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Benign Tumors and Tumor-Like Lesions of the Breast

Robbins, Cotran & Kumar: Pathologic Basis of Disease

Overview: Breast Structure and Lesion Classification

The breast is composed of ducts, lobules, two epithelial cell types (luminal and myoepithelial), and two stromal compartments (intralobular and interlobular). Each element is a source of both benign and malignant disease.

BREAST ANATOMY → Sources of Disease
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                    BREAST PARENCHYMA
                          │
          ┌───────────────┴────────────────┐
     EPITHELIAL CELLS               STROMAL CELLS
          │                               │
    ┌─────┴──────┐                 ┌──────┴──────┐
  Luminal    Myoepi-           Intralobular  Interlobular
  cells      thelial           stroma        stroma
    │          cells               │               │
    │                          Fibroadenoma    Lipoma
    │                          Phyllodes Tumor Myofibro-
    │                                          blastoma
    │                                          Fibromatosis
    │                                          Angiosarcoma
    │
    ├─ Fibrocystic Changes (non-neoplastic)
    ├─ Proliferative Disease Without Atypia
    ├─ Proliferative Disease With Atypia
    └─ Carcinoma (DCIS → Invasive)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Key point: >90% of symptomatic breast lesions are benign. Most benign palpable masses are fibroadenomas or cysts.

FLOWCHART 1: Clinical Presentation of Breast Disease

Patient presents with breast complaint
              │
   ┌──────────┼──────────────┬──────────────┐
   ▼          ▼              ▼              ▼
Lumpiness   Pain        Palpable        Nipple
(diffuse    (cyclic/    Mass            Discharge
nodularity) non-cyclic)   │              │
   │           │           │             │
Fibrocystic Premenstrual  ┌─┴──┐       ┌──┴───┐
Changes     edema /     Benign Malignant Milky  Bloody/
            ruptured    (~95%) (~5%    (galac- Serous
            cyst              ↑with    torrhoea)(papilloma
                              age)            /cancer)
                         │
              ┌──────────┴────────────┐
         Round/oval,           Irregular,
         rubbery, mobile       hard (scirrhous),
         (Fibroadenoma,        nonmobile
         Cyst)                 (Carcinoma)

SECTION 1: Nonproliferative Breast Changes (Fibrocystic Changes)

Age group: 30-50 years | Cause: Hormonal fluctuations during menstruation | Risk: NO increased cancer risk

Three Principal Morphological Features:

FIBROCYSTIC CHANGES
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
         ┌──────────────────────┐
         │  FIBROCYSTIC CHANGES │
         └──────────────────────┘
                   │
    ┌──────────────┼──────────────┐
    ▼              ▼              ▼
CYSTIC CHANGE   FIBROSIS      ADENOSIS
    │              │              │
Dilation of    Cyst rupture   Increase in
lobules →      → chronic       acini per
larger cysts   inflammation    lobule
    │
Lined by flat
or APOCRINE
cells
("Blue-dome     Fibrosis = palpable
 cysts")        nodularity
    │
Calcifications
common
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Apocrine Cysts (Fig. 23.5) - Cysts lined by apocrine cells with granular eosinophilic cytoplasm; blue-tinged turbid fluid; confirmed by FNA causing mass to disappear:

Fig. 23.5 Apocrine cysts - (A) clustered calcifications, (B) gross dark turbid fluid, (C) histology showing apocrine lining with calcifications

Special variant - Lactational Adenoma: Palpable mass in pregnant/lactating females; normal-appearing breast tissue with lactational changes; regresses after breastfeeding stops. Considered an exaggerated local response to gestational hormones rather than a true neoplasm.

SECTION 2: Proliferative Breast Disease Without Atypia

Risk: Small increase (~1.5-2x) in subsequent carcinoma in BOTH breasts | Usually detected as mammographic densities, calcifications, or incidental findings

Subtypes:

