Lesions of Intralobular Stroma

Slide 1 · Lesions of Intralobular Stroma

Fibroadenoma and phyllodes tumor

Breast fibroepithelial pathology

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain the biphasic composition and stromal origin of both tumors.

Recognize the gross and microscopic patterns of fibroadenoma.

Interpret MED12, RARA and related molecular findings cautiously.

Distinguish fibroadenoma from the three grades of phyllodes tumor.

Connect stromal grading and surgical margins to clinical behavior.

Slide 3 · Two stromal compartments of the breast

Intralobular stroma surrounds duct–lobular epithelial structures.

Interlobular stroma lies between lobular units and supports connective tissue.

The compartment of origin predicts the tumor’s epithelial association.

Lobular unit + intralobular stroma

Interlobular stroma surrounds the unit

Slide 4 · Stromal tumors: Two different families

Intralobular origin: Fibroadenoma and phyllodes tumor.

Interlobular origin: Mesenchymal tumors, including lipoma and angiosarcoma.

The distinction is an origin-and-composition framework, not simply a location label.

Lobular unit + intralobular stroma

Interlobular stroma surrounds the unit

Slide 5 · Fibroepithelial tumors are biphasic

Fibroadenoma and phyllodes tumor contain neoplastic stroma.

Both also contain epithelial elements that are nonneoplastic.

Stromal-derived growth factors may stimulate the accompanying epithelium.

Neoplastic stromal cells

Accompanying epithelial ducts

Slide 6 · What “biphasic” does not mean

Two visible components do not establish that both are neoplastic.

In these tumors, stromal proliferation is the neoplastic driver.

Epithelial lining can become compressed, stretched, or displaced by the stroma.

Neoplastic stromal cells

Accompanying epithelial ducts

Slide 7 · Histology separates the two major tumors

Fibroadenoma usually has relatively delicate intralobular-type stroma.

Phyllodes tumor tends toward greater stromal cellularity and mitotic activity.

Architecture alone is insufficient: Stromal features must be considered.

Fibroadenoma

Phyllodes

More delicate stroma

Greater cellularity

Slide 8 · Fibroadenoma: The defining concept

Most common benign stromal tumor of the female breast.

Benign fibroepithelial neoplasm derived from intralobular stroma.

Produces a discrete lesion containing ducts embedded in proliferative stroma.

Neoplastic stromal cells

Accompanying epithelial ducts

Slide 9 · Typical age at presentation

Most fibroadenomas occur in women in their 20s and 30s.

Palpability is a common route to recognition in younger patients.

The same tumor may instead be found incidentally by imaging.

Typical age at presentation

Fibroadenoma: 20s–30s

Phyllodes: Sixth decade

Overlap is possible

Morphology decides

Slide 10 · Fibroadenomas may be multiple

Multiple lesions can occur within one breast.

Bilateral fibroadenomas are also frequent.

Multiplicity alone does not establish malignant disease.

Circumscribed stromal nodule

Slide 11 · Hormone responsiveness is clinically visible

Fibroadenomas respond to hormonal conditions.

Growth may occur during pregnancy.

Regression may occur after menopause.

Reproductive hormonal context

Growth during pregnancy

Regression after menopause

Slide 12 · Rapid growth in pregnancy: A diagnostic trap

Accelerated enlargement may occur during pregnancy.

Rapid expansion can sometimes lead to infarction.

Rapid growth and infarction may falsely suggest carcinoma.

Reproductive hormonal context

Growth during pregnancy

Regression after menopause

Slide 13 · Fibroadenoma as an imaging abnormality

In a young patient, a palpable mass is typical.

Imaging may demonstrate a well-circumscribed mammographic density.

Radiodensity relates to the absence of adipose tissue within the nodule.

Circumscribed stromal nodule

Slide 14 · Older patients: Calcification may dominate

Some fibroadenomas are first detected by mammography in older women.

Clustered calcifications may be the presenting imaging finding.

Associated stromal hyalinization alters the lesion’s appearance.

