Mucoepidermoid Carcinoma

Slide 1 · Mucoepidermoid Carcinoma

Salivary gland pathology · morphology, molecular basis and clinical behavior

A carcinoma with mucous, intermediate and epidermoid differentiation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Recognize the three characteristic epithelial populations and their growth patterns.

Interpret the CRTC1::MAML2 alteration without overstating its significance.

Relate morphology and histologic grade to local behavior and prognosis.

Apply a practical differential diagnosis and reporting framework.

Slide 3 · Defining the entity

Primary malignant epithelial neoplasm of a salivary gland

Combines mucous, intermediate and epidermoid (squamoid) cells.

Architecture ranges from cystic structures to cords and solid sheets.

Biologic behavior is heterogeneous: Low-grade and high-grade tumors differ substantially.

Slide 4 · Place among salivary gland tumors

Benign lesions

Pleomorphic adenoma and Warthin tumor are important benign reference entities.

Mucoepidermoid carcinoma

Most common primary malignant salivary-gland neoplasm.

Other carcinomas

Adenoid cystic and acinic cell carcinoma provide useful morphologic contrasts.

A salivary gland mass cannot be classified from location alone.

Slide 5 · How common is mucoepidermoid carcinoma?

Among all salivary gland tumors

~15%

The proportion cited; Denominator is benign plus malignant tumors.

Among primary malignancies

Most common

The ranking refers to primary malignant salivary-gland tumors.

Epidemiologic figures follow the supplied Robbins edition; Not a population-incidence estimate.

Slide 6 · Anatomic distribution

Parotid gland

60–70%

Most MECs develop in the parotid gland.

Other glands

A substantial contribution to salivary tumors elsewhere, particularly minor salivary glands.

“Most tumors arise in the parotid” ≠ “most parotid tumors are MEC.”

Slide 7 · Minor salivary glands are clinically important

MEC contributes a large fraction of the neoplasms arising outside the parotid.

A minor-gland site does not exclude a salivary primary.

Accurate site assignment matters when interpreting resection boundaries and local extension.

Slide 8 · Why the name is informative

Muco-

Mucous cells with intracellular and/or luminal mucin.

-epidermoid

Squamoid epithelial differentiation within the same neoplasm.

Intermediate

A third, often less distinctive population bridges the recognized appearances.

Cellular heterogeneity is an identification clue, not a grade by itself.

Slide 9 · Three epithelial populations

Mucous

Intermediate

Epidermoid / squamoid

Schematic cell forms — concept illustration; Not a photomicrograph.

Slide 10 · How the cells contribute to architecture

Mucous cells

Contribute mucin to intracellular vacuoles and lumina

Intermediate cells

Contribute to compact epithelial nests

Epidermoid cells

Produce squamoid areas within mixed structures

Interpret cell type together with architectural pattern and invasion.

Slide 11 · A tumor with several growth patterns

Cords

Elongated or branching strands of tumor cells.

Sheets

Broad, densely cellular expanses of tumor.

Cystic structures

Luminal spaces may contain mucin and vary in size.

Slide 12 · Gross dimensions and appearance

Reported lesions can reach approximately 8 cm in diameter.

Typical cut surface: Pale gray-white tissue.

Small mucin-containing cysts may punctuate the solid component.

Slide 13 · Apparent circumscription can be deceptive

Irregular nests across a tissue interface

Macroscopic impression

The lesion may look demarcated.

A well-defined true capsule is absent.

Growth may extend into nearby tissue.

Slide 14 · What a mucin-filled cyst means

Pathologic connection

Cystic spaces are a recognized growth configuration.

Mucin may collect within small lumina.

Cysts do not imply a benign lesion.

Slide 15 · Low-power histology: Diagnostic search pattern

Scan architecture before committing to cytologic interpretation.

Locate lumina

Look for cystic or microcystic spaces

Map solid areas

Identify cords, nests and sheets

Inspect borders

Assess invasion in adjacent tissue

Schematic teaching workflow derived morphology; Not a formal diagnostic algorithm.

