Abstract
Atypical adenomatous hyperplasia (AAH) and adenocarcinoma in situ (AIS) are generally treated as different lesions, depending on the differences in lesion size and histological findings. However, these differences are not absolute; thus, AAH and AIS are often difficult to distinguish. Moreover, whether AAH and AIS can be regarded as different lesions remains unknown because cytological specimens, especially those of AAH, are rare. In this study, we examined these uncommon cytological specimens and compared the cytological findings between AAH and AIS. We observed many common cytological features with no obvious differences between AAH and AIS. These findings suggest that these two distinct lesions can be grouped into a single category. Therefore, we propose creating a new cytological category.