SSTR Expressing Mediastinal Ectopic Thyroid: A Rarity Unveiled

Case and Discussion

A 31-year-old female presented with exertional dyspnea and retrosternal chest pain for 3 months. Contrast-enhanced computed tomography (CECT) thorax showed a mass measuring ~8 cm × 5.5 cm in the anterior and superior mediastinum abutting the left lobe of thyroid, a possible differential of lymphoma, neurogenic tumor, germ cell tumors, ectopic thyroid tissue, thymoma, or neuroendocrine tumor (NET) were considered by the clinician. Her thyroid profile was within normal limits (thyroid-stimulating hormone – 2.07 uIU/mL, T3 – 137 ng/dl, and T4 – 6.8 ug/dl). Serum chromogranin was normal (45.4 ng/mL). Other tumor markers including Beta-human chorionic gonadotropin (beta-hCG), Cancer antigen-125 (CA-125), Alpha-fetoprotein (AFP), Carcino-embryogenic antigen (CEA), and Carbohydrate antigen 19.9 (CA19.9) were also within normal limits. ^99mTechnetium-pertechnetate scan with single-photon emission CT/CT [Figure 1a and d] showed tracer uptake in the normal cervical thyroid gland with no significant uptake in mediastinal mass. On ⁶⁸Ga-DOTANOC positron emission tomography–CT (PET/CT) [Figure 1b and e], a well-defined, heterogeneous density tracer-avid mass measuring ~ 8 cm × 5.6 cm was noted in the superior and anterior mediastinum on the left side extending from D1–D6 vertebrae level abutting the left lobe of the thyroid with low fluorodeoxyglucose (FDG) avidity on ¹⁸F-FDG PET/CT [Figure 1c and f]. Both lobes of the thyroid gland in the cervical region appeared normal. In view of increased somatotostatin (SSTR) expression and low FDG avidity, a high possibility of a neuroendocrine tumor was considered. She underwent an en bloc resection of the mass with excision of adherent adjacent thymic tissue. On histopathology [Figure 2], gross images showed a well-circumscribed nonencapsulated mass with cut sections showing areas of hemorrhage, scarring, and brown colloid-filled nodular areas [Figure 2a]. Microscopy showed thyroid follicles of varying sizes with evidence of old hemorrhage and scarring [Figure 2b and c]. The left lobe of the thymus attached with the specimen showed thymic parenchyma within normal histological limits for age [Figure 2d]. Thus, a final impression of the ectopic thyroid gland with adenomatous changes was made. Ectopic thyroid tissue can be described as functional thyroid tissue that is located anywhere other than its anatomic position including the mediastinum, heart, diaphragm, and esophagus.[1] Intrathoracic ectopic thyroid tissue is very rare and is usually located in the anterosuperior aspect of the mediastinum.[234] Intrathoracic thyroid is often secondary to the extension of the cervical portion of the thyroid gland; however, in the present case, the mass was separate from the thyroid gland in the neck.[4] In symptomatic cases, surgery is the treatment of choice.[56] Ectopic thyroid tissue presents as intensely enhancing mass on CECT scan; however in our case, it showed heterogeneous density mass with mild enhancement, while noncontrast CT showed attenuation less than normal cervical thyroid gland likely due to necrosis.[7] Technetium-99m or I-123 thyroid scintigraphy is the most useful diagnostic tool in localizing ectopic thyroid tissue; however, absent iodine content can be due to necrosis or underlying malignancy.[3] Our patient did not have uptake in thyroid scintigraphy attributable to necrosis (after ruling out contrast interference). It is well known that normal thyroid expresses SSTRs, as seen in ⁶⁸Ga-DOTANOC scan.[8] To our knowledge, no published literature could be found where SSTR expression in the ectopic thyroid gland is documented. Thus, patients with SSTR expressing mediastinal masses may also have ectopic thyroid tissue as a possible differential.[9]

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Figure 1 (a,d) 99mTc-pertechnetate scan with SPECT/CT shows tracer uptake in normal cervical thyroid gland with no significant uptake in mediastinal mass. (b,e) On 68Ga-DOTANOC PET/CT a well-defined, heterogenous density tracer avid mass was noted in the superior and anterior mediastinum on the left side extending with low FDG avidity on 18F-FDG PET/CT (c,f)

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Figure 2 On histopathology, gross images show a well circumscribed non-encapsulated mass with cut sections showing areas of hemorrhage, scarring and brown colloid-filled nodular areas (a). Microscopy showed thyroid follicles of varying sizes with evidence of old hemorrhage and scarring (b, c). Left lobe of the thymus attached with the specimen (d) showed thymic parenchyma within normal histological limits for age

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