Slide 16 · High-power histology: Three-cell survey

Mucous

Intermediate

Epidermoid / squamoid

Schematic cell forms — concept illustration; Not a photomicrograph.

Survey at least several representative areas; One dominant cell population may obscure the others.

Slide 17 · Mucous differentiation

Mucous cells produce the component that gives the tumor its “muco-” designation.

Intracellular mucin and mucin-bearing lumina may be easier to appreciate than rare isolated mucous cells.

Mucin positivity supports differentiation; It is not a standalone diagnosis of MEC.

Slide 18 · Intermediate-cell differentiation

Intermediate cells constitute one of the three morphologic populations.

They can dominate compact portions of the tumor and make the lesion appear monotonous.

Search for associated mucous and epidermoid differentiation before classifying a predominantly intermediate-cell tumor.

Slide 19 · Epidermoid or squamoid differentiation

Squamoid areas contribute a second identifiable epithelial phenotype.

The coexistence of mucous and intermediate cells is more informative than squamoid morphology alone.

A predominantly solid, squamoid carcinoma invites careful differential diagnosis.

Slide 20 · Mucin stains: Complementary evidence

H&E

Shows cystic architecture and mucinous cell vacuoles, when sufficiently evident.

Histochemical mucin stain

Can highlight intracellular or luminal mucin in equivocal areas.

Interpretation

A supportive phenotypic finding; Retain architectural and cytologic context.

The uploaded chapter provides no image file or mucin-staining protocol; This is ancillary diagnostic context.

Slide 21 · Mucin accumulation: A schematic model

Three linked observations

Mucous epithelial differentiation

Mucin within luminal structures

Grossly appreciable small cysts

Slide 22 · Compare three growth configurations

Cords

Linear, branched epithelial groupings; Examine relationships with stroma.

Sheets

Confluent tumor cells with reduced luminal emphasis.

Cysts

Spaces containing mucin and/or lined by mixed tumor cells.

Slide 23 · Cords require interface assessment

Irregular nests across a tissue interface

Read the edge of the lesion

Cords may approach or infiltrate the surrounding salivary tissue.

Overall demarcation does not prove encapsulation.

The interface is critical when evaluating malignant growth.

Slide 24 · Sheets alter the visual impression

Broad sheets can make some MECs appear largely solid.

At high magnification, cytologic blandness or striking anaplasia may be seen depending on grade.

Sample cystic and solid regions to avoid interpreting one area as the entire tumor.

Slide 25 · Cystic architecture can be prominent

MEC frequently includes small cystic and microcystic configurations.

A cyst-rich appearance should prompt a search for the defining cellular mixture.

A seemingly circumscribed cystic lesion still requires evaluation for infiltrative foci.

Slide 26 · Cytologic variability is unusually broad

Relatively bland

Uniform nuclei and limited atypia may predominate

Intermediate appearance

Cytologic findings vary with tumor region

Highly anaplastic

Conspicuously malignant cells may predominate

Morphologic grade provides the link between appearance and expected behavior.

Slide 27 · Bland cells do not establish benignity

Robbins explicitly describes cytologically bland MECs.

A malignant neoplasm may be infiltrative despite deceptively mild nuclear atypia.

Architecture, tumor borders and the recognized cell mixture must be read together.

Slide 28 · Infiltration is a decisive morphologic issue

Irregular nests across a tissue interface

Interpret the interface

No continuous, well-defined capsule.

Tumor may invade neighboring structures.

Invasion helps explain the potential for local recurrence.

Slide 29 · The molecular hallmark

Recurrent translocation

T(11;19)

Balanced rearrangement involving 11q21 and 19p13.

Resulting fusion

CRTC1::MAML2

A chimeric transcription-related protein identified in more than half of Robbins cases.

Slide 30 · Reading the chromosome notation

T(11;19)

A translocation between chromosomes 11 and 19.

(q21;p13)

Named breakpoints on the long arm of chromosome 11 and short arm of chromosome 19.

Fusion product

Genomic rearrangement brings portions of CRTC1 and MAML2 together.

