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PA30.{3,5} | Phyllodes Tumor & Breast Morphology — SDL Guide (Part 3)

Microscopic Morphology Part 4 — Paget Disease, Inflammatory Carcinoma, and Special IHC

Paget disease of the nipple (microscopy):
Large malignant cells (Paget cells) with abundant pale vacuolated cytoplasm and large nuclei scattered individually within the nipple epidermis — they do NOT form glands, they sit as isolated cells in the squamous epithelium. This creates the clinical picture of nipple eczema that fails to respond to steroid creams.

Mechanism: malignant cells from an underlying DCIS or invasive carcinoma migrate up through the ductal system and invade the nipple epidermis via a pagetoid spread mechanism (LCIS can do this too — 'pagetoid spread' is named after this disease).

Key IHC: Paget cells are CK7+, HER2+, mucin+ (CEA positive). Distinguishes from Toker cells (normal clear cells in nipple epidermis, CK7+, HER2−, mucin−) and from melanoma (Melan-A/S100 positive, CK7−).

Three-panel medical illustration showing 40× histology of Paget disease of the nipple: large panel depicting scattered pale Paget cells within squamous epithelium with labels, a cytological detail inset of a single Paget cell, and a lower panel showing underlying DCIS in a subepithelial duct. — **Panel A:** Paget cells (scattered, clear vacuolated cytoplasm, large vesicular nuclei); surrounding squamous keratinocytes (pink, polygonal, intercellular bridges); keratinizing surface layer; absence of gland formation annotation; dermo-epidermal boundary line. **Panel B:** Single Paget cell cytological detail — pale glassy cytoplasm, large nucleus, prominent nucleolus, distinct nuclear membrane; blue zoom-box border linking back to Panel A. **Panel C:** Subepithelial duct cross-section; DCIS (comedo pattern) — pleomorphic epithelial cells filling lumen with central necrotic debris; basement membrane of duct; surrounding fibrous stroma.

Inflammatory carcinoma (microscopy):
There is no classic microscopic pattern of the underlying tumor — it is most commonly high-grade invasive ductal NST. The diagnostic microscopic finding is dermal lymphatic invasion — clusters of tumor cells within dilated lymphatic channels in the dermis. This is confirmed on a skin punch biopsy. Note: the 'inflammation' of the name is a misnomer — there is NO true inflammatory infiltrate in the dermis (the red, warm, edematous appearance is lymphedema from obstructed lymphatics).

Two-panel illustration showing medium-power histology of inflammatory breast carcinoma with tumor emboli within dilated dermal lymphatic channels in papillary and reticular dermis (Panel A), and a schematic skin cross-section for anatomical orientation (Panel B). — **Panel A:** Dilated lymphatic channel (thin endothelial-lined wall), Tumor emboli (cohesive clusters of hyperchromatic carcinoma cells within lumen), Papillary dermis (upper, loose collagen), Reticular dermis (lower, dense coarse collagen bundles), Collagenous stroma (eosinophilic interstitium), Thin endothelial lining, Callout: 'No inflammatory infiltrate'. **Panel B:** Epidermis, Papillary dermis, Reticular dermis, Subcutaneous fat, Dermal lymphatics (schematic, blue-green), Red highlight box with arrow indicating region shown in Panel A.

Nottingham Histological Grade (Bloom-Richardson-Elston Grade):
All invasive breast carcinomas should be graded using three parameters, each scored 1–3:
1. Tubule/gland formation (> 75% = 1; 10–75% = 2; < 10% = 3)
2. Nuclear pleomorphism (small uniform = 1; moderate variation = 2; marked variation = 3)
3. Mitotic count (per 10 HPF, calibrated to field area — thresholds vary by microscope)

Total score → Grade:
- 3–5 = Grade 1 (well differentiated) — best prognosis
- 6–7 = Grade 2 (moderately differentiated)
- 8–9 = Grade 3 (poorly differentiated) — worst prognosis

