SAT502 Choristoma: A Very Rare Cause of Thyroid Nodule. Clinical Case and Literature Review

Disclosure: J. Rocha: None. R. Moiteiro da Cruz: None. M. Alexandre: None. A. Gomes: None. D. López-Presa: None. M.J. Bugalho: None. Introduction: A choristoma is a well delimitated benign lesion formed by normal tissue in an unusual location. Choristomas are rare lesions that are mainly reported in the eye, tongue or ears. The etiology of the choristoma is not well established but congenital defects in cell migration and exposure to chronic inflammation are two of the suggested hypothesys. Diagnosis is made after surgical removal of the mass, usually with no other associated abnormalities. To our knowledge, to this day there is only one other case of thyroid choristoma described in literature. Clinical case: A 70-year-old man presented to the emergency department with hyperglycemia, polydipsia and polyuria. Personal history was notable for non-Hodgkin lymphoma in remission after stem cell autologous transplant, currently under high dose prednisolone treatment due to sudden cervical enlargement. Cervical imaging with CT scan showed a 47mm hypodense nodule of the right thyroid lobe. The patient was refered to an Endocrinology appointment.Thyroid function was normal, with normal serum calcitonin and negative anti TPO and antiTG antibodies. Fine-needle aspiration biopsy was notable for folicular lesion of undetermined significance. During the following weeks there was a noticeable thyroid enlargement causing positional dyspnoea and dysphonia. Ultrassound revealed a 73mm nodule of the right lobe, predominantly cystic and isoechoic in the solid regions. At this point the patient underwent total thyroidectomy without complications.Histopathological examination revealed a choristoma composed of cysts lined by squamous epitelium, smooth muscle bundles, mature adipose tissue, areas of calcification and ossification and extramedullary hematopoiesis; there were also signs of old and recent haemorrhage and foreign body granulomas. No cytological atypia or tumoral necrosis were found. Conclusion: Thyroid choristomas are extremely rare entities. Unlike the previously described case our patient had a rapid growth of the cervical mass. This case is specially relevant due to its clinical course, existing comorbidities and rarity of the final diagnosis. Presentation Date: Saturday, June 17, 2023