Schwannomas arise from Schwann cells, the glial cells that wrap peripheral nerves in insulating myelin sheaths. They can occur anywhere in the body, with an estimated annual incidence of roughly 0.6 per 100,000 people, and about 25 to 45 percent of them appear in the neck. Most follow an indolent, slow-growing course and are cured by simple surgical excision. In the neck, they can spring from the sympathetic chain, the glossopharyngeal, vagus, accessory, recurrent laryngeal, or hypoglossal nerves. But schwannomas that grow within the thyroid parenchyma itself are vanishingly rare, and those in the thyroid bed, outside the gland but pressed against it, are rarer still. Because they abut the thyroid capsule, they are routinely misidentified on ultrasound as nodules growing inside the gland, and that single misclassification cascades into the wrong biopsy strategy, the wrong surgical plan, and sometimes unnecessary removal of healthy thyroid tissue.

The Chinese patient’s story began nine years before her admission, when a first ultrasound spotted a nodule in her left thyroid lobe. She was asymptomatic and declined intervention, but the mass slowly enlarged. Three years before admission, follow-up imaging showed a large mixed cystic and solid mass occupying nearly the entire left lobe, obscuring normal gland tissue, measuring about 58 by 44 by 59 millimeters, with an estimated volume of roughly 78.8 cubic centimeters. It had relatively well-defined borders, regular morphology, heterogeneous internal echoes, and punctate linear blood flow. Based on those features, radiologists strongly suspected a follicular neoplasm, a category of thyroid tumor that includes both benign follicular adenomas and follicular carcinomas, which are notoriously difficult to tell apart without examining the entire excised specimen. The patient again deferred surgery and chose surveillance.

Twelve days before admission, an outside hospital repeated the ultrasound and found essentially the same lesion, now measured at 61 by 44 by 50 millimeters and predominantly solid, with punctate patchy hyperechoic foci casting posterior acoustic shadows, the signature of coarse calcification. The ratio of its anteroposterior to transverse diameter was below one, a reassuring feature, and the findings were read as slow progression of the same lesion. On examination in Chengdu, clinicians palpated a well-circumscribed, roughly 6 by 4 by 5 centimeter mass in the left neck that moved synchronously with swallowing, a classic sign of a lesion attached to the thyroid or its immediate surroundings. No cervical lymph nodes were enlarged, and thyroid function tests were normal.

The diagnostic workup then deployed the full modern ultrasound arsenal. Gray-scale B-mode imaging showed a 62 by 35 by 47 millimeter mass at the inferior portion of the left lobe, adjacent to the peripheral capsule, with a slightly irregular shape, relatively clear borders, no peripheral hypoechoic halo, mixed cystic and solid echoes, and coarse calcifications; capsular invasion could not be definitively excluded. Color Doppler imaging revealed only scant intralesional blood flow, classified as Adler grade one. Elastography indicated intermediate stiffness. Contrast-enhanced ultrasound, which tracks the wash-in of microbubble agents through tissue, showed heterogeneous enhancement: partial central areas enhanced at the same intensity as surrounding tissue, while most of the lesion showed absent or reduced enhancement, consistent with cystic degeneration. The lesion was categorized as TI-RADS 3, an intermediate-suspicion category, and the working diagnosis remained a follicular neoplasm.

Computed tomography added an important clue that was, in hindsight, easy to overlook. The scan showed a slightly hypodense mass of about 5.0 by 4.3 centimeters at the left root of the neck with scattered calcifications, ill-defined boundaries with the left thyroid lobe, and marked heterogeneous enhancement with scattered intralesional vascular channels on contrast scanning, with tentative feeding supply traced to the left common carotid artery. The mass compressed and displaced the trachea and esophagus, partially extended into the superior mediastinum, and pressed on the brachiocephalic vein and proximal left cervical vessels. The radiology team judged the findings highly suggestive of a neoplastic lesion, but the anatomical origin remained ambiguous, precisely the ambiguity that defines this disease.

