Thyroid carcinoma is the most common endocrine malignancy, with papillary thyroid carcinoma (PTC) representing 50–90% of differentiated thyroid cancers. Anaplastic large cell lymphoma (ALCL) is a rare T-cell lymphoma characterized by CD30 expression and distinctive growth patterns, comprising about 2% of all lymphomas and 9% of adult non-Hodgkin lymphomas. Extranodal ALCL involving the thyroid is rare, and only 2 cases have been reported on ALCL coexistence with PTC. We present the first documented case of ALK-negative ALCL coexisting with PTC within the thyroid gland.

A 78-year-old Filipino woman presented with a two-month history of a painless, enlarging anterior neck mass. Thyroid function tests showed mildly suppressed TSH (0.32 uIU/mL) and normal FT4 and FT3. Ultrasound revealed multiple thyroid nodules, with two TIRADS-4 nodules in the left superior and right middle poles. Fine-needle aspiration biopsy was done with cytology results revealing suspicious for malignancy (Bethesda Category V). The patient underwent total thyroidectomy. Histopathology examination revealed round cell malignancy right mid inferior, papillary thyroid carcinoma, infiltrative follicular subtype , isthmus (0.1cm). Atypical follicular lesions left lobe (0.7cm and 0.5cm), along with chronic lymphocytic thyroiditis. Further evaluation with immunohistochemistry showed: CD 30 positive, ALK negative and a high Ki-67 (80-90%) .The immunohistochemical staining profile and histomorphological features of the previously identified round cell malignancy supported the diagnosis of ALK-negative anaplastic large cell lymphoma. The patient was referred to oncology and started on Brentuximab vedotin, Doxorubicin, and Cyclophosphamide (CHP-BV regimen).

The coexistence of PTC and ALK-negative ALCL is extremely uncommon, with only two cases previously reported, none involving both entities in the thyroid itself. The pathogenesis remains unclear, though theories propose monotypic expansion of activated T cells that occurs with solid tumors, T cell lymphoma originating from cytotoxic CD30+ T cell expansion and secondary genetic alterations leading to clonal T-cell transformation . ALK-negative ALCL typically affects older adults, with a poorer prognosis than ALK-positive variants.

To our knowledge, this is the first reported case of coexisting PTC and ALK-negative ALCL both originating in the thyroid. This case highlights the importance of histopathology with combined immunohistochemical evaluation in diagnosis of thyroid malignancies with atypical features since accurate diagnosis is necessary for appropriate management. Further investigation is needed to identify potential association between PTC and ALCL.