Consensus and conflict in invasive micropapillary carcinoma: a case report and review of the literature

Invasive micropapillary carcinoma (IMPC) is an aggressive histologic subtype of adenocarcinoma that has been gaining increased attention over the past twenty years. It is important to recognize IMPC due to its strong association with early lymphovascular invasion (LVI), high risk of lymph node metastasis, perineural invasion and poor prognosis. Controversies regarding IMPC include differentiating from retraction artifact and mimics, clinical significance of proportion of micropapillary component (MC), pathogenesis, biologic nature of the entity and consequently terminology, etc. We herein present a case of rectal IMPC arising from a tubulovillous adenoma. Since HER2 over-expression has been reported in IMPC of the breast and the bladder but never in the colorectum, given the availability of HER2 targeted therapy, HER2 protein expression in our case is examined by immunohistochemical study which shows weak incomplete membrane staining in less than 5% of cells. Literature is reviewed with emphasis on colorectal IMPC as well as aforementioned controversial topics. In summary, more study is needed to resolve the conflicts and build consensus on IMPC.