Invasive Micropapillary Carcinoma of the Breast: Key Points for Boards, Inside-Out and Cup-Shaped Staining, MCQs

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Invasive Micropapillary Carcinoma of the Breast: Key Points for Boards, Inside-Out and Cup-Shaped Staining, MCQs

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Pathology MCQs

22 May 2021

Subtitle: Invasive micropapillary carcinoma of the breast: key points for boards, morphology, inside-out (reverse polarity) EMA/MUC1 staining, cup-shaped E-cadherin and HER2 staining, receptor profile, nodal metastasis, mimics and 7 MCQs.

Author: PathologyMCQ Editorial Board

Category: Breast Pathology

Last Updated: October 2026

Read Time: 5 minutes

At-a-Glance

Morphology: hollow or morula-like clusters without fibrovascular cores in clear stromal spaces.
Inside-out staining: EMA, MUC1 and CD15s on the stroma-facing surface.
Cup-shaped staining: E-cadherin, HER2 and p120 stain basolateral membranes with apical sparing.
Very frequent lymphovascular invasion and nodal metastasis.
Mostly ER positive; HER2 positive in a minority.

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Morphology

Tumour cells form small tufts, morules or hollow clusters (pseudopapillae) without fibrovascular cores. Each cluster lies in an empty space separated from the fibrocollagenous stroma, mimicking lymphovascular invasion. WHO 2019 uses the term invasive micropapillary carcinoma when this pattern forms more than 90% of the tumour; otherwise report it as a mixed tumour and state the proportion.

Invasive micropapillary breast carcinoma microscopy — Micropapillary clusters in clear stromal spaces.

IHC Patterns

Inside-out (reverse polarity): EMA, MUC1 and CD15s stain the apical membrane, which faces the stroma, so the outer rim of each cluster is highlighted.

Inside-out EMA and CD15s staining in micropapillary carcinoma — Inside-out staining: EMA and CD15s on the stroma-facing surface.

Cup-shaped (U-shaped) staining: basolateral markers such as E-cadherin, HER2 and p120 stain the cell borders except the outward apex.

Cup-shaped basolateral membrane staining with apical sparing — Cup-shaped staining: basolateral membranes stained, apical surface spared (E-cadherin, HER2, p120 pattern).

Behaviour and Receptors

Invasive micropapillary carcinoma: key facts

Feature	Typical finding
Lymphovascular invasion	Very common
Nodal metastasis at presentation	Up to about 70–90%
ER / PR	ER positive in about 90%; PR in about 50–70%
HER2	Positive in about a quarter to a third
HER2 scoring pitfall	Basolateral (cup-shaped) staining can be scored as complete membranous if strong

Differential Diagnosis

Lymphovascular invasion: spaces lack endothelium (CD31, D2-40 negative) in IMPC.
Mucinous carcinoma with micropapillary pattern: clusters float in mucin.
Metastatic micropapillary carcinoma from bladder (GATA3+, also breast!), lung (TTF-1), ovary (PAX8, WT1) or colon (CDX2, SATB2).
Papillary carcinoma: true fibrovascular cores.

Video

Test Yourself: 7 MCQs

Micropapillary carcinoma: 7 MCQs

Morphology, inside-out and cup-shaped staining, and behaviour, with instant explanations.

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Exam Pearls

No fibrovascular cores; clusters in clear spaces.
Inside-out: EMA, MUC1, CD15s outward.
Cup-shaped: E-cadherin, HER2, p120 basolateral.
High LVI and nodal metastasis.

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