Three-panel comparison of Nottingham grading in invasive breast carcinoma: Grade 1 shows well-formed tubules, uniform nuclei, and rare mitoses (scores 1+1+1); Grade 2 shows moderate tubule formation and pleomorphism (scores 2+2+2); Grade 3 shows solid sheets, marked nuclear pleomorphism, and brisk mitoses (scores 3+3+3), with a summary strip showing the total score ranges for each grade. — **Panel A:** Grade 1 invasive carcinoma — >75% well-formed tubular structures with central lumina; small uniform round nuclei (low pleomorphism); rare mitotic figures; Tubule score 1, Nuclear score 1, Mitotic score 1 — Total 3. **Panel B:** Grade 2 invasive carcinoma — mixture of tubules and solid nests (10–75% tubule formation); moderate nuclear enlargement and contour irregularity; scattered mitotic figures; Tubule score 2, Nuclear score 2, Mitotic score 2 — Total 6. **Panel C:** Grade 3 invasive carcinoma — solid sheets with <10% tubule formation; markedly enlarged irregular nuclei with prominent nucleoli and coarse chromatin; brisk mitoses with arrows; Tubule score 3, Nuclear score 3, Mitotic score 3 — Total 9. **Footer strip:** Nottingham scoring formula: Grade 1 = 3–5, Grade 2 = 6–7, Grade 3 = 8–9; three feature categories listed with score ranges.

SELF-CHECK

A microscopic section from an invasive breast carcinoma shows tumor cells streaming in single-file lines through the fibrous stroma with a targetoid pattern around normal ducts. E-cadherin immunostain is negative. Which type of invasive carcinoma is this?

A. Invasive ductal carcinoma NST

B. Tubular carcinoma

C. Invasive lobular carcinoma

D. Mucinous carcinoma

Reveal Answer

Answer: C. Invasive lobular carcinoma

The Indian-file (single-file) pattern, discohesive cells streaming through stroma in targetoid arrangement around normal structures, and NEGATIVE E-cadherin are the classic features of invasive lobular carcinoma (ILC). E-cadherin loss is the defining molecular event — ILC lacks E-cadherin, making the cells discohesive. Ductal NST forms nests and sheets. Tubular forms angulated glands. Mucinous shows cells in mucin pools.

Master Recognition Table: Practical Exam Pattern Summary

Use this table as a rapid-revision reference. In the practical exam, your answer should cover: lesion name → key gross feature → key microscopic feature → one distinguishing point.

Lesion	Gross	Microscopy — Key Feature	Distinguishing Point
Fibroadenoma	Firm, rubbery, white, encapsulated, lobulated	Intracanalicular (slit-like ducts) + pericanalicular (round ducts); hypocellular myxoid stroma	Hypocellular stroma vs phyllodes (hypercellular)
Phyllodes tumor (benign)	Large, firm, leaf-like cross-section	Leaf-like fronds; mildly hypercellular stroma; pushing margins; < 5 mitoses/10HPF	Same architecture as fibroadenoma but stroma is hypercellular
Phyllodes tumor (malignant)	Large, necrotic, haemorrhagic	Marked stromal cellularity, pleomorphism; ≥10 mitoses/10HPF; stromal overgrowth; infiltrative margins	Stromal overgrowth = epithelium replaced by stroma in ≥1 ×4 field
Fibrocystic change	Bilateral cysts (Bloodgood blue-domed) + fibrosis	Cysts + apocrine metaplasia + fibrosis ± ductal hyperplasia	Apocrine snouts; ± ADH if nuclear atypia
Intraductal papilloma	Small friable intraluminal wart; bloody nipple discharge	Fibrovascular core + bilayer (epithelial + myoepithelial)	Myoepithelial layer present (lost in papillary carcinoma)
DCIS low-grade	Often invisible grossly	Cribriform/micropapillary; uniform cells; intact BM; myoepithelial layer present	No necrosis; low nuclear grade
DCIS high-grade (comedo)	Toothpaste-like necrosis from ducts	Large pleomorphic cells; central necrosis; microcalcification	Central comedonecrosis + coarse calcification on mammogram
LCIS	No mass	Uniform discohesive cells distending acini; no necrosis	E-cadherin negative; bilateral risk marker
IDC NST	Gritty, grey-white, stellate, hard	Irregular infiltrating nests/cords; dense desmoplastic stroma	Desmoplasia — the hallmark
ILC	Often no mass; diffuse induration	Indian-file (single-file) infiltration; targetoid growth; E-cadherin negative	Single-file = ILC; E-cad negative
Mucinous carcinoma	Soft, gelatinous	Tumor cells in extracellular mucin pools	Gelatinous gross; mucin Alcian blue+; good prognosis
Medullary carcinoma	Soft, well-circumscribed	Syncytial sheets; dense lymphocytic infiltrate at periphery; pushing margins	BRCA1; lymphocytic infiltrate = good prognosis despite high-grade
Tubular carcinoma	Small, stellate	Well-formed angulated tubules, single layer; no myoepithelium	Best prognosis; angulated lumens = tubular
Paget disease	Eczematous nipple	Large pale Paget cells in nipple epidermis; CK7+/HER2+	Nipple involvement; does not spare nipple (unlike eczema)
Inflammatory carcinoma	Peau d'orange; no mass	Dermal lymphatic invasion — tumor emboli in dermal lymphatics	Not truly inflammatory; T4d staging

