Incidentally Detected Gastrointestinal Stromal Tumor in a Patient with Carcinoma Prostate: ⁶⁸Ga-Prostate-Specific Membrane Antigen Versus ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography

A 67-year-old male, postrobotic-assisted radical prostatectomy for carcinoma prostate, was referred to our department for ⁶⁸Ga-prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) in view of slowly rising prostate-specific antigen (0.024 ng/ml–0.138 ng/ml). ⁶⁸Ga-PSMA PET/CT showed a large heterogeneous density mass with increased PSMA uptake in the left subhepatic region, adjacent to the lesser curvature of the stomach [Figure 1]. Contrast-enhanced CT (CECT) showed heterogeneous enhancement of this mass. ¹⁸F-fluorodeoxyglucose (FDG) PET/CT showed no significant FDG uptake in the mass [Figure 1]. Ultrasound (USG)-guided biopsy was performed, which was suggestive of gastrointestinal stromal tumor (GIST) [Figure 2].

Close

Figure 1 MIP image of ⁶⁸Ga-PSMA PET/CT (a) shows increased tracer uptake in the left subhepatic region (white arrow). Axial fused ⁶⁸Ga-PSMA PET/CT and corresponding NCCT images (b and c) show a large PSMA avid soft tissue density mass (white arrows) in the left subhepatic region. The mass is closely abutting the lesser curvature of the stomach and left lobe of the liver. Axial fused 18F-FDG PET/CT and corresponding CECT images (d and e) show a heterogeneously enhancing soft-tissue density mass (white arrows) with a peripheral necrotic component in the left subhepatic region with no significant FDG uptake. No abnormal FDG uptake was noted in the 18F-FDG PET/CT MIP image (f). MIP: Maximum intensity projection, PET/CT: Positron emission tomography/computed tomography, CECT: Contrast-enhanced CT, FDG: Fluorodeoxyglucose

Export to PPT

Close

Figure 2 USG-guided biopsy showing tumor cells arranged in loose ill-defined fascicles and sheets with interspersed blood vessels, H and E ×100 (a) and ×200 (b). The tumor cells are spindled and epithelioid shaped containing oval and round nuclei, respectively, with finely granular nuclear chromatin, inconspicuous nucleoli, and moderate eosinophilic cytoplasm, H and E ×400 (c). The tumor cells demonstrated immunopositivity for DOG1, ×400 (d). Overall findings were suggestive of GIST. GIST: Gastrointestinal stromal tumor

Export to PPT

GIST is the most common mesenchymal tumor of the digestive tract. It originates from the interstitial cells of Cajal and is characterized by overexpression of the tyrosine kinase receptor KIT with 95% staining positive for CD117 (c-KIT) and 70% for CD34.[1] GISTs are now considered to be potentially malignant and all nonmetastatic GISTs should be resected.[2] Small GISTs are usually asymptomatic and are usually incidentally detected either during investigations or surgical procedures for unrelated diseases.[3]

GISTs usually appear as rounded soft-tissue masses, arising from the wall of a hollow viscus (most commonly the stomach) with an endoluminal or exophytic growth pattern. It may have a central necrotic zone with peripheral enhancement on CECT. Although GISTs are usually FDG avid,[4] few cases of GISTs with low FDG uptake have been reported.[5] In these cases, FDG PET/CT cannot be used for monitoring response to therapy.[6] Few cases of GIST, incidentally detected on ⁶⁸Ga-PSMA PET/CT imaging have been reported in the literature.[789] However, heterogeneity of PSMA and FDG uptake in gastric GIST have never been reported before. In such cases of GISTs which are FDG negative and PSMA positive, ⁶⁸Ga-PSMA PET/CT should be preferred over 18F-FDG PET/CT for response assessment.