Fine-needle aspiration biopsy, the standard first-line invasive test for thyroid nodules, was performed and returned Bethesda category I, meaning the specimen was nondiagnostic. That failure is not unusual. According to the literature reviewed by the authors, FNA achieves a diagnostic sensitivity of only 0 to 40 percent for schwannomas, with inadequate specimen rates as high as 36 to 50 percent, because the tumor’s spindle cells and dense fascicular architecture do not exfoliate well through a thin needle. With a high preoperative suspicion of thyroid cancer and no useful cytology, the team proceeded to a left thyroid lobectomy combined with isthmectomy. Intraoperatively, the tumor proved to possess an intact capsule and was closely adherent to the thyroid capsule; on cut section it was grayish-brown, predominantly solid, with cystic spaces containing old dark brown fluid.

Definitive answers came from the microscope. Hematoxylin-eosin staining revealed densely packed fascicles of abundant spindle-shaped tumor cells, the histological hallmark of schwannoma, and, critically, no tumor infiltration within the thyroid parenchyma. Immunohistochemistry sealed the diagnosis: the cells were strongly positive for S100 and SOX10, the two most reliable markers of Schwann-cell lineage, partially positive for D2-40, negative for CD117, CD34, DOG1, Desmin, SMA, and Pan-CK, and showed scattered Ki-67 positivity in fewer than 1 percent of cells, indicating a very low proliferative rate, with retained H3 K27Me3 expression. The patient recovered well, maintained normal thyroid function after losing her left lobe, and remained free of abnormalities on serial ultrasound follow-up one year after surgery.

The case also carries a subtle lesson about measuring tumor growth. The team calculated lesion volumes with the standard ellipsoid formula, multiplying the three diameters and the factor pi divided by six. By that calculation, the volume appeared to shrink from about 78.8 cubic centimeters to 53.4 cubic centimeters over three years, even as the maximum longitudinal diameter grew from 58 to 62 millimeters, a relative increase of 6.9 percent. Prior evidence shows that for well-circumscribed thyroid nodules larger than one centimeter, volume changes below 13.1 percent and diameter changes below 7.3 percent should not be interpreted as true biological progression, because scanning-plane variation, axis alignment, and observer variability dominate the measurements. Moreover, cystic schwannomas can spontaneously shrink as cystic fluid is absorbed and degenerated tissue remodels, a phenomenon documented in hypoglossal schwannomas and a key reason these lesions fluctuate on serial scans.

The authors’ prescription is straightforward: when imaging is atypical, FNA is inconclusive, or the thyroid margins are ill-defined, pathologists, radiologists, and surgeons should confer before the operating room, and ultrasound-guided core needle biopsy should be used when needed. Unlike FNA, core needle biopsy yields intact tissue cores that preserve histological architecture and support immunohistochemistry, and international guidelines already endorse it as a routine approach for cervical masses. Getting the diagnosis right before surgery changes everything about treatment. Most thyroid-bed schwannomas, unlike intrathyroidal ones, can be enucleated alone without any thyroidectomy; hemithyroidectomy is needed only for dense adhesion, and total thyroidectomy is rarely required. Complete surgical excision remains first-line therapy with an excellent prognosis. For the growing number of patients found to have thyroid nodules each year, this case is a reminder that not every mass next to the thyroid belongs to it, and that a needle of the right caliber, aimed with the right question in mind, can spare an entire gland.

Subject of Research: A case report of a thyroid-bed schwannoma preoperatively misdiagnosed as a thyroid follicular neoplasm

Article Title: Thyroid-bed schwannoma mimicking a thyroid neoplasm: a case report

Article References: Wang, M., & Yu, B. (2026). Thyroid-bed schwannoma mimicking a thyroid neoplasm: a case report. Frontiers in Medicine, 13, Article 1982014. https://doi.org/10.3389/fmed.2026.1982014

Image Credits: AI Generated

DOI: 10.3389/fmed.2026.1982014

Keywords: schwannoma, thyroid bed, thyroid nodule, ultrasound, fine-needle aspiration, core needle biopsy, follicular neoplasm, contrast-enhanced ultrasound, thyroidectomy, immunohistochemistry, case report, head and neck tumors