Overexpression of CD 133 and BCL‐2 in non‐small cell lung cancer with neuroendocrine differentiation after transformation in ALK rearrangement‐positive adenocarcinoma

: Transformation to small cell lung cancer is one phenomenon of acquired resistance to anaplastic lymphoma kinase (ALK) tyrosine kinase inhibitors in ALK rearrangement-positive non-small cell lung cancer (NSCLC). Few case reports have focused on other types of histological transformation. We report a case of transformation of ALK rearrangement-positive adenocarcinoma to NSCLC with neuroendocrine differentiation during alectinib therapy. A 36-year-old woman presented with a tumor in the left lower lobe and bone metastases. She was diagnosed with ALK rearrangement-positive adenocarcinoma by histopathology of the primary tumor. Alectinib had been effective for 8 months before new lesions appeared. Histopathological re-examination of a recurrent tumor revealed poorly differentiated carcinoma with insulinoma-associated protein 1 (INSM1) expression, which remained ALK-positive. Expression of CD133, BCL-2, and SOX2 was positive in comparison to the initial tumor. Expression of SOX2 became more strongly positive than it was before treatment. The immunohistochemical findings of these markers associated with cancer stem-like cells and/or neuroendocrine differentiation suggest that cancer stem cells play a role in the mechanisms of histological transformation and acquired resistance of ALK rearrangement-positive cancer. To our knowledge, this is the first report to suggest an association between cancer stem-like cells and histological transformation in ALK rearrangement-positive lung cancer.