MKI67 and papillary adenoma: (2) Plmonary papillary adenoma is a specific subtype of invasive lung adenocarcinoma that shows indistinct demarcation from the surrounding area and fibrous capsule, histological features of infiltrative growth into the peripheral tissues, a complex branching papillary structure, obvious heterogeneity of the lining tumor cells, multiple nuclear mitosis images, and a high KI67 proliferation index [8].