Solid-variant primary pulmonary adenoid cystic carcinoma with pleural metastasis and malignant pleural effusion: a rare case report.

Background: Pulmonary adenoid cystic carcinoma (PACC) is a rare salivary gland-type malignancy, accounting for 0.04%-0.2% of primary lung neoplasms. The solid variant with pleural metastasis and malignant pleural effusion is exceptionally uncommon, posing significant diagnostic and therapeutic challenges. Case presentation: We report a 65-year-old man whose lung biopsy was initially interpreted as squamous cell carcinoma without keratinization in the setting of basaloid morphology and p40/p63 positivity. The diagnosis was revised to solid-variant primary PACC after an expanded immunohistochemical workup demonstrated biphasic epithelial-myoepithelial differentiation, including expression of SOX10 and c-Myb. Given the advanced stage, the patient received a systemic combination of tislelizumab, cyclophosphamide, pegylated liposomal doxorubicin, and nedaplatin, initially combined with local intrapleural therapy. This treatment was followed by a partial response and marked symptomatic improvement without grade ≥3 treatment-related adverse events. Conclusion: This case highlights a diagnostic pitfall in lung tumors exhibiting squamoid immunophenotypes and underscores the necessity of incorporating myoepithelial markers into the diagnostic workup. Furthermore, it provides a cautiously interpreted clinical observation of immune checkpoint inhibitor-based combination therapy in advanced PACC.