therefore seem to be a natural progression towards optimal assessment of skeletal metastases, incorporating both functional information for sensitivity and anatomical information for specificity in a single comprehensive test. Hybrid MRI scanners are also in development and may have more to offer in view of MRI’s particular sensitivity for bone-marrow-based disease. Hybrid SPECT/CT with diagnostic quality CT has partly been driven by the success of PET/CT, and as a relatively recently available modality, the evidence for incremental benefit in imaging bone metastases is only just starting to accrue [4–7]. Utsunomiya et al. have shown an incremental benefit of fused SPECT/CT compared to separate SPECT and CT images or planar and SPECT images [4]. Fused images allowed greater diagnostic certainty with fewer equivocal lesions, better interobserver agreement and greater overall diagnostic accuracy in diagnosing benign and malignant bone lesions as measured by ROC analysis. Fused SPECT/ CT allowed indeterminate lesions to be classified as benign or malignant in 17/82 lesions for one observer and 8/82 for another observer. The correct classification of benign lesions was more frequent for both observers, thus predominantly improving specificity. In a similar retrospective study of bone SPECT/CT, Romer et al. analysed 52 indeterminate skeletal lesions on SPECT in 44 patients being staged for malignancy [5]. Approximately two-thirds of lesions were classified as benign and in only four lesions was it not possible to classify them as benign or malignant with SPECT/CT. As bone scintigraphy remains in frequent use and is readily available for staging many cancers, it seems likely that it will continue to have a significant and enhanced clinical role in its new form of SPECT/CT as combined SPECT/CT scanners become more widely available. However, interest in evaluating skeletal metastases with PET/CT techniques is increasing. It is possible to take advantage of two aspects of metabolic changes associated with bone metastases with two readily available PET tracers, namely F-fluoride and FDG. With a similar mechanism of uptake to Tc-labelled bisphosphonates, F-fluoride has been available as a bone-specific tracer for many years and the properties of FDG as a tumourspecific agent are well recognised. This Editorial Commentary refers to the article doi:10.1007/s00259009-1181-2