Lesion	Morphology	Key Features	Clinical Presentation
Usual Ductal Hyperplasia (UDH)	Mixed luminal + myoepithelial cells filling ducts; irregular peripheral slit-like lumens	Mixed population (CK14-positive myoepithelial cells within mass)	Usually incidental
Sclerosing Adenosis	Increased acini, compressed by stromal fibrosis; "swirling" pattern with well-circumscribed outer border	May mimic invasive carcinoma; calcifications within lumens	Palpable mass, radiologic density, calcifications
Radial Scar (Complex Sclerosing Lesion)	Stellate lesion; entrapped glands in hyalinized elastotic stroma surrounded by radiating projections	Mimics invasive carcinoma radiologically + histologically. NOT caused by trauma/surgery. >1 cm = complex sclerosing lesion	Mammographic abnormality
Papilloma (Large Duct)	Multiple branching fibrovascular cores in dilated duct; UDH and apocrine metaplasia common	Solitary; central (lactiferous sinuses)	Nipple discharge - bloody (torsion/infarction) or serous; >80% produce discharge
Papilloma (Small Duct)	Same structure, peripheral ducts	Multiple; peripheral	Clinically occult; small palpable masses or mammographic densities

PROLIFERATIVE DISEASE WITHOUT ATYPIA - RISK PATHWAY
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Normal Breast → Fibrocystic Change (nonproliferative)
     │
     └─→ Proliferative Without Atypia ────────────────►
              (UDH, Sclerosing Adenosis,                RISK:
               Radial Scar, Papilloma)               ~1.5-2x
                                                      (both
                                                       breasts)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

SECTION 3: Proliferative Breast Disease WITH Atypia (Precursor Lesions)

Risk: 4-5x increased risk of carcinoma | Regarded as precursors, not just risk markers

Flowchart: Low-Grade Neoplasia Pathway

MOLECULAR PROGRESSION - LOW GRADE PATHWAY
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Normal TDLU
     │
     ▼
Columnar Cell Lesion (CCL)
  - Clonal proliferation
  - Earliest recognizable precursor
  - Variably dilated acini
  - Columnar epithelial cells
  - Microcalcifications on mammography
     │
     ▼
Flat Epithelial Atypia (FEA)
  - CCL + CYTOLOGIC ATYPIA
  - No architectural complexity (unlike ADH)
     │
     ▼
Atypical Ductal Hyperplasia (ADH)
  - Clonal; has SOME but NOT ALL features of
    low-grade DCIS (found in ~10% of calcification biopsies)
  OR
Atypical Lobular Hyperplasia (ALH)
  - Incidental finding; found in <5% of biopsies
     │
     ▼
Low-grade DCIS / LCIS
     │
     ▼
Invasive Carcinoma (same morphologic/molecular features)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

SECTION 4: Relative Risk of Breast Cancer by Lesion Type

Category	Lesion	Relative Risk vs. General Population
Nonproliferative	Fibrocystic changes, adenosis, mild hyperplasia	No increased risk (1x)
Proliferative without atypia	UDH, sclerosing adenosis, papilloma, radial scar	~1.5-2x
Complex fibroadenoma	With cysts >0.3 cm, sclerosing adenosis, epithelial calcifications, papillary apocrine change	Slightly increased
Proliferative with atypia	Atypical ductal hyperplasia (ADH)	~4-5x
Proliferative with atypia	Atypical lobular hyperplasia (ALH)	~4-5x
LCIS / DCIS	Lobular/ductal carcinoma in situ	~8-10x

(Derived from Robbins, Cotran & Kumar Table 23.1 data)

SECTION 5: Fibroadenoma

The most common benign tumor of the female breast. Arises from intralobular stroma.

Molecular Pathogenesis:

FIBROADENOMA - PATHOGENESIS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                  INTRALOBULAR STROMAL CELL
                           │
             ┌─────────────┴────────────┐
             ▼                          ▼
    MED12 mutation (~2/3)          RARA mutation (~1/3)
    (Mediator complex,              (Retinoic acid receptor α;
    regulates RNA Pol II            estrogen target gene;
    transcription)                  cooperates with ER)
             │                          │
             └────────────┬─────────────┘
                          ▼
          Altered expression of sex hormone-regulated genes
          → stromal cell proliferation + survival
                          │
                          ▼
                    FIBROADENOMA
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Note: Uterine leiomyoma also has MED12 mutations - both arise from hormone-responsive stromal cells.

