Unexpected Lung Scan Uncovers Rare Pancreatic Schwannoma Mimicking Neuroendocrine Tumor

Incidental Finding Leads to Rare Diagnosis

A 67‑year‑old woman in Japan underwent a CT scan to further evaluate an abnormal spot on her lung. The examination revealed an unexpected mass measuring 31 × 23 mm in the pancreas, a finding not initially sought by the clinicians.

Initial Imaging Suggested a Neuroendocrine Tumor

On contrast‑enhanced CT, the lesion displayed well‑defined borders with a heterogeneous internal pattern. Peripheral areas enhanced more prominently, and several small regions within the mass showed uptake comparable to nearby blood vessels. This appearance is characteristic of a pancreatic neuroendocrine tumor (pNET), an uncommon malignancy arising from the hormone‑producing cells of the pancreas.

Before surgery, two nuclear medicine scans were performed to refine the differential diagnosis. A somatostatin receptor scan, which often highlights pNETs, was negative. A second scan using the tracer MIBG demonstrated only faint uptake, making a paraganglioma unlikely. Despite these results, the imaging findings remained suggestive of a pNET, and the surgical team proceeded with a laparoscopic enucleation to remove the tumor while preserving the surrounding pancreatic tissue.

Pathology Reveals a Schwannoma

Microscopic examination of the excised mass showed extensive internal hemorrhage and degeneration. The tumor consisted of wavy, spindle‑shaped cells arranged in alternating dense (Antoni A) and loose (Antoni B) patterns, a hallmark of schwannoma. Immunohistochemical staining was positive for S100, a marker of Schwann cells, and negative for synaptophysin, chromogranin A, and INSM1, which are typically expressed in neuroendocrine tumors.

Why the Misdiagnosis Occurred

Pancreatic schwannoma is exceedingly rare, representing approximately 0.03 % of all pancreatic neoplasms. Its preoperative identification is challenging because it frequently mimics other pancreatic tumors. In this case, the extensive bleeding within the schwannoma created a patchy, necrotic‑like appearance on imaging, closely resembling a pNET with central necrosis.

Clinical Implications

The patient recovered uneventfully and was discharged seven days after the operation. This case underscores that, even with advanced imaging, definitive diagnosis often requires histopathological evaluation. While pancreatic masses can be benign, they may also be malignant; therefore, any incidental finding should be discussed with a clinician to determine appropriate follow‑up.

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