Delicate / myxoid

Hyalinized / atrophic

Slide 15 · Gross pathology: The characteristic nodule

Cut surface is gray-white and rubbery.

Borders are well circumscribed; The mass bulges above the adjacent tissue.

Slit-like epithelial-lined spaces may be seen on sectioning.

Circumscribed stromal nodule

Slide 16 · Fibroadenoma spans a wide size range

Size may be less than 1 cm.

Some lesions become sufficiently large to replace much of the breast.

Size alone should not be used as a proxy for stromal malignancy.

Circumscribed stromal nodule

Slide 17 · Microscopy: The core components

Fibrous, often myxoid stroma resembles native intralobular stroma.

Ductal epithelial structures are dispersed within the stroma.

Stromal proliferation changes the contour of the epithelial profiles.

Neoplastic stromal cells

Accompanying epithelial ducts

Slide 18 · Pericanalicular growth pattern

Stroma grows around ducts but leaves them patent.

Ducts remain relatively rounded or oval.

The named pattern refers to the spatial relationship between stroma and ducts.

Patent, round-to-oval ducts

Slide 19 · Intracanalicular growth pattern

Expanding stroma compresses and distorts epithelial ducts.

Lumina become elongated and cleft-like.

The distortion is produced by stromal growth rather than epithelial invasion.

Compressed, cleft-like lumina

Slide 20 · Pericanalicular versus intracanalicular

Pericanalicular: Open epithelial lumina surrounded by stroma.

Intracanalicular: Ducts are compressed into slits or clefts.

Both patterns demonstrate the same benign biphasic tumor concept.

Fibroadenoma

Phyllodes

Patent ducts

Cleft-like ducts

Slide 21 · Architecture reflects stromal pressure

The stromal component determines duct contour.

Less-compressed ducts stay patent; Stronger compression creates clefts.

Recognizing a cause–effect relationship is more useful than memorizing silhouettes.

Compressed, cleft-like lumina

Slide 22 · Aging fibroadenoma: Stromal change

Stroma commonly becomes densely hyalinized in older patients.

The initially delicate/myxoid component is replaced by more collagenized tissue.

Appearance may therefore differ markedly from the textbook young-adult pattern.

Delicate / myxoid

Hyalinized / atrophic

Slide 23 · Aging fibroadenoma: Epithelial change

Accompanying epithelium is often atrophic in older patients.

Epithelial attenuation occurs alongside stromal hyalinization.

Interpret the epithelial change within the otherwise benign fibroepithelial lesion.

Delicate / myxoid

Hyalinized / atrophic

Slide 24 · MED12 alterations: Common stromal drivers

Approximately two-thirds of fibroadenomas harbor driver mutations in MED12.

MED12 encodes a mediator-complex subunit.

The mediator complex participates in RNA polymerase II–dependent gene regulation.

Recurrent stromal alterations

MED12

RARA

Transcriptional regulation

Mechanism remains incompletely explained

Slide 25 · Mediator biology links mutation to transcription

Mediator helps regulate RNA polymerase II–dependent transcription.

MED12 mutation may disrupt the expression of specific stromal growth/survival genes.

This is a proposed mechanism, not a completely established causal pathway.

Recurrent stromal alterations

MED12

RARA

Transcriptional regulation

Mechanism remains incompletely explained

Slide 26 · Hormone-sensitive stroma and MED12

Both breast fibroadenoma and uterine leiomyoma are stromal tumors.

Both arise in organs responsive to female sex hormones.

Their shared MED12 association supports a possible hormone-regulated transcriptional mechanism.

Breast intralobular stroma

MED12 alterations

Hormone-responsive growth

Mechanism proposed by Robbins

Slide 27 · RARA is a second recurrently altered gene

Around one-third of fibroadenomas harbor RARA mutations.

RARA encodes retinoic acid receptor alpha.

The gene is an estrogen target and may cooperate with estrogen receptor in transcription.

Recurrent stromal alterations

MED12

RARA

Transcriptional regulation

Mechanism remains incompletely explained

Slide 28 · MED12 and RARA: Interpret the evidence

Mutation frequencies are reported independently in the source.

