Background: Follicular tumors include follicular thyroid adenomas and carcinomas; however, it is difficult to distinguish between the two when the cytology or biopsy material is obtained from a portion of the tumor. The presence or absence of invasion in the resected material is used to differentiate between adenomas and carcinomas, which often results in the unnecessary removal of the adenomas. If nodules that may be follicular thyroid carcinomas are identified preoperatively, active surveillance of other nodules as adenomas is possible, which reduces the risk of surgical complications and the expenses incurred during medical treatment. Therefore, we aimed to identify biomarkers in the invasive subpopulation of follicular tumor cells. Methods: We performed a spatial transcriptome analysis of a case of follicular thyroid carcinoma and examined the dynamics of CD74 expression in 36 cases. Results: We identified a subpopulation in a region close to the invasive area, and this subpopulation expressed high levels of CD74. Immunohistochemically, CD74 was highly expressed in the invasive and peripheral areas of the tumor. Conclusions Although high CD74 expression has been reported in papillary and anaplastic thyroid carcinomas, it has not been analyzed in follicular thyroid carcinomas. Furthermore, the heterogeneity of CD74 expression in thyroid tumors has not yet been reported. The CD74-positive subpopulation identified in this study may be useful in predicting invasion of follicular thyroid carcinomas.

A 68-year-old man was admitted to our hospital with complaints of fatigue, polyuria, and loss of appetite, and was diagnosed with diabetic ketosis. Chest and abdominal computed tomography (CT) showed a pulmonary tumor on the right S3 and multiple liver tumors. Blood chemistry revealed elevated levels of amylase and hepatobiliary enzymes. Pathological examination of a biopsy specimen from the liver tumor showed a small cell carcinoma. Based on the imaging and pathological findings, we made a diagnosis of extensive disease small-cell lung cancer (ED-SCLC), cT1aN3M1b (HEP, ADR). Treatment with carboplatin and etoposide evoked partial response and the serum level of amylase decreased. Immunohistochemical staining of liver biopsy specimen was positive for amylase, leading to a diagnosis of SCLC with amylase production. About 22 months after the diagnosis of SCLC, he was admitted to our hospital with fatigue, muscular weakness, edema, and hyperpigmentation. Laboratory findings showed elevated serum levels of hepatobiliary enzymes, adrenocorticotropic hormone (ACTH), and cortisol, and a decreased serum potassium level. Urinary potassium level was elevated. Pituitary magnetic resonance imaging showed a normal morphology. We made a diagnosis of SCLC complicated by Cushing’s syndrome. We report this rare case of SCLC with amylase and ACTH production, which was detected in the course of treatment of SCLC.