Anaplastic Lymphoma Kinase (ALK)-Rearranged Lung Cancer That Showed Exclusively Scattered Isolated Cells Devoid of Mucin Production in Cytology

We present a rare case of Anaplastic Lymphoma Kinase (ALK)-rearranged lung cancer characterized by isolated scattered mucin-free cancer cells forming no clusters in the cytology of endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) samples from a paratracheal lymph node. A female patient in her late 40s underwent chest and abdominal CT scan, revealing a 6 cm diameter tumor in the upper lobe of the left lung along with enlargement of mediastinal and hilar lymph nodes, bilateral pleural effusion, and an additional 5.5 cm diameter tumor in the right greater psoas muscle. EBUS-TBNA was performed to obtain samples for cytological and histological examination. Cytology showed exclusively solitary cancer cells that were negative for Periodic Acid-Schiff (PAS) and Alcian blue staining, without clusters. Immunohistochemical analysis of cell block and histology specimens demonstrated positive expression of TTF-1, ALK, and vimentin, while E-cadherin expression was absent. Genetic analysis of samples obtained by EBUS-TBNA confirmed the presence of EML4-ALK fusion. The tumor in the right greater psoas muscle was identified as a metastatic tumor from the lung tumor based on ALK-positivity and the EML4-ALK fusion. The absence of E-cadherin expression and the presence of vimentin expression suggest that this ALK-rearranged lung cancer may have undergone epithelial-mesenchymal transition, resulting in the loss of cellular adhesiveness.