Slide 31 · Fusion formation: Conceptual sequence

CRTC1 locus

Gene contribution from chromosome 19

MAML2 locus

Gene contribution from chromosome 11

Fusion transcript

CRTC1::MAML2 chimeric product

The translocation is recurrent, not universal.

Slide 32 · The fusion protein is not normal MAML2

The incorporated portion of MAML2 cannot interact with Notch signaling components as intact MAML2 does.

The encoded fusion is therefore functionally distinct from its parent proteins.

Robbins treats the downstream tumorigenic mechanism as a hypothesis, not a settled linear pathway.

Slide 33 · Hypothesized signaling effects

CRTC1::MAML2

Rearranged transcription-associated protein

Potential perturbation

Notch- and cAMP-dependent pathways

Tumor development

Proposed contribution to salivary carcinogenesis

Causal pathway is shown as a hypothesis, consistent with Robbins (“believed”, “possibly”).

Slide 34 · What can—and cannot—be inferred

Established observation

Interpretive boundary

CRTC1::MAML2 occurs in many MECs

Not detected in every MEC

Fusion protein is structurally altered

Full downstream mechanism remains uncertain

A recurrent molecular feature exists

Presence alone does not specify grade or stage

Slide 35 · An additional fusion is recognized

Classic rearrangement

CRTC1::MAML2 associated with t(11;19).

Alternative rearrangement

CRTC3::MAML2 is an additional recognized fusion partner combination.

These represent related molecular routes; Do not replace morphologic classification.

Slide 36 · Molecular testing: Diagnostic role

MAML2 rearrangement is a desirable diagnostic feature in the WHO 5th-edition framework.

Molecular assessment can assist when morphology is ambiguous or a variant pattern mimics another entity.

A negative assay does not, by itself, exclude MEC; Interpret the test within sample and assay limitations.

Slide 37 · Avoid overinterpreting fusion status

Fusion presence has been investigated in relation to histologic type and outcome.

Morphologic grade and pathologic stage remain clinically important independent domains.

Avoid a categorical prognostic claim based on the rearrangement alone.

Slide 38 · Recognize morphologic variants

Classical pattern

Mixed populations in typical cystic and solid arrangements.

Cytologic variants

Clear/oncocytic and other unusual dominant cytoplasmic appearances.

Stromal or architectural variants

Examples include sclerosing and Warthin-like appearances.

Variant names are supplementary literature context; Not an exhaustive classification list.

Slide 39 · Low-grade MEC: Morphologic orientation

Cystic growth and recognizable mucous differentiation commonly feature in low-grade lesions.

Cells can be relatively uniform rather than frankly anaplastic.

Low grade does not mean noninvasive or incapable of recurrence.

Slide 40 · High-grade MEC: Morphologic orientation

A more solid architecture and conspicuous cytologic atypia can predominate.

High-grade examples may be highly anaplastic and unmistakably malignant.

Infiltration and the difficulty of complete excision are clinically important.

Slide 41 · Intermediate grade: The diagnostic middle

Robbins recognizes low, intermediate and high histologic grades.

Intermediate-grade tumors lie between the morphologic extremes.

Assigning grade requires assessment of the composite specimen, not a single isolated feature.

Slide 42 · What grading schemes examine

Structural features

Cytologic / activity features

Cystic versus solid growth

Nuclear anaplasia / pleomorphism

Invasive border

Mitotic activity

Perineural or lymphovascular involvement

Tumor necrosis

Feature list is educational; Not a point-based grading formula.

Slide 43 · Why grading systems can disagree

Published MEC grading methods weight architectural and cytologic findings differently.

The same specimen can receive different grades under different grading algorithms.

Specify the grading system when using formal scoring criteria and avoid mixing cutoffs across systems.

Slide 44 · Grade is linked to clinical course

Low grade

Local invasion possible; Metastasis rare

Intermediate grade

Risk must be judged within full tumor context

High grade

More invasive; Greater recurrence and metastasis risk

Grade describes tumor behavior; Stage separately records anatomic extent.

Slide 45 · Low-grade outcome data

Local recurrence

~15%

Can recur after treatment despite relatively favorable biology.

