INTRODUCTION

The term “Pagetoid” is a major dermatopathology pattern-recognition concept.

It refers to:

• Upward scatter of atypical cells within the epidermis.

This pattern is classically associated with:

• Paget disease of the breast

• Extramammary Paget disease

• Melanoma

• Mycosis fungoides

The essential histologic idea:

• Abnormal cells appear individually or in nests high within the epidermis where they normally should not be.

PAGET DISEASE

Definition

Paget disease is an intraepidermal adenocarcinoma characterized by malignant glandular epithelial cells (“Paget cells”) infiltrating the epidermis.

Two major forms:

1. Mammary Paget disease (MPD)

2. Extramammary Paget disease (EMPD)

FOUNDATIONS (First Principles)

Normal Epidermal Histology

The epidermis normally contains:

• Keratinocytes

• Melanocytes

• Langerhans cells

• Merkel cells

Normally:

• Large pale mucin-containing glandular cells are absent.

The epidermis is avascular and organized into:

• Basal layer

• Spinous layer

• Granular layer

• Cornified layer

Normal Mammary & Apocrine Structures

Mammary Paget Disease

Related to:

• Lactiferous ducts of breast

Normal ductal epithelium:

• Forms glandular structures

• Produces secretory material

Extramammary Paget Disease

Arises in:

• Apocrine-rich areas

Common sites:

• Vulva

• Perianal region

• Scrotum

• Penis

• Axilla

Apocrine glands normally:

• Produce glandular secretions

• Express mucin-related proteins

INITIATING EVENT

Mammary Paget Disease

Most commonly:

• Epidermotropic spread of underlying ductal carcinoma cells into epidermis.

Underlying breast carcinoma is present in most cases.

Extramammary Paget Disease

May arise:

1. Primarily within epidermal/apocrine structures
   or

2. Secondary to underlying visceral adenocarcinoma

PATHOGENESIS (Cause → Effect Chain)

Mammary Paget Disease

Step 1: Underlying Breast Carcinoma Develops

Usually ductal carcinoma.

Step 2: Malignant Cells Migrate Along Ducts

Tumor cells extend through lactiferous ducts.

Step 3: Epidermotropism

Tumor cells infiltrate nipple epidermis.

Step 4: Intraepidermal Spread

Large malignant glandular cells proliferate within epidermis.

Produces:

• Erythematous scaly plaque clinically

Extramammary Paget Disease

Step 1: Malignant Glandular Transformation

Occurs in apocrine-related epithelium.

Step 2: Intraepidermal Proliferation

Paget cells spread through epidermis.

Step 3: Potential Dermal Invasion

Advanced lesions may become invasive adenocarcinoma.

HISTOPATHOLOGY EXPLAINED

1. FOUNDATIONS

Normal Epidermis

Normally:

• Keratinocytes are cohesive

• No mucin-producing glandular cells exist within epidermis

Thus:

• Presence of large pale atypical glandular cells is abnormal.

2. KEY HISTOLOGIC FEATURE: PAGET CELLS

Description

Paget cells are:

• Large round cells

• Pale or amphophilic cytoplasm

• Large pleomorphic nuclei

• Prominent nucleoli

They occur:

• Singly

• In nests
  within epidermis.

Why Do Paget Cells Look Pale?

Their cytoplasm contains:

• Mucin

• Glycoproteins

Mucin displaces normal cytoplasmic staining.

Thus on H&E:

• Cytoplasm appears pale/clear.

Why Are They Large?

Malignant glandular transformation causes:

• Increased cytoplasmic organelles

• Secretory differentiation

• Nuclear enlargement

Pagetoid Spread

Paget cells ascend through epidermis individually and in clusters.

This creates:

• Scattered intraepidermal malignant cells

This upward spread is termed:
“Pagetoid spread.”

Epidermal Changes

Associated epidermis may show:

• Acanthosis

• Hyperkeratosis

• Parakeratosis

Sometimes ulceration.

Dermal Changes

Dermis may contain:

• Chronic inflammatory infiltrate

• Underlying carcinoma

• Invasive adenocarcinoma

3. TEMPORAL EVOLUTION

Early Disease

Few scattered Paget cells.

Established Disease

Diffuse epidermal involvement.

Advanced Disease

Dermal invasion and metastatic potential.

4. NAMING LOGIC & TERMINOLOGY

“Paget Cell”

Named after Sir James Paget.

“Pagetoid”

Means:

• Resembling Paget disease pattern

NOT necessarily Paget disease itself.

Refers specifically to:

• Upward intraepidermal scatter.

5. STAINING & MARKERS

H&E

Shows:

• Large pale intraepidermal cells

Mucin Stains

PAS

Positive due to mucin/glycoprotein.

Often diastase resistant.

Mucicarmine

Highlights intracellular mucin.

Alcian Blue

Detects acidic mucopolysaccharides.

Immunohistochemistry

CK7

Strongly positive in Paget disease.

Most important marker.

EMA

Positive.

Supports glandular differentiation.

CEA

Usually positive.

HER2

Often positive in mammary Paget disease.

GCDFP-15

Supports apocrine differentiation.

DIFFERENTIATING MAMMARY VS EXTRAMAMMARY PAGET

Mammary Paget Disease

Usually associated with:

• Underlying breast carcinoma

Nipple/areola involvement.

Extramammary Paget Disease

Occurs in:

• Apocrine-rich anogenital skin

May be:

• Primary cutaneous

• Secondary visceral spread

CLINICO-PATHOLOGICAL CORRELATION

Why Lesions Are Eczematous

Epidermal infiltration disrupts barrier function.

Produces:

• Scaling

• Erythema

• Crusting

Why Lesions Are Chronic

Malignant intraepidermal cells persist and expand slowly.

