Anaplastic transformation of papillary thyroid carcinoma in multiple lung metastases presenting with a malignant pleural effusion: a case report

INTRODUCTION Anaplastic transformation of well-differentiated papillary thyroid carcinoma at distant metastasis sites is rare. To the best of our knowledge, this is the first report of an autopsy case of anaplastic transformation of papillary thyroid carcinoma in multiple lung metastases presenting with a malignant pleural effusion. CASE PRESENTATION We report an autopsy case of a 61-year-old Japanese man with anaplastic transformation of papillary thyroid carcinoma with multiple lung metastases presenting with a malignant pleural effusion, which was difficult to diagnose by cytological examination before the autopsy. He presented with a 1-month history of progressive dyspnea, and examination of the left pleural effusion revealed a bloody exudate with an increase in thyroglobulin; however, malignant cells in the pleural fluid were negative for thyroglobulin. CONCLUSION It is important to be aware that anaplastic transformation of differentiated thyroid carcinoma could develop in lung metastases and could be a cause of a malignant pleural effusion.