Eight-panel histopathology mosaic summarising key microscopic features of breast lesions: fibroadenoma (intracanalicular), phyllodes (hypercellular stroma), comedo DCIS (central necrosis), IDC NST (desmoplasia), ILC (Indian-file), mucinous carcinoma (mucin pools), medullary carcinoma (syncytial sheets and lymphocytes), and Paget disease of the nipple (intraepidermal Paget cells), each panel labelled with lesion name and distinguishing morphological marker. — **Panel A:** Fibroadenoma — compressed slit-like ducts, paucicellular fibrous stroma (intracanalicular pattern). **Panel B:** Phyllodes Tumor — leaf-shaped fronds, densely hypercellular spindle-cell stroma. **Panel C:** Comedo DCIS — dilated duct with central necrosis/ghost nuclei, intact myoepithelial rim, basement membrane. **Panel D:** IDC NST — irregular invasive nests in desmoplastic stroma, absent myoepithelial layer, mitotic figures. **Panel E:** ILC — single-file linear infiltration of monotonous small cells, targetoid perductal arrangement. **Panel F:** Mucinous Carcinoma — extracellular mucin pools with floating clusters of low-grade epithelial cells. **Panel G:** Medullary Carcinoma — syncytial sheets of large pleomorphic cells, dense lymphoplasmacytic infiltrate, pushing border. **Panel H:** Paget Disease — large pale intraepidermal Paget cells (clear cytoplasm, large nucleus) within squamous nipple epidermis.

CLINICAL PEARL

The two most important practical-exam distinctions — say these out loud until they are automatic:

Fibroadenoma vs Phyllodes: Both are fibroepithelial. The single discriminator at low power is stromal cellularity. Fibroadenoma stroma is pale, sparse, hypocellular. Phyllodes stroma is cellular, crowded, dark. Then check for leaf-like architecture, mitoses, and margins. If you have to say one thing, say: 'The stroma is markedly hypercellular — this is phyllodes, not fibroadenoma.'

DCIS vs Invasive carcinoma: Stand back and look at the duct contour. In DCIS, the duct is a smooth oval with an outer myoepithelial rim — the tumor fills the inside but the container is intact. In invasive carcinoma, there IS no container — the cells burst out into the stroma in irregular jagged nests without any surrounding myoepithelial layer. These two things cannot look the same once you have trained your eye on them.

SELF-CHECK

Which parameter, when present in phyllodes tumor, most strongly predicts risk of distant metastasis?

A. Mitotic count of 5–9 per 10 HPF

B. Infiltrative tumor margins

C. Stromal overgrowth (stroma replacing epithelium in ≥1 ×4 field)

D. Moderate stromal nuclear atypia

Reveal Answer

Answer: C. Stromal overgrowth (stroma replacing epithelium in ≥1 ×4 field)

Stromal overgrowth — defined as stroma completely replacing the epithelial component in at least one low-power (×4) field — is the single parameter most strongly associated with distant metastasis in phyllodes tumor. It signals that the stroma is behaving as a pure sarcoma. Mitotic count, margin status, and nuclear atypia all contribute to overall grading, but stromal overgrowth is the most ominous individual finding. Its presence automatically places the lesion in the malignant grade.