Distant metastasis

Rare

Uncommon in low-grade MEC as.

5-year survival

>90%

A textbook summary estimate; Not individualized prognosis.

Slide 46 · High-grade outcome data

Recurrence

Up to 30%

Clinically consequential local or regional relapse can occur.

Distant metastasis

30%

Reported in the Robbins high-grade summary.

5-year survival

50%

Substantially below the low-grade figure.

Slide 47 · Contrast: Grade and prognosis

Clinical dimension

Low grade

High grade

Local behavior

May invade

Often highly invasive

Recurrence

Approximately 15%

Up to 30%

Distant metastasis

Rare

Approximately 30%

5-year survival

More than 90%

Approximately 50%

Textbook summary figures; These are not patient-specific estimates or matched study cohorts.

Slide 48 · Histologic grade is not pathologic stage

Grade

Degree of structural and cellular aggressiveness in a histologic sample.

Stage

Anatomic disease extent using the relevant AJCC framework.

Report both

Do not substitute microscopic appearance for anatomic spread—or vice versa.

AJCC Version 9 applies to salivary-gland carcinoma staging from 2026 (CAP/AJCC).

Slide 49 · Resection pathology: Key data domains

Document site and tumor size, histologic type and histologic grade.

Assess tumor extent, resection margins and relevant invasive features.

Evaluate regional nodes where submitted and apply current site-specific staging.

Slide 50 · The margin problem begins at the border

Irregular nests across a tissue interface

Pathology questions

How far does tumor extend?

Is a true capsule present?

Does tumor reach an inked margin?

Slide 51 · Comparative diagnosis: Pleomorphic adenoma

Property

Mucoepidermoid carcinoma

Pleomorphic adenoma

Nature

Malignant epithelial tumor

Benign mixed tumor

Characteristic elements

Mucous + intermediate + squamoid

Ductal / myoepithelial cells and chondromyxoid stroma

Margins

Often infiltrative, no definite capsule

Usually circumscribed; protrusions possible

Slide 52 · Comparative diagnosis: Warthin tumor

Property

Mucoepidermoid carcinoma

Warthin tumor

Behavior

Malignant

Benign

Architecture

Cords / sheets / mucin-containing cysts

Papillary-cystic oncocytic epithelium

Stroma

May be infiltrative

Lymphoid tissue often with follicles

Site

Many salivary sites

Predominantly parotid

Slide 53 · Comparative diagnosis: Adenoid cystic carcinoma

Property

Mucoepidermoid

Adenoid cystic

Cellular differentiation

Mucous / intermediate / squamoid

Basal myoepithelial and epithelial

Typical architecture

Cysts and solid areas

Often cribriform / tubular or solid

Characteristic biology

MAML2 rearrangement recurrent

MYB / MYBL1 alteration recurrent

Clinical behavior

Strong grade-related spectrum

Perineural infiltration and late recurrence prominent

Slide 54 · Comparative diagnosis: Acinic cell carcinoma

Property

Mucoepidermoid carcinoma

Acinic cell carcinoma

Cell population

Mixed mucous, intermediate, epidermoid

Serous acinar-type cells

Cytoplasm

May include mucin-rich cells

Often granular / zymogen-rich

Patterns

Cords, sheets, cysts

Sheets, microcysts, glands or papillae

Molecular concept

CRTC1::MAML2 in many

NR4A3 alteration recognized (WHO 2022)

Slide 55 · When the lesion resembles squamous carcinoma

A squamoid-dominant MEC may resemble a conventional squamous carcinoma.

Search for convincing mucous cells and intermediate-cell differentiation.

Consider MAML2 testing as supportive evidence when morphologic overlap is substantial.

Slide 56 · Diagnostic comparison matrix

Clue

MEC

Key contrast

Mucous + squamoid cells

Characteristic mixed differentiation

Usually not the defining pair in pleomorphic adenoma

Chondromyxoid matrix

Not a defining MEC feature

Supports pleomorphic adenoma

Double oncocytic layer + lymphoid stroma

Not typical MEC pattern

Favors Warthin tumor

Cribriform hyaline spaces

Not the defining architecture

Suggests adenoid cystic carcinoma

Slide 57 · FNA and limited biopsy: What is knowable?

