Figure 1 The profile of a 48-year-old man with bladder carcinoma. 18F-Sodium fluoride (NaF) positron emission tomography/computed tomography (PET/CT) is performed after injecting 3.9 mCi of 18F-NaF with urinary catheter in place and images are acquired 60 minutes post-injection. a) 18F-NaF maximum intensity projection (MIP), b) zoom lateral and, c) anterior 18F-NaF MIP images show two foci of intense activity in the urinary bladder (arrow) and rest of the urinary bladder is filled with mild homogenous tracer uptake (arrow head), findings appear as a double density of increase uptake in the urinary bladder. d, e, f) Non-contrast CT image in the bone window demonstrates soft tissue mass with marginal calcification and increased tracer uptake on fused images. The surveyed skeleton shows multiple aggressive lytic lesions with osteoblastic activity at D11 vertebra, sacrum, pelvic bones, and left proximal femur consistent with osteoblastic bone metastases. Double density foci (hotter spot within hot area) are due to soft tissue contrast uptake at the primary calcified urinary bladder mass and relative mild tracer uptake in the non-radioactive urine in the bladder.

Figure 2 Diagnostic CT a) pre-contrast, b) contrast and, c) delayed portovenous phase images showing focal lesions of varying sizes in the urinary bladder. The largest lesion was at the right uretero-vesicle junction with evidence of extra mural and perivesical fat stranding with marginal interrupted calcification. Another suspicious lesion is seen in the left anterolateral wall. (d) Fused 18F-NaF PET/CT images in soft tissue window show 18F-NaF uptake at the calcification. Bladder transitional cell carcinoma is the second most common urinary tract malignancy (1). Common sites of metastasis are nodal, skeletal, hepatic, pulmonary, and adrenal glands (2). Among these, the spinal axis where these are usually lytic (3). 18F-NaF is an excellent bone-seeking tracer with predominant skeletal uptake due to quick first-pass extraction, minor plasma protein binding and rapid renal excretion. After one hour of administration, around 10% of this tracer is found in plasma (4). 18F-NaF PET/CT is highly sensitive in characterizing both blastic and lytic lesions. Due to its excellent spatial resolution, this tracer is can also identify increased bone turnover along the thin reactive border around bone lysis (5). Thus, this was a case with depicting lytic bone metastases and extra osseous 18F-NaF uptake in calcified urinary bladder mass (double density sign). 18F-NaF can be localized in extraosseous calcifying lesions. Skeletal primary and metastatic foci can absorb 18F-NaF because of either tumoral calcification or calcifications within necrotic foci (6). In literature, there are examples of extraosseous 18F-NaF uptake for both malignant (brain, cardiac, lymph nodes, liver metastases) and benign etiologies (cardiac amylodosis, meningioma and lung amyloidosis) (7,8,9). The mechanisms of extraosseous 18F-NaF uptake are unclear. Several postulates reveal that the process underlying active calcium turnover demonstrates 18F-NaF uptake. Extraosseous 18F-NaF uptake is often encountered. Thus, the recognition of extra osseous 18F-NaF uptake is essential for precise diagnosis and interpretation.