Morphology:

Well-circumscribed, rubbery, gray-white nodule that bulges above surrounding tissue
Slit-like spaces lined by epithelium
Pericanalicular pattern: stroma surrounds patent ducts
Intracanalicular pattern: stroma compresses/distorts ducts into cleft-like spaces
In older women: densely hyalinized stroma + atrophic epithelium

Fig. 23.24 Fibroadenoma - (A) well-circumscribed mass on radiograph, (B) rubbery white gross specimen, (C) intralobular stroma compressing epithelium:

Fig. 23.24 Fibroadenoma showing radiograph, gross specimen, and histology with intracanalicular pattern

Subtypes of Fibroadenoma:

Subtype	Features	Association
Sporadic	Standard morphology	MED12/RARA mutations
Myxoid	Myxoid stroma	Most sporadic; small proportion: Carney complex (PRKAR1A germline mutations, AD)
Complex	Cysts >0.3 cm, sclerosing adenosis, epithelial calcifications, or papillary apocrine change	Slightly increased cancer risk (likely due to co-existing at-risk lesions in surrounding breast)

Clinical Features:

Feature	Details
Age	Mainly 20s-30s
Number	Frequently multiple and bilateral
Hormonal response	Grow during pregnancy; regress after menopause; rapid growth/infarction in pregnancy may mimic carcinoma
Special association	~50% of cyclosporin A recipients after renal transplant develop multiple bilateral fibroadenomas (regress after drug cessation)
Malignant potential	Extremely rare; no significant risk unless complex type

SECTION 6: Phyllodes Tumor

Arises from intralobular stroma, like fibroadenoma, but much less common (~2.5% of fibroepithelial lesions).

Pathogenesis Comparison:

FIBROADENOMA vs. PHYLLODES TUMOR - MOLECULAR COMPARISON
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Gene          Fibroadenoma        Phyllodes Tumor
─────────────────────────────────────────────────────────
MED12         ✓ (majority)        ✓ (majority)
RARA          ✓ (~1/3)            ✓
TERT          ✗                   ✓ (additional)
TP53          ✗                   ✓ (additional)
RB            ✗                   ✓ (additional)
─────────────────────────────────────────────────────────
→ Shared origin (intralobular stroma) BUT phyllodes has
  additional genomic instability → more aggressive behavior
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Grading:

PHYLLODES TUMOR GRADING
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
BENIGN (~75%)          BORDERLINE           MALIGNANT (<25%)
  │                        │                     │
Resembles              More prominent        Widely infiltrative;
fibroadenoma           stromal atypia,       difficult to distinguish
but stroma is          cellularity,          from sarcoma
more cellular +        mitotic               (marked stromal
mitotically            activity              overgrowth, few
active                                       epithelial elements)
  │                        │                     │
Occasional local       Increased             Hematogenous
recurrence             recurrence            metastasis in ~1/3
No metastasis          (margin status        Lymphatic spread
                        is key predictor)     RARE
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Fig. 23.25 Phyllodes Tumor - (A) leaf-like architecture at low power, (B) increased stromal cellularity, atypia and mitotic activity compared to fibroadenoma:

Fig. 23.25 Phyllodes tumor - leaf-like architecture and high-power view showing stromal cellularity

Key clinical points:

Peak age: 6th decade (10-20 years later than fibroadenoma)
"Phyllodes" = Greek for leaf-like (due to bulbous leaf-like projections into cystic spaces)
Axillary lymph node dissection is contraindicated (lymphatic spread is rare regardless of grade)
Only the neoplastic stromal component is present in metastases (not the epithelial component)

SECTION 7: Benign Lesions of Interlobular Stroma

These tumors consist of stromal cells only (no accompanying epithelial component).

Tumor	Key Features
Myofibroblastoma	Composed of myofibroblasts; unique - only breast tumor equally common in both males and females
Lipoma	Palpable; fat-containing lesion on mammography
Fibromatosis	Clonal fibroblast/myofibroblast proliferation; irregular infiltrating mass; may involve muscle; locally aggressive but does NOT metastasize; associated with prior trauma/surgery, or FAP / hereditary desmoid syndrome / Gardner syndrome

SECTION 8: Other Benign Breast Conditions (Inflammatory/Structural)

Gynecomastia (Male Breast)

The only benign lesion of any frequency in the male breast.