Fine-needle aspiration may provide an initial cytologic classification; The Milan system is referenced by the WHO update.

A core or aspirate samples only part of a heterogeneous neoplasm.

Full invasive boundaries and definitive grade may be difficult to assess on limited material.

Slide 58 · A practical diagnostic workflow

Recognize tumor

Salivary epithelial malignancy in clinical context

Classify morphology

Cell populations, cysts, sheets, invasion

Use support when needed

Mucin / molecular testing if equivocal

Complete report

Grade, site, extent, margins, nodes

Use this as a teaching framework, not a substitute for local diagnostic protocols.

Slide 59 · Case 1: A deceptively bland lesion

A parotid mass contains mucin-bearing cysts and mixed epithelial cells.

Cells show little cytologic atypia, but irregular nests extend into adjacent salivary tissue.

Question: Does bland nuclear morphology justify a benign diagnosis?

Question: Which features should enter the grading discussion?

Slide 60 · Case 1: Interpretation

Classification

The combined mucous, intermediate and squamoid cells support MEC.

Evidence of malignancy

Infiltration is not negated by bland nuclei or a cyst-rich appearance.

Next step

Assess the full specimen for histologic grade, margins and extent.

Slide 61 · Case 2: A densely cellular salivary malignancy

A salivary-gland resection reveals broad solid sheets and marked nuclear atypia.

Only focal mucous differentiation is identified on further sampling.

Question: What diagnosis should enter the differential?

Question: What is the clinical relevance of a high-grade pattern?

Slide 62 · Case 2: Interpretation

Diagnostic consideration

Consider high-grade MEC alongside other high-grade epithelial malignancies.

Grade-related risk

High grade to greater recurrence and distant metastasis.

Workup

Complete sampling and ancillary tests may clarify a morphologically difficult case.

Slide 63 · Pattern-recognition exercise

Finding A

Mixed cell types with cystic spaces containing mucin.

Finding B

Infiltrative cords lacking a discrete true capsule.

Finding C

High-grade sheet-like growth with pronounced anaplasia.

Identify: Diagnostic identity, malignant growth pattern, and grade-related feature.

Slide 64 · Pattern-recognition: Answers

Finding

Principle

Why it matters

A

Characteristic differentiation

Supports MEC when cell types are documented

B

Invasive interface

Explains why apparent circumscription is misleading

C

High-grade morphology

Raises concern for aggressive behavior

Slide 65 · Four recurring diagnostic pitfalls

Cysts → benign

False: Low-grade MEC may appear cystic.

Bland cells → benign

False: Malignant invasion may coexist with bland cytology.

Fusion → grade

False: A molecular feature does not replace histologic grading.

Site → identity

False: Parotid predominance alone cannot classify a tumor.

Slide 66 · Integrated clinicopathologic model

Cellular identity

Mucous + intermediate + epidermoid

Architecture and invasion

Cysts, cords, sheets, tumor border

Histologic grade

Bland ↔ anaplastic / structure

Clinical risk

Recurrence, spread, survival

Relations summarize source-supported observations; This is not a quantitative prediction tool.

Slide 67 · Core concepts to retain

MEC is the most common primary malignant salivary-gland neoplasm.

Mixed epithelial differentiation and variable cystic/solid growth define the morphologic concept.

CRTC1::MAML2 is recurrent, but neither universal nor a substitute for morphology.

Histologic grade strongly influences recurrence, distant spread and survival.

Slide 68 · References and teaching evidence

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. Ch 16, pp 681–682.

Skálová A, Hyrcza MD, Leivo I. WHO 5th-edition salivary gland update. Head Neck Pathol. 2022;16:40–53.

Fehr A, et al. MEC grading and CRTC1/3–MAML2 genotyping. Virchows Arch. 2021;479:975–985.

Nakano S, et al. MEC variants, grading systems and fusions. Histopathology. 2022;80:729–735.

College of American Pathologists. Salivary Gland Cancer Protocol (April 2026).