A Rare Dual Pathology of Papillary Thyroid Carcinoma with Incidental Unicentric Castleman's Disease: Case Report and Literature Review.

Background: Papillary thyroid carcinoma (PTC) is the most common form of thyroid malignancy, often presenting with regional lymph node involvement. Castleman's disease is a rare, non-clonal lymphoproliferative disorder, typically presenting as unicentric or multicentric disease. The incidental coexistence of PTC and Castleman's disease is exceptionally rare and can complicate the diagnostic and therapeutic approach, especially when lymphadenopathy is involved. Case Presentation: We report the case of a 36-year-old male who presented with a left-sided neck mass and was diagnosed with multifocal classic and diffuse sclerosing variants of PTC, with cervical lymph node metastases. Surgical pathology following total thyroidectomy and lymph node dissection unexpectedly revealed features of unicentric hyaline-vascular variant Castleman's disease in a non-metastatic lymph node. Immunohistochemical staining confirmed the diagnosis. The patient was asymptomatic with regard to Castleman's disease and was managed conservatively. PTC was managed with surgery and suppressive levothyroxine therapy, with plans for adjuvant radioactive iodine therapy. Conclusion: This case illustrates a rare dual pathology of PTC with incidental unicentric Castleman's disease. It highlights the importance of thorough histopathological evaluation in lymphadenopathy associated with thyroid carcinoma and suggests a potential need for increased awareness of concurrent benign lymphoproliferative disorders in oncologic surgery. Further studies are needed to understand the clinical significance and possible pathophysiological link between these two conditions.