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E2024. Building a Differential in Technetium MDP Bone Scintigraphy: It Might Be Hot, But Malignant it’s Not
Authors
  1. Courtney Dey; Duke University; Eastern Virginia Medical School
  2. Katherine Johnson; Eastern Virginia Medical School
  3. Sarah Shaves; Eastern Virginia Medical School
  4. Lester Johnson; Eastern Virginia Medical School
Background
Bone scintigraphy is a common, important study for the detection of osteoblastic metastases. The technetium-99m-methylene diphosphonate (99mTc-MDP) tracer avidly attaches to the hydroxyapatite crystals on the surface of bone by a process known as chemisorption, and so serves as a marker of osteoblastic activity. Bone scintigraphy is very sensitive for osteoblastic metastases, when numerous scattered sites of abnormal uptake are evident in the axial and proximal appendicular skeleton, the pattern can be quite specific for osseous metastatic disease. However, the relatively poor specificity of bone scan for individual lesions leads to false positives when only one or a few abnormal sites of focally increased activity are seen that may reflect a variety of other conditions. There are numerous benign conditions that may focally increase osteoblastic activity and thus 99mTc-MDP uptake. Therefore, recognition of the characteristic findings associated with some of these conditions is important in order to build an accurate differential, to limit unnecessary additional diagnostic studies while appropriately recommending other imaging exams for the indeterminate/nonspecific activity, and to optimize the course of treatment for patients.

Educational Goals / Teaching Points
Review the mechanism of 99mTc-MDP bone scintigraphy, non-malignant causes of increased radiotracer accumulation on 99mTc-MDP bone scintigraphy & 'Aunt Minnie' imaging appearance which allows confident diagnosis by bone scintigraphy.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Osteoblastic metastatic disease most often produces sites of focally increased uptake on bone scintigraphy due to stimulation of osteoblasts by local tumor growth. We present cases of benign processes which may demonstrate increased uptake on 99mTc-MDP bone scintigraphy & should be recognized given their characteristic appearances. These include fibrous dysplasia, Paget disease, sacral insufficiency fracture, some traumatic injuries, & metabolic disorders such as hyperparathyroidism. In contrast, many benign causes of abnormally increased uptake on bone scan are indeterminate by bone scan and require correlation with radiographs, CT, and/or MRI of the site of uptake for specific diagnosis. For some lesions biopsy is required for definitive diagnosis.

Conclusion
Bone scintigraphy with 99mTc-MDP is a sensitive study for detection of altered osteoblast activity and is especially useful for early detection of osteoblastic metastatic disease. In such cases tracer uptake is typically focally increased corresponding to osseous reaction to tumor presence. However, a variety of benign conditions may mimic metastatic disease. By becoming familiar with characteristic patterns on bone scintigraphy, one may better differentiate the causes of focally increased radiotracer uptake, thereby increasing exam accuracy and improving outcomes for patients.