Micropapillary Mucinous Adenocarcinoma of the Lung: A Brief Case Report

In 2011, the International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society (IASLC/ATS/ERS) classification added micropapillary predominant adenocarcinoma of the lung as a new histologic subtype and reclassified the former mucinous bronchioloalveolar carcino-ma (BAC) as a variant of invasive mucinous adenocarcinoma. 1 Recent studies regarding epidermal growth factor receptor (EGFR) and K-ras mutations in lung cancer demonstrated that BAC with a lepidic or hobnail growth pattern is more typically associated with EGFR mutation, while mucinous differentiation is more common in cases with K-ras mutation. 2-4 However, these studies were based on the 2004 World Health Organization (WHO) classification, and there had been a few reports on the micropapillary pattern of this disease and its correlation with EGFR and K-ras mutations. According to the report of the IASLC, 1 micropapillary predominant adenocarcinoma is known to be related to K-ras (33%), EGFR (20%), and BRAF (20%) mutations. Hypothesizing that micropapillary predominant ad-enocarcinoma with mucinous differentiation may be correlated with K-ras mutation, we performed a histological review of a case of micropapillary mucinous adenocarcinoma with associated K-ras mutation. A 69-year-old woman presented with persistent cough for one month previously. The patient had a 13 pack-year smoking history and hypertension. Chest computed tomography revealed predominant ground glass opacity with patchy consolidation occupying almost the entire left lower lobe of the lung (Fig. 1A). It was histologically confirmed as adenocarcinoma of the lung by transbronchial lung and lymph node biopsy. Analysis of EGFR and K-ras mutation by polymerase chain reaction-se-quencing was performed on tissue from the lymph node biopsy, which revealed missense mutation of K-ras in the 12th codon, c.34G>T (p.G12C), without EGFR mutation. After neoadju-vant concurrent chemoradiation, a left lower lobectomy was performed. The gross examination revealed a poorly defined, large, mass-like lesion with scattered dilated bronchus, which was similar in appearance to lobar pneumonia and well correlated with radiologic findings (Fig. 1B). Microscopically, the tumor demonstrated a predominantly micropapillary growth pattern in the alveolar cavity and a lep-idic growth pattern along the septa with hobnail features (Fig. 1C). Neoplastic cells with abundant clear or amphophilic cyto-plasm intermingled with each other (Fig. 1D). Special stains (periodic acid Schiff, mucicarmine, and Alcian blue pH 1.0 and 2.5) revealed positivity in the cytoplasm of a few tumor cells (Fig. 1D, inset). Tumor cells had well-defined cell borders and large, pleomorphic nuclei (Fig. 1E), with occasional horseshoe shaped nuclei and perinuclear clearing. Their nuclei …