Sarcoidosis or sarcoid-like reaction developed during the follow-up of malignant tumors: difficulties in differentiation from recurrent or metastatic diseases

Sarcoidosis or sarcoid-like reaction can occur or manifest several years after completion of treatment during the long-term follow-up of malignant tumors, often creating difficulty in differentiating these lesions from recurrent or metastatic malignant tumors on imaging studies. In recent years, CT and positron-emission tomography (PET) have been widely used in follow-up of malignant tumors; and findings in these studies are similar for sarcoidosis/sarcoid-like reaction and recurrent/metastatic diseases, special caution is needed in diagnosing either of these disorders. In patients who show results incompatible with recurrent or metastatic disease clinically and in imaging studies, even if increased FDG uptake on PET images is noted, biopsy should be performed, thereby allowing unnecessary treatments to be avoided.