Metastatic superscan in prostate carcinoma on gallium-68-prostate-specific membrane antigen positron emission tomography/computed tomography scan.

We describe the imaging features of a metastatic superscan on gallium-68 Glu-NH-CO-NH-Lys-(Ahx)-[Ga-68(HBED-CC)], abbreviated as gallium-68-prostate-specific membrane antigen ((68)Ga-PSMA) positron emission tomography/computed tomography (PET/CT) imaging. (68)Ga-PSMA is novel radiotracer undergoing evaluation for PET/CT imaging of prostate carcinoma. This patient had a superscan of metastases on conventional bone scintigraphy and was referred for (68)Ga-PSMA PET/CT to evaluate the feasibility of (177)Lu-PSMA therapy.

A 55-year-old case of metastatic prostatic adenocarcinoma on docetaxel chemotherapy, with rising serum prostate-specific antigen (sPSA) level, underwent bone scintigraphy. Gleason score was 9 (5 + 4), and sPSA level was 340 ng/mL. Findings on bone scintigraphy showed diffusely increased skeletal accumulation with increased bone-to-soft tissue (renal uptake) ratio suggestive of a metastatic superscan [Figure 1a - anterior and b - posterior view]. He further underwent gallium-68-prostate-specific membrane antigen (68Ga-PSMA) PET/CT to evaluate feasibility for 177Lu-PSMA therapy. Maximum intensity projection image [Figure 2a] showed generalized increased tracer uptake in entire axial and appendicular skeleton with reduced physiological uptake in bilateral lacrimal and salivary glands, spleen, small bowel, and kidneys. Sagittal [Figure 2b] and transaxial positron emission tomography/computed tomography (PET/CT) fusion images [Figure 2c–e] showed sclerotic changes in the entire axial skeleton with increased tracer uptake. All features were suggestive of metastatic superscan on 68Ga-PSMA PET/CT.

Figure 1

Findings on bone scintigraphy showed diffusely increased skeletal accumulation with increased bone-to-soft tissue (renal uptake) ratio suggestive of a metastatic superscan (a) anterior, (b) posterior view

Figure 2

Maximum intensity projection image (a) showed generalized increased tracer uptake in entire axial and appendicular skeleton with reduced physiological uptake in bilateral lacrimal and salivary glands, spleen, small bowel, and kidneys. Sagittal (b) and transaxial positron emission tomography/computed tomography fusion images (c-e) showed sclerotic changes in the entire axial skeleton with increased tracer uptake. All features were suggestive of metastatic superscan on gallium-68-prostate-specific membrane antigen positron emission tomography/ computed tomography

Superscan is a well-known phenomenon on bone scintigraphy characterized by increased bone-to-soft tissue ratio with a symmetrical increased skeletal uptake and reduced tracer uptake in nonaffected bones and soft tissue.[1] Common conditions include metastatic cancers and metabolic bone disease that give rise to this appearance on skeletal scintigraphy.[2] This phenomenon has also been described previously in the literature on 18F-fluorodeoxyglucose PET/CT and 68Ga-labeled [1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid]-1-NaI3-octreotide PET/CT scan where a superscan implies a high contrast between metastatic and nonmetastatic organs.[3456] In this case, there was increased tracer accumulation in entire axial and appendicular skeleton and reduced tracer uptake in the salivary and lacrimal glands, spleen, proximal small gut, and kidneys which normally show the physiological distribution of 68Ga-PSMA. This case demonstrates the altered biodistribution pattern in a metastatic superscan of 68Ga-PSMA PET/CT, which further predicts a poor prognosis.[78]

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