a carcinoma, subtype of adenocarcinoma, thought to be derived from epithelium of terminal bronchioles, in which the neoplastic tissue extends along the alveolar walls and grows in small masses within the alveoli; involvement may be uniformly diffuse and massive, or nodular, or lobular; microscopically, the neoplastic cells are cuboidal or columnar and form papillary structures; mucin may be demonstrated in some of the cells and in the material in the alveoli, which also includes denuded cells; metastases in regional lymph nodes, and even in more distant sites, are known to occur, but are infrequent. SYN: bronchiolar adenocarcinoma, bronchiolar carcinoma, bronchioloalveolar adenocarcinoma, bronchioloalveolar carcinoma, bronchoalveolar carcinoma.