AbstractRarely, salivary gland tumors such as mucoepidermoid carcinoma, mammary analogue secretory carcinoma and mucinous carcinoma arise as primary tumors from ectopic or metaplastic salivary gland tissue adjacent to or within the thyroid gland. We report for the first time a case of primary salivary acinic cell carcinoma (AcCC) adjacent to the thyroid gland in a 71-year-old female patient with Crohns disease and a previous history of malignant melanoma. Following the development of a nodule adjacent to the left thyroid lobe, a fine-needle aspiration biopsy was reported as consistent with a follicular lesion of undetermined significance (Bethesda III). A left-sided hemithyroidectomy was performed. A circumscribed lesion measuring 33 mm was noted adjacent to the thyroid and trapping parathyroid, it was composed of solid nests and glands with microcystic and follicular patterns. The tumor was negative for thyroid, parathyroid and paraganglioma markers, but positive for pan-cytokeratins, CK7, CD10, CD117, androgen receptor and HNF-beta. A metastasis of a thyroid-like renal cell carcinoma was suspected but ruled out, and the patient had no evident lesions on extensive radiology of the urogenital, pulmonary and GI tracts. Based on the morphology, a diagnosis of AcCC was suggested, and confirmed with DOG1 and PAS-diastase staining. Molecular analyses pinpointed a constitutional ASXL1 variant of uncertain significance, but no fusion events. The patient had no radiological or clinical evidence of parotid, submandibular or sublingual tumors postoperatively, and the excised lesion was therefore assumed to be a primary tumor. We here detail the morphological and immunophenotypic profile of this previously undescribed perithyroidal tumor.