They establish recurrent molecular associations, not a stand-alone histologic diagnosis.

Do not assume all tumors carry both alterations or that their co-occurrence is obligatory.

Recurrent gene alterations

MED12: ~⅔ of fibroadenomas

RARA: ~⅓ of fibroadenomas

Shared with phyllodes

No stand-alone diagnostic test

Slide 29 · Genes do not replace morphology

MED12 and RARA are relevant to tumor pathogenesis.

Routine distinction between fibroadenoma and phyllodes relies on morphology.

Histologic stromal features as the basis for separation.

Fibroadenoma

Phyllodes

More delicate stroma

Greater cellularity

Slide 30 · Cyclosporin A: A striking clinical association

Multiple bilateral fibroadenomas in almost half of women receiving cyclosporin A after renal transplantation.

Regression after cessation of treatment is described.

This is an association; Causation and neoplastic status remain unsettled.

Cyclosporin A exposure

Multiple / bilateral lesions

Regression after cessation

Causal nature remains uncertain

Slide 31 · Cyclosporin-associated lesions: Uncertainty

The lesions may be true neoplasms or reactive hyperplasia.

Regression after discontinuation favors a potentially reversible proliferative process.

Do not classify the mechanism as settled when the source identifies uncertainty.

Cyclosporin A exposure

Multiple / bilateral lesions

Regression after cessation

Causal nature remains uncertain

Slide 32 · Recognized fibroadenoma variants

Myxoid fibroadenoma and complex fibroadenoma are named variants.

A variant designation reflects a particular histologic constellation.

These categories are not equivalent to malignant transformation.

Two named subtypes

Myxoid fibroadenoma

Complex fibroadenoma

Sporadic or syndromic setting

Slide 33 · Myxoid fibroadenoma: Stromal phenotype

Myxoid fibroadenomas show a myxoid stromal character.

Most cases are sporadic.

A minority are associated with Carney complex.

Delicate / myxoid

Hyalinized / atrophic

Slide 34 · Carney complex: Inherited association

A minority of myxoid fibroadenomas occur in Carney complex.

Carney complex is an autosomal dominant condition.

It is usually associated with inactivating germline PRKAR1A mutations.

Myxoid fibroadenoma

Minority: Carney complex

Germline PRKAR1A

Autosomal dominant syndrome

Slide 35 · Complex fibroadenoma: Operational definition

Cysts larger than 0.3 cm are one defining change.

Other qualifying findings: Sclerosing adenosis, epithelial calcifications, or papillary apocrine change.

A complex fibroadenoma is defined by one or more of these tissue changes.

One or more qualifying changes

Cysts >0.3 cm

Sclerosing adenosis

Epithelial calcifications

Papillary apocrine change

Slide 36 · Complex fibroadenoma: The four findings

Cysts larger than 0.3 cm.

Sclerosing adenosis.

Epithelial calcifications or papillary apocrine change.

One or more qualifying changes

Cysts >0.3 cm

Sclerosing adenosis

Epithelial calcifications

Papillary apocrine change

Slide 37 · Complex fibroadenoma and later carcinoma risk

A slightly increased subsequent breast carcinoma risk is described.

The risk may partly reflect concurrent “at-risk” lesions in surrounding breast.

Association should not be simplified into direct malignant progression of fibroadenoma.

Fibroadenoma

Surrounding breast tissue

At-risk lesions

Association may reflect context

Slide 38 · Adjacent tissue can explain observed risk

Atypical hyperplasia may occur in surrounding breast tissue.

Such associated at-risk pathology could drive part of the observed risk association.

Histologic context outside the nodule matters when interpreting risk.

Fibroadenoma

Surrounding breast tissue

At-risk lesions

Association may reflect context

Slide 39 · Fibroadenoma: Integrating clinical and morphology

Young patient, discrete mass, and benign biphasic histology are concordant.

Patent ducts and compressed clefts are acceptable within fibroadenoma.

Atypical presentation requires evaluation of the whole clinicopathologic