2 Bone Scintigraphy: Bone scintigraphy continues to be the most widely used radionuclide technique for investigation of skeletal metastasis primarily due to its widespread availability (2) . Radiotracer uptake depends on local blood flow, osteoblastic activity and extraction efficiency. Once accumulated in bone diphosphonate are absorbed by hydroxyapatite crystals on mineralizing bone surfaces (3) . A major advantage of radionuclide bone scanning is that imaging of the whole skeleton can be

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... ormed this is important given that metastatic lesions can occur in regions of the appendicular skeleton that are not routinely included in a skeletal survey (4) . A further advantage relates the high sensitivity of scintigraphy which enables earlier detection of osseous metastases. The sensitivity and specificity of bone scintigraphy for detection of bone metastasis is 78% and 48%, respectively. In particular, studies indicate that only a 5%-10% alteration in the ratio of lesion to normal bone is necessary to manifest abnormal tracer accumulation on a bone scan. As a result, osteosclerotic bone metastases can be detected on bone scintigraphy up to 18 months earlier than on plain radiographs (5) . Limitations of skeletal scintigraphy: Bone scintigraphy is non-specific and multiple benign osseous lesions, such as eosinophilic granuloma fibrous dysplasia and enchondroma, can lead to a false positive diagnosis with similar pattern as bone metastasis (6) . Interpreting focal accumulation of radiotracer in the spine can be particularly problematic as degenerative disease may be indistinguishable from bone metastases. Consequently, other imaging modalities such as plain radiography, CT or MRI are often required for correlation to exclude benign causes. Secondly, the spatial resolution of scintigraphy is poor measuring approximately 1 cm and can result in difficulty determining the precise location of a lesion within a bone which can be of diagnostic significance (2) . Thirdly, bone scintigraphy assesses osteoblastic processes rather than tumour proliferation and, consequently, false negative results can occur (7) . Frangos S, et al,. Giannopoulou, and S. Lutz, -Bone metastases: assessment of therapeutic response through radiological and nuclear medicine imaging modalities.‖ Clin. Oncol. vol. 23, no. 9, pp. 632-645; 2011. Wondergem M, et al,. Accuracy of 18 F-NaF PET/CT in bone metastasis detection and its effect on patient management in patients with breast carcinoma, Nucl. Med. Commun., vol. 39, no. 4, pp. 325-333; 2018. 33. Liu Y. The diagnostic performance of 18F-fluoride PET/CT in bone metastases detection: a meta-analysis, Clin.