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PA30.2 | Carcinoma of the Breast — SDL Guide (Part 3)

Special Invasive Types: Medullary, Mucinous, and Tubular Carcinoma

Several invasive carcinoma types have better prognosis than IDC NST when diagnosis is strict. They account for ~5–10% of invasive breast carcinomas collectively.

Side-by-side histology comparison of medullary carcinoma (left, showing syncytial high-grade cells with dense lymphoplasmacytic stroma) and mucinous carcinoma (right, showing low-grade cell clusters floating in abundant pale extracellular mucin pools). — **Panel A:** Medullary carcinoma: syncytial tumor cell sheets with indistinct borders, vesicular open nuclei with prominent nucleoli, dense lymphoplasmacytic stromal infiltrate, pushing (non-infiltrative) tumor margin. **Panel B:** Mucinous carcinoma: small uniform low-grade tumor cell clusters, abundant pale extracellular mucin lakes surrounding cell nests (not intracellular), thin fibrous septae dividing mucin pools, minimal stromal component.

1. Medullary Carcinoma (Classic)
• Rare; more common in BRCA1 mutation carriers
• Soft, well-circumscribed, fleshy ("pusher" margins — not invasive-stellate)
• Histology: Syncytial sheets (>75%) of high-grade vesicular-nucleus cells + prominent lymphoplasmacytic infiltrate in stroma
• Triple-negative (ER−/PR−/HER2−) yet paradoxically better prognosis than grade-matched IDC NST
• Good prognosis relates to the immune infiltrate — tumour-infiltrating lymphocytes (TILs) are a favourable prognostic marker

2. Mucinous (Colloid) Carcinoma
• Mostly in older women (>60 yrs)
• Gross: Glistening, gelatinous, grey-blue tumour
• Histology: Small, uniform, low-grade tumour cells floating in large pools of extracellular mucin (mucin surrounds cell clusters, not intracellular)
• ER+/PR+, low proliferation, very favourable prognosis (>90% 10-year survival for pure mucinous)

3. Tubular Carcinoma
• Detected by screening mammography as a tiny spiculated lesion
• Histology: Well-formed, open, angulated tubules lined by a single layer of bland low-grade cells, infiltrating stroma; no myoepithelium around tubules (distinguishes from sclerosing adenosis)
• Grade 1 by definition; ER+/PR+
• Excellent prognosis — 10-year survival >95%

Three-panel H&E histology diagram comparing tubular carcinoma (Panel A, medium power) showing angulated open tubules with absent myoepithelial layer in desmoplastic stroma against sclerosing adenosis (Panel B) with intact outer myoepithelial rim, and a high-power split (Panel C) annotating the diagnostic difference. — **Panel A:** Angulated open tubules; single-layer bland luminal epithelium; desmoplastic fibrous stroma with fibroblasts; absent myoepithelial layer (annotated with dashed bracket and red label). **Panel B:** Compressed distorted glands within lobular architecture; inner luminal epithelial cells; outer myoepithelial cell rim (grey-blue); intralobular fibrosis; intact myoepithelial layer (annotated with green label). **Panel C:** Left — tubular carcinoma gland cross-section: single epithelial ring, open lumen, no outer rim, label 'No myoepithelial rim'; Right — sclerosing adenosis gland: two-layer ring, inner pink epithelium + outer grey-blue myoepithelium, label 'Myoepithelial rim intact'; colour key for cell types.

Inflammatory Carcinoma and Paget Disease of the Nipple

These two entities have distinctive clinical presentations and are must-know for examinations.

Inflammatory Carcinoma

Inflammatory carcinoma is a clinical diagnosis, not a histologic subtype. It is the most aggressive presentation of breast carcinoma.

