Mutually exclusive driver mutations identifies 2 separate primaries in a collision tumor initially interpreted as a solitary lung adenocarcinoma with tumor heterogeneity.

Distinction of histologically heterogenous, single primary tumor from two or more collision tumors with different primaries could represent a challenge to practicing pathologists. Histologic variations including differences in degree of differentiating within a tumor, are typically interpreted as tumor heterogeneity in a contiguous small size tumor biopsy. The authors report a case of adult former smoker female who presented with lung mass and a metastatic lytic lesion of acetabulum. A needle biopsy of a lung mass revealed an adenocarcinoma with well and moderately differentiated components. Next generation sequencing studies proved 2 different primaries in this small needle biopsy.