This patient was diagnosed with an exophytic GIST tumor in 2013 (images not available) and underwent surgical resection and had follow-up 18F-FDG PET/CT in 2014 to evaluate for recurrence/relapse, which reported no recurrent/residual disease. Similarly, follow-up CT arterial portography (AP) done in 2015 and octreoscan (111In-DTPA-octreotide) done in May 2017 reported no disease recurrence (Figure 1). Patient subsequently underwent NETSPOT (68Ga DOTATATE) PET/CT imaging in November 2017, which reported an incidental focus of intense uptake in the right superior frontal lobe (Figure 2). This was further confirmed on dedicated MRI with contrast of the brain which found a 1.5 x 1.3 x 1.2 cm avidly enhancing, dural-based mass in the right superior frontal lobe consistent with meningioma (Figure 3).
Figure 1: Anterior and posterior whole body planar imaging using octreoscan (111In- DTPA-octreotide). Figure 2A: MIP image of the 68Ga-DOTATATE. Physiological tracer distribution is seen in the pituitary gland, spleen, adrenal glands, kidney and urinary bladder. Incidental uptake is visualized in the right superior frontal lobe. Figure 2C: Non-enhanced CT and fused PET/CT axial images confirm anatomic location of the hyperdense/tracer-avid focus. Coronal slice of T1-weighted brain MRI with contrast image. A contrast-enhancing, dural-based mass is seen in the right superior frontal lobe, most consistent with meningioma.
Meningiomas are the most common primary intracranial neoplasms and represent about 36.1% of all intracranial tumors (1). They arise from the meningothelial cells of the arachnoid membranes. Conventional imaging modalities like CT and MRI are part of the routine diagnostic workup of patients who are being evaluated for meningiomas. Nuclear medicine studies--SPECT and PET--could provide complementary information to CT and MRI.
Somatostatin receptors (SSTRs) are expressed on the cell membrane of various central nervous and peripheral tissues in high density to a varying extent, including neuroendocrine tumors and intracranial tumors (2). There are 5 different subtypes of SSTRs, of which Subtype 2 is most often expressed on the surface (3, 4). There are two isoforms of SST-2: SSTR-2A and SSTR-2B. 111In-DTPA-octreotide binds with high affinity to SSTR2, SSTR3, and SSTR5; overexpression of the SSTR2A receptor may explain the high tracer uptake observed in meningiomas (5). Even though many studies have reported 100% sensitivity of detecting all meningioma lesions, a major drawback of 111In-octreotide SPECT is its difficulty in detecting meningiomas with a diameter <2.7 cm or a volume <10 mL (6).
68Ga-DOTATATE PET/CT offers high-contrast images of meningiomas measuring as small as 7-8 mm in diameter and can also be clearly separated from both surrounding brain and bone tissue (7). Since meningiomas have a tendency to local osseous invasiveness, this unique capability of PET/CT is particularly helpful for evaluating meningiomas of the skull base where biopsy has a high risk of hemorrhage and the findings of CT and MRI are often unclear (8).
Two potential reasons for false-negative results on octreoscan are (9)
Presence of different somatostatin receptor subtypes that have different affinities for the radioligand;
Presence of unlabeled somatostatin, either because of octreotide therapy or resulting from production of somatostatin by the neuroendocrine tumor.
Since 111In-DTPA-octreotide has higher affinity for SSTR-2A, while 68Ga-DOTATATE has high affinity for both SSTR-2A and SSTR-2B, this might explain the incidental finding on NETSPOT exam but lack of it on octreoscan.
Benign hepatic adenomas may be FDG-avid and a cause of a false positive finding for malignancy.