Uncommon Presentation of Pulmonary Adenoid Cystic Carcinoma: A Case of Brain Metastasis in a Young Smoker

Abstract Introduction/Objective Pulmonary adenoid cystic carcinoma (PACC) is a rare entity, accounting for only 0.04-0.2% lung cancers, and mainly originating from the small salivary glands in the tracheobronchial tree. Due to slow-growing pattern and unspecific symptoms, the diagnosis is often delayed. While metastasis in PACC is uncommon, reports of brain metastasis are exceedingly rare. We present a case of PACC with metastasis to the brain in a young smoker. Methods/Case Report A 29-year-old female with history of smoking presented with headache, hypersomnia, episodes of vomiting and falling. Physical examination revealed multiple palpable nodes on the mid head, left temporal and right parietal area, left shoulder area were found. CT scan demonstrated complex cystic mass with a mural nodule within the left cerebellar hemisphere, with displacement of the cerebellar tonsils, as well as a 7.4 cm left infrahilar mass with heterogenous enhancement. Patient underwent craniotomy with tumor resection and left lower lobe endobronchial biopsy. Touch smear of the brain lesion showed clusters of hyperchromatic cells with myxoid like and hyalinized stroma. H&E stain showed cords of neoplastic cells embedded in a basophilic/myxoid stroma. Lung biopsy showed similar histological patterns. IHC stain showed the tumor cells were positive for epithelial and myoepithelial cells markers(CAM5.2, CK7, p63, Sox 10), retained INI1/BAF47 and BRG1 expression, negative for NUT and BCOR. Salivary Gland NGS Fusion Panel was performed and detected a MYBL1::NFIB [t(8;9)(8q13.1;9p23)] fusion alteration, which confirmed the diagnosis of adenoid cystic carcinoma. Considering no lesion was found in the patient’s head and neck, diagnosis of PACC was made. Results (if a Case Study enter NA) NA Conclusion PACC manifests three histologic subtypes: cribriform (most common), trabecular, and solid. In this case, brain and lung lesions exhibited similar patterns - nests of small, monomorphic cells within a myxoid-like and basophilic stroma background, consistent with trabecular pattern. Cytologically, densely packed cellular clusters were embedded in a myxoid like matrix. Distinguishing PACC from other salivary gland tumors is challenging. Immunostains for epithelial and myoepithelial cells, along with morphology, aid in diagnosis. The MYB::NFIB fusion helps differentiate from other lung salivary gland malignancies, but is found in a lower percentage of PACC cases (~40%) compared to other primary adnoid cystic carcinoma (~85%).