Two Cases of ALK-Altered Cancers of Unknown Primary Diagnosed by Immunohistochemistry

Cancer of unknown primary (CUP) accounts for 5% of all malignancies. Patients with CUP may live averagely for 8 months after diagnosis, and thus, rapid and reasonable diagnosis is necessary. Among patients with CUP, anaplastic lymphoma kinase (ALK)-overexpressing CUPs, whose primary sites were confirmed to be the lungs (Lung-CUP) by using antibodies against cytokeratin 7, thyroid transcription factor-1, and Napsin A, along with clinical characteristics progressed rapidly and were very sensitive to the ALK inhibitor alectinib. The incidence of ALK alteration in Lung-CUP is 19%. Consequently, it is advised that Lung-CUP be examined by immunohistochemistry (IHC) with an anti-ALK antibody. Alternative examinations, such as a cancer genome test, require as much as 2 months to complete, whereas IHC can be completed within days. In this report, a rapid assessment by IHC led to alectinib treatment, which resulted in good outcomes in 2 cases of Lung-CUP. Alectinib was effective for ALK-altered Lung-CUPs.