Background and Aim It is of importance to search for prognostic indicators supplementing the tumour–node–metastasis stage for surgically resected early-stage lung adenocarcinomas. The roles of stromal invasion and micropapillary pattern in categorising histopathology and predicting the prognosis of stage I lung adenocarcinomas are explored.

Methods We retrospectively examined 212 consecutive surgically resected stage I lung adenocarcinomas to propose a new histopathology-based categorical classification. Category A tumours have pure lepidic growth pattern without stromal invasion (ie, adenocarcinoma in situ). Stromal invasion in the form of central fibrotic focus is absent in category B tumours and present in category C tumours. Category B is subclassified into B1, which has areas of lepidic growth, and B2, which does not. Category C is subclassified into C1, which has invasive tumour cells in the periphery of central fibrotic focus, and C2, which has invasive tumour cells in the centre of central fibrotic focus. Based on the absence or presence of micropapillary pattern, the C2 tumours are further subclassified into C2a and C2b, respectively.

Results The 5-year recurrence-free probabilities for categories B1 (17 cases), B2 (10 cases), C1 (nine cases), C2a (114 cases) and C2b (62 cases) are 100%, 78.8%, 100%, 67.5% and 53.1%, respectively (p<0.001).

Conclusions Based on stromal invasion and micropapillary pattern, the histopathological categorical classification proposed here provides a concise and precise scheme for outcome prediction in early-stage lung adenocarcinomas.