Version June 2024
| Definition | |
| Preinvasive glandular proliferations | |
| Atypical adenomatous hyperplasia | Proliferation of atypical pneumocytes or Clara cells along alveolar walls, with cytology that falls short of carcinoma and is ≤5 mm. |
| Adenocarcinoma in situ (AIS) | Malignant epithelial cells growing along intact alveolar walls (lepidic pattern) with no evidence of invasion, measuring ≤3 cm. |
| Invasive adenocarcinoma subtypes | |
| Minimally invasive | Lepidic growth present in the majority of the tumor, but with ≤5 mm of invasion. Tumors are generally ≤3 cm. |
| Lepidic | Lepidic growth present in the majority of the tumor, but with at least one focus of invasion that is >5 mm. |
| Acinar | Tumors cells form gland-like spaces (acini), sometimes with cribriform architecture. Typically associated with desmoplastic stromal response. |
| Papillary | Tumor cells coat papillae with central fibrovascular core. |
| Micropapillary | Tumor cells coat papillae but do not have a fibrovascular core. Tumor cells seem to float in alveolar spaces. |
| Solid | Tumors lack recognizable glandular features or papillae. The cells grow in solid-appearing sheets. Mucin generally present with special stains. Also, typically express pneumocytic differentiation by immunoperoxidase studies, eg, TTF-1. |
| Invasive mucinous | Tumor cells have goblet cell morphology or columnar cells with abundant mucin. Many of the cells grow along intact alveolar septae, but will also show areas of invasion. Most of these tumors were formerly known as mucinous bronchioloalveolar carcinomas. |
| Colloid | Malignant cells are associated with abundant extracellular mucin. Tumor cells may be hard to find and may also be found floating in the mucin. |
| Fetal | The overall low appearance of the tumor resembles fetal lung. The tumors resemble endometrioid adenocarcinoma. |
| Enteric-like | The tumors resemble those arising in the colorectum. Central necrosis is typically present in the tumor cell nests. |
| Signet ring and clear cell features | Both of these patterns may be seen as components of other tumor types, most commonly solid forms and are no longer considered as stand-alone diagnoses. |
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