Thymoma in Ectopic Thymus: An Overlooked Differential and a Pathologist's Dilemma

Thymomas arising in thymic tissue located in an abnormal location are rarely described in medical literature and present with a wide range of diagnostic dilemmas. Amongst the abnormal locations for ectopic thymus, the thyroid serves as the most common site; however, case reports of ectopic thymic tissue have also been seen in rarer locations, such as the middle ear, base of the skull, submandibular gland, and tonsils. Myasthenia gravis (MG), a paraneoplastic syndrome associated with thymomas, is very rare in cases of ectopic site thymomas. In this report, we present one such case of a 44-year-old female patient who initially presented with generalised weakness, difficulty breathing, and loss of appetite for one week. Differential diagnoses of tuberculosis, lymphoma, and metastatic malignancies were considered clinically. A contrast-enhanced computed tomography of the neck, thorax, and abdomen indicated the presence of a mild homogenously enhancing mediastinal mass (6.6x5.4x4.0 cm) in the right para-tracheal region abutting the left distal brachio-cephalic vein and the superior vena cava. Prompt laboratory examination, specifically fine needle aspiration cytology and cell block evaluation of the mediastinal mass, helped categorise the lesion as type B2 thymoma. The patient was advised of a thymectomy but refused and presented months afterwards with a diagnosis of MG. Ectopic thymic foci along with the thymus were removed, and a diagnosis of type B2 thymoma in the mediastinal mass was substantiated. The reported case highlights the rarity of MG in patients with ectopic thymoma and the importance of a strong clinical suspicion and laboratory findings in making a correct diagnosis, eventually leading to better patient management.