Three-panel diagram: Panel A shows a clinical illustration of inflammatory breast carcinoma with labelled diffuse erythema, peau d'orange, nipple retraction and absence of a discrete mass; Panel B illustrates the dermal lymphatic obstruction mechanism producing peau d'orange; Panel C compares key distinguishing features of inflammatory carcinoma versus acute mastitis. — **Panel A:** Diffuse erythema (whole-breast skin redness), peau d'orange texture (orange-peel dimpling), nipple retraction, generalised swelling/oedema, label: no discrete palpable mass — diffuse involvement. **Panel B:** Epidermis, dermis, dermal lymphatic vessel distended by tumour emboli, lymphoedema of dermis, sweat-gland openings, skin tethering at sweat-gland openings producing surface dimpling, arrows showing obstruction-to-dimple causal chain. **Panel C:** Left: Acute Mastitis — wedge-shaped erythema, central abscess, fever icon, responds to antibiotics; Right: Inflammatory Carcinoma — diffuse whole-breast involvement, no discrete abscess, no fever, no antibiotic response, T4d Stage IIIB+ label.

Presentation: Rapid onset (weeks) of breast erythema, warmth, tenderness, swelling, and skin oedema resembling acute mastitis — but without fever and without response to antibiotics
Peau d'orange (orange-peel skin): Dermal lymphatic obstruction by tumour emboli causes lymphoedema of the skin → tethering of skin at sweat-gland openings → orange-peel texture
Key histological finding: Dermal lymphatic invasion — tumour cell emboli within dermal lymphatics. This is required for diagnosis (not just carcinoma with secondary skin oedema)
Usually high-grade, ER-negative, HER2+ or triple-negative
Staging: T4d (Stage IIIB minimum) regardless of tumour size
Treated with neoadjuvant chemotherapy first (not primary surgery)

Two-panel H&E histology diagram of inflammatory breast carcinoma skin punch biopsy showing dilated dermal lymphatic channels filled with tumour cell emboli in oedematous dermis, with a high-power inset detailing nuclear pleomorphism within the embolus. — **Panel A:** Dermal lymphatic vessel (thin-walled dilated channel), tumour emboli within lumen (intraluminal pleomorphic cell clusters), oedematous dermis (pale loosely arranged collagen stroma). **Panel B:** Endothelial lining (flat cells forming vessel wall), tumour cell embolus (cohesive cluster of malignant cells), nuclear pleomorphism (irregular hyperchromatic nuclei with prominent nucleoli).

Paget Disease of the Nipple

Clinical: Eczematous, crusted, itchy, erythematous lesion of the nipple-areola complex, may ooze/bleed; mimics eczema but does NOT respond to topical steroids
Histology: Large, pale, vacuolated malignant cells (Paget cells) individually or in clusters within the squamous epithelium of the nipple epidermis, with a clear halo around each cell
Pathogenesis: Malignant ductal cells migrate from an underlying DCIS (or invasive carcinoma) up the nipple ducts into the epidermis
Key point: Paget disease of the nipple always indicates underlying carcinoma (DCIS in ~30–40%, invasive carcinoma in ~60–70%)

SELF-CHECK

A 60-year-old woman presents with a 3-week history of erythema, warmth, and oedema of the entire right breast with peau d'orange skin change. Skin punch biopsy shows tumour cell emboli in dermal lymphatics. What is the minimum TNM T-stage for this lesion?

A. T1 (tumour ≤2 cm)

B. T2 (tumour 2–5 cm)

C. T3 (tumour >5 cm)

D. T4d (inflammatory carcinoma)

Reveal Answer

Answer: D. T4d (inflammatory carcinoma)

Inflammatory carcinoma with dermal lymphatic invasion is classified as T4d regardless of tumour size. This places it at Stage IIIB minimum. The defining histological requirement is tumour cell emboli in dermal lymphatics — the clinical syndrome alone (erythema + peau d'orange) is insufficient without biopsy confirmation.

Molecular/Intrinsic Subtypes and IHC-Based Classification

Gene expression profiling (microarray) has identified intrinsic molecular subtypes of breast carcinoma with distinct biology, prognosis, and treatment sensitivity. In routine practice, surrogate IHC markers (ER, PR, HER2, Ki-67) approximate these subtypes.