Why EMPD Occurs in Apocrine Areas

Tumor demonstrates glandular/apocrine differentiation.

DIFFERENTIAL DIAGNOSIS

Histologic Differential

Melanoma in situ

Pagetoid melanocytes.

Bowen disease

Atypical keratinocytes throughout epidermis.

Mycosis fungoides

Atypical lymphocytes with epidermotropism.

Clear cell papulosis

Benign CK7+ pagetoid cells.

PAGETOID SPREAD

Definition

Pagetoid spread refers to:

• Upward migration/scatter of atypical cells within epidermis.

It is a pattern, not a diagnosis.

CLASSIC CONDITIONS WITH PAGETOID SPREAD

1. Paget disease

2. Melanoma

3. Mycosis fungoides

4. Bowen disease occasionally

5. Langerhans cell histiocytosis

PAGETOID SPREAD IN MELANOMA

FOUNDATIONS

Melanocytes normally reside:

• Along basal layer.

They do NOT normally ascend high into epidermis.

HISTOPATHOLOGY

Pagetoid Melanocytes

Atypical melanocytes ascend upward through epidermis.

Seen as:

• Single cells

• Nests

Often above basal layer.

Why Does This Happen?

Loss of normal growth regulation and adhesion allows abnormal upward migration.

Cytology

Melanocytes show:

• Hyperchromatic nuclei

• Nuclear atypia

• Variable melanin pigment

IMMUNOSTAINS

Positive:

• S100

• SOX10

• Melan-A

• HMB-45

CLINICAL IMPORTANCE

Pagetoid scatter is characteristic of:

• Superficial spreading melanoma

Board-favorite point.

PAGETOID SPREAD IN MYCOSIS FUNGOIDES

Definition

Mycosis fungoides is a cutaneous T-cell lymphoma with epidermotropism.

HISTOPATHOLOGY

Epidermotropism

Atypical lymphocytes migrate into epidermis.

Pagetoid Reticulosis Variant

Marked epidermal infiltration by atypical lymphocytes in pagetoid pattern.

Pautrier Microabscesses

Collections of atypical lymphocytes within epidermis.

Board-favorite feature.

Why Do Lymphocytes Enter Epidermis?

Neoplastic T cells express skin-homing receptors:

• CLA

• CCR4

These interact with epidermal adhesion molecules.

IMMUNOHISTOCHEMISTRY

Typically:

• CD3+

• CD4+

• Loss of CD7 common

PATTERN RECOGNITION & DIAGNOSTIC LOGIC

If Large Pale Mucinous Cells

→ Think Paget disease.

CK7+, mucin+.

If Atypical Pigmented Melanocytes

→ Think melanoma.

S100+, SOX10+.

If Small Cerebriform Lymphocytes

→ Think mycosis fungoides.

CD3+, CD4+.

DIFFERENTIATING PAGETOID CONDITIONS

Paget Disease

• Large pale mucinous cells

• CK7+

• PAS+

Melanoma

• Melanocytic atypia

• Pigment may be present

• SOX10+

Mycosis Fungoides

• Small atypical lymphocytes

• Epidermotropism

• Pautrier microabscesses

Bowen Disease

Bowen disease:

• Full-thickness keratinocyte atypia

• Dyskeratosis

• No mucin

MANAGEMENT

Mammary Paget Disease

Requires evaluation for:

• Underlying breast carcinoma

Treatment:

• Surgery

• Oncologic management

Extramammary Paget Disease

• Wide local excision

• Mohs surgery

• Evaluation for associated internal malignancy

Recurrence common.

EXAM-FOCUSED INSIGHTS

• Paget cells are large pale mucin-containing intraepidermal adenocarcinoma cells.

• CK7 positivity is classic for Paget disease.

• PAS and mucicarmine positivity reflect intracellular mucin.

• Mammary Paget disease is usually associated with underlying ductal carcinoma.

• EMPD occurs in apocrine-rich skin.

• Pagetoid spread means upward scatter of atypical cells within epidermis.

• Pagetoid spread is a pattern, not a diagnosis.

• Superficial spreading melanoma classically shows pagetoid melanocytic spread.

• Pautrier microabscesses are characteristic of mycosis fungoides.

• SOX10 and S100 support melanoma diagnosis.

MUST-KNOW BOARD EXAM QUESTIONS

1. What are Paget cells?

Large pale intraepidermal malignant glandular cells containing mucin.

2. Which immunostain is classically positive in Paget disease?

CK7.

3. Why do Paget cells appear pale on H&E?

Because intracellular mucin displaces normal cytoplasmic staining.

4. What does “pagetoid spread” mean?

Upward scatter of atypical cells within the epidermis.

5. Which breast malignancy is usually associated with mammary Paget disease?

Underlying ductal carcinoma.

6. Which areas are commonly affected in extramammary Paget disease?

Apocrine-rich anogenital regions.

7. Which melanoma subtype classically shows pagetoid spread?

Superficial spreading melanoma.

8. Which stains highlight mucin in Paget disease?

PAS, mucicarmine, Alcian blue.

9. Which feature is characteristic of mycosis fungoides histology?

Epidermotropism of atypical lymphocytes.

10. What are Pautrier microabscesses?

Collections of atypical lymphocytes within the epidermis.

11. Which markers are positive in melanoma?

S100, SOX10, Melan-A, HMB-45.

12. What is the major histologic differential diagnosis of Paget disease?

Melanoma in situ.

13. Why are Paget lesions clinically eczematous?

Intraepidermal tumor disrupts epidermal barrier function.

14. Which disease demonstrates CD3+, CD4+ epidermotropic lymphocytes?

Mycosis fungoides.

15. Is pagetoid spread itself a diagnosis?

No. It is a histologic pattern.