The relationships between gross, microscopic and ultrastructural features of pulmonary adenocarcinomas and the morphologic significance of lepidic growth are studied using nine cases of peripheral adenocarcinomas with lepidic growth. They include three bronhioloalveolar carcinomas ( BAC's), two acinar adenocarcinomas, two papillary adenocarcinoms and two metastatic adenocarcinomas. Ill-defined consolidating mass was seen in three cases of BAC's and one acinar adenocarinoma. Other cases of acinar, papillary types and metastatic tumors were classified into secretory type or poorly differentiated type and metastatic adenocarcinomas showed mixture of the types. Mucin secreting cell by light microscopy showed apical clustering of electron-lucent mucin vacuoles by electron microscopy, but non-secretory type showed variable features of electron microscopy, but non-secretory type showed variable features of Clara cell, type II pneumocytes of indeterminate cells. Tumor growing along the alveolar wall was encountered in every type of the cases byt the distribution and microscopic findings were different according to the light microscopic types. The lepidic pattern was seen homogeneously in the whole tumor field in BAC's but was seen only at the peripheral zone of the tumor in the other instances. We could classify the microscopic features of lepidic growth into three patterns based on the findings in BAC's. Regular lepidic growth confined to the alveolar wall was seen only in BAC. One paillary adenocarcinoma and one BAC showed multicentric nodules which was composed of micropapillary projections into the alveolar spaces in addition to the tumor cells covering the preexisting alveolar wall. Other cases showed intraluminal polypoid growth and shedding into the lumina and numerous tumor balls were scattered. The significance of these morphologic patterns of lepidic growth is undetermined, however we could speculate that first and second patterns are well differentiated forms but polypoid shedding pattern is poorly differentiated form with rapid aerogenous spread and has more frequent chance to be detected by sputum cytology.
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