Four-quadrant IHC composite of breast carcinoma showing Panel A ER-positive nuclear staining, Panel B PR-positive nuclear staining, Panel C HER2 3+ complete membranous staining, and Panel D Ki-67 high proliferation index, each annotated with the marker name and scoring method. — **Panel A:** ER (Estrogen Receptor) — strong brown nuclear DAB staining in majority of carcinoma nuclei; Allred Score / H-score annotation; nuclear-pattern label with leader line. **Panel B:** PR (Progesterone Receptor) — moderate-to-strong brown nuclear staining in subset of tumor nuclei; Allred Score annotation; nuclear-pattern label with leader line. **Panel C:** HER2 (HER2/neu, ERBB2) — strong complete circumferential brown membranous staining (3+ pattern); ASCO/CAP 0–3+ scoring annotation; small inset comparing 0/1+ patterns for context. **Panel D:** Ki-67 (MIB-1 clone) — numerous brown proliferating nuclei among blue non-proliferating nuclei; high proliferation index ≥30% annotation; fraction counter visual illustrating hot-spot counting method.

Molecular Subtype	ER	PR	HER2	Ki-67	Prognosis	Treatment
Luminal A	+	+	−	Low (<20%)	Best	Endocrine therapy only
Luminal B (HER2−)	+	+/−	−	High (≥20%)	Intermediate	Endocrine + chemo
Luminal B (HER2+)	+	+/−	+	Any	Intermediate	Endocrine + anti-HER2 + chemo
HER2-enriched	−	−	+	High	Poor	Anti-HER2 + chemo
Basal-like / Triple-negative (TNBC)	−	−	−	High	Worst	Chemo (± PARP inhibitors if BRCA+)

Key IHC scoring rules:
• ER/PR: Positive if ≥1% of nuclei show staining (Allred score ≥3 or ≥1% per ASCO/CAP 2020 guidelines). Positive = hormone-dependent.
• HER2: Scored 0/1+/2+/3+. 3+ (strong, complete membranous staining in >10% of cells) = positive. 2+ (equivocal) → reflex FISH/CISH for HER2 gene amplification. 0/1+ = negative.
• Ki-67: Proliferation marker (% positive nuclei). Cutoff varies (~14–20%); high Ki-67 = aggressive biology.

Three-panel diagram showing breast cancer molecular subtypes on an ER/HER2 quadrant bubble chart with prognosis arrows and treatment icons, a TNBC feature callout, and a HER2 IHC-to-FISH testing algorithm flowchart. — **Panel A:** Quadrant bubble chart: Luminal A (ER+/PR+/HER2−, ~40%, best prognosis, hormone therapy), Luminal B (ER+/HER2+, ~20%, intermediate, hormone + anti-HER2), HER2-Enriched (ER−/HER2+, ~15%, poor, anti-HER2), Triple-Negative (ER−/PR−/HER2−, ~15–20%, worst prognosis, chemotherapy); bubble size ∝ relative frequency; prognosis arrows color-coded green/yellow/orange/red. **Panel B:** TNBC spotlight: prevalence ~15–20%, BRCA1 mutation enrichment, basal CK5/6 expression marker, current treatment landscape including PARP inhibitor standard for BRCA-mutant TNBC. **Panel C:** HER2 testing decision tree: IHC 0/1+ → negative; IHC 3+ → positive (trastuzumab eligible); IHC 2+ equivocal → reflex FISH; FISH ratio >2.0 or >6 copies/cell → amplified (trastuzumab eligible); FISH ratio ≤2.0 → not amplified.

Triple-negative breast cancer (TNBC) merits special attention:
• Comprises ~15–20% of breast carcinomas
• Highly aggressive, high proliferation, early haematogenous spread
• Enriched for BRCA1 mutations (hereditary) and basal cytokeratin expression (CK5/6+)
• No targeted therapy currently (no hormone receptor, no HER2) — but PARP inhibitors for BRCA-mutant TNBC are now standard

CLINICAL PEARL

HER2 2+ equivocal — the FISH reflex: When IHC returns HER2 2+ (equivocal), the pathologist must order reflex fluorescence in situ hybridisation (FISH). FISH counts the number of HER2 gene signals per cell (and the chromosome 17 centromere as reference). A ratio >2.0 or absolute copy number >6 signals/cell = amplified = positive. Trastuzumab is only effective in HER2-amplified tumours. This is why the case in the hook scenario required a FISH test — the IHC alone was inconclusive.