Apocrine Features in Invasive Ductal Carcinoma
Apocrine features in invasive ductal carcinoma are defined by distinctive morphological characteristics including abundant granular eosinophilic cytoplasm, apical cytoplasmic projections (snouts), and large nuclei with prominent nucleoli, confirmed by the presence of specific ultrastructural granules and immunohistochemical markers. 1
Morphological Characteristics
The key histologic features that define apocrine differentiation in invasive ductal carcinoma include:
- Cytoplasmic features: Uniformly fine granular, pale, eosinophilic cytoplasm with abundant volume 2
- Apical projections: Characteristic cytoplasmic "snouts" or blebs protruding from the apical surface, similar to those seen in benign apocrine metaplasia 2
- Nuclear features: Large nuclei with prominent nucleoli and at least threefold variation in nuclear size when atypia is present 1
Ultrastructural Confirmation
At the electron microscopic level, apocrine differentiation is definitively characterized by:
- Membrane-bound vesicles: Numerous 400-600 nm membrane-bound vesicles containing dense homogeneous osmophilic cores 2
- Apical clustering: These granules cluster toward the apex of the cytoplasm in the majority of epithelial cells 2
Immunohistochemical Markers
Apocrine differentiation can be confirmed using specific immunohistochemical stains:
- GCDFP-15 positivity: This protein marker is present in apocrine epithelium and serves as a reliable antigenic marker for confirming apocrine differentiation 3
- Androgen receptor (AR): Typically positive in apocrine cells 4
- Hormone receptor status: Usually estrogen receptor (ER) and progesterone receptor (PR) negative 5, 2
Diagnostic Threshold
For classification as "carcinoma with apocrine differentiation" as a special type, the WHO 2019 criteria require apocrine morphology in at least 90% of the tumor. 1
When apocrine features are present focally (less than 90%), this should be documented but the tumor is classified as invasive ductal carcinoma with focal apocrine differentiation rather than as a distinct special type 3
Clinical Pitfalls
A critical caveat is that morphologically distinct apocrine regions within a tumor can harbor different genetic aberrations compared to non-apocrine areas, including unique gains and losses on chromosome 9p and 9q, demonstrating intra-tumor genetic heterogeneity 6. This has implications for molecular testing, as sampling only one region may not capture the full genetic landscape.
The hormone receptor profile differs significantly from typical invasive ductal carcinoma, with higher rates of ER and PR negativity 5, which impacts treatment planning and should prompt consideration of androgen receptor-targeted therapies in the "pure apocrine carcinoma" subset (ER/PR-negative, AR-positive) 1.