Mucinous micropapillary pattern in lung adenocarcinomas: a unique histology with genetic correlates

In lung adenocarcinoma (ADC), micropapillary carcinomas (MPCs) are associated with poor prognosis because these tumours exhibit higher metastatic potential. Despite this, there are no studies investigating the differences between mucinous and non-mucinous MPC.We evaluated the proportion of micropapillary components in lung ADCs, and compared the differences with respect to the presence or absence of associated mucin. Tumour specimens from 694 patients with consecutively resected primary lung ADC were reviewed, and 37 cases of invasive mucinous ADCs were excluded. A significant (≥5%) micropapillary component was noted in 320 (48.7%) of 657 evaluable cases. When the cases with micropapillary component were divided into 67 (20.9%) mucinous and 253 (79.1%) non-mucinous subtypes, tumours with mucinous micropapillary component exhibited significantly more aggressive pathological features, a higher proportion of HER2 mutations (P = 0.002) and ALK rearrangements (P < 0.001), and a lower proportion of EGFR mutations (P = 0.038) compared to those with a non-mucinous micropapillary component. In survival analyses, mucinous MPC tended to be more aggressive compared with non-mucinous MPC, but its prognostic value was not statistically significant (P = 0.076).Mucinous micropapillary pattern is an under-recognized unique growth associated significantly with HER2 mutation and ALK rearrangement.