GYNECOMASTIA - PATHOGENESIS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
IMBALANCE: Estrogen ↑ / Androgen ↓
                  │
    ┌─────────────┼─────────────────┐
    ▼             ▼                 ▼
PHYSIOLOGIC    PATHOLOGIC        DRUG-INDUCED
  │              │                  │
Puberty     Cirrhosis           Alcohol, Marijuana,
Old age     (↓ estrogen         Heroin, Antiretrovirals,
            metabolism)         Anabolic steroids
            Klinefelter (47,XXY)
            Leydig cell tumor
            Sertoli cell tumor
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Morphology: Dense collagenous connective tissue +
            epithelial hyperplasia of duct lining
            (Lobule formation almost NEVER seen)

Duct Ectasia vs. Squamous Metaplasia of Lactiferous Ducts

Feature	Duct Ectasia	Squamous Metaplasia of Lactiferous Ducts (Zuska Disease)
Age	5th-6th decade, multiparous	Any age
Smoking	NOT associated	>90% are smokers
Presentation	Periareolar mass, thick white nipple secretions, occasional skin retraction	Painful erythematous subareolar mass; recurrent abscess; fistula tract
Morphology	Dilated ducts with inspissated secretions; lipid-laden macrophages; periductal lymphoplasmacytic inflammation; granulomas	Keratinizing squamous metaplasia extends into nipple ducts; keratin plugs → duct rupture → granulomatous inflammation
Key risk	May mimic invasive carcinoma clinically/radiologically	Recurrences common; may cause nipple inversion
Treatment	Observation	En bloc surgical removal of duct + fistula

MASTER FLOWCHART: Benign Breast Disease - Overview and Cancer Risk

BENIGN BREAST DISEASE - RISK STRATIFICATION (ROBBINS)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

FEMALE BREAST LESION
         │
         ├─── INFLAMMATORY / STRUCTURAL (No cancer risk)
         │         ├── Duct Ectasia
         │         ├── Fat Necrosis
         │         └── Squamous Metaplasia of Lactiferous Ducts (Zuska)
         │
         ├─── NON-PROLIFERATIVE (No increased cancer risk)
         │         └── Fibrocystic Changes
         │               ├── Cysts (with apocrine metaplasia)
         │               ├── Fibrosis
         │               ├── Adenosis
         │               └── Lactational Adenoma
         │
         ├─── PROLIFERATIVE WITHOUT ATYPIA (RR ~1.5-2x)
         │         ├── Usual Ductal Hyperplasia
         │         ├── Sclerosing Adenosis
         │         ├── Radial Scar / Complex Sclerosing Lesion
         │         └── Papilloma (large duct / small duct)
         │
         ├─── PROLIFERATIVE WITH ATYPIA (RR ~4-5x)
         │         ├── Columnar Cell Lesions
         │         ├── Flat Epithelial Atypia
         │         ├── Atypical Ductal Hyperplasia (ADH)
         │         └── Atypical Lobular Hyperplasia (ALH)
         │                       │
         │                       ▼
         │               DCIS / LCIS (RR ~8-10x)
         │                       │
         │                       ▼
         │              INVASIVE CARCINOMA
         │
         └─── STROMAL TUMORS (Neoplasms)
                   │
                   ├─ INTRALOBULAR STROMA
                   │       ├── Fibroadenoma (BENIGN - most common)
                   │       └── Phyllodes Tumor (Benign/Borderline/Malignant)
                   │
                   └─ INTERLOBULAR STROMA
                           ├── Myofibroblastoma (BENIGN)
                           ├── Lipoma (BENIGN)
                           ├── Fibromatosis (locally aggressive, no metastasis)
                           └── Angiosarcoma (MALIGNANT - rare)

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

QUICK COMPARISON TABLE: Major Benign Breast Tumors

Feature	Fibroadenoma	Phyllodes Tumor	Papilloma (Large Duct)	Sclerosing Adenosis
Origin	Intralobular stroma	Intralobular stroma	Ductal epithelium	Lobular epithelium + stroma
Age	20s-30s	6th decade	Any	Reproductive age
Gross	Well-circumscribed, rubbery, gray-white	Variable size, leaf-like bulbous protrusions	Intraductal	Stellate/glandular
Key mutations	MED12, RARA	MED12, RARA, TERT, TP53, RB	-	-
Hormonal response	Yes (grows in pregnancy, regresses at menopause)	Less prominent	No	No
Key symptom	Palpable mobile mass	Palpable mass ± rapid growth	Nipple discharge (bloody/serous)	Palpable mass, calcifications
Cancer risk	None (slight if complex type)	0% (benign) to ~33% metastasis (malignant)	Slight increase	Slight increase
Metastasis	No	Rare; only malignant type; hematogenous	No	No
LN dissection	N/A	Contraindicated (lymphatic spread rare)	N/A	N/A

Source: Robbins, Cotran & Kumar Pathologic Basis of Disease, Chapter 23 - The Breast (pp. 945-985)

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