Pulmonary Oligometastasis From the Kidney Masquerading as Primary Carcinoma of the Lung

A biopsy of a solitary lung mass was diagnosed as a primary large cell carcinoma, favoring adenocarcinoma, based on positive epithelial markers, solid growth pattern, absence of squamous or endocrine differentiation, and relatively abundant eosinophilic cytoplasm. The immunohistochemical evaluation was initially confounded by misleading positive results for CD68 and calretinin. The diagnosis was changed to metastatic renal cell carcinoma, when further history and immunohistochemical stains were performed. This report underscores the utility of history and imaging findings in differentiating primary versus metastatic lung tumors and the need for judicious use of immunohistochemical stains. The differential diagnosis of carcinomas of the lung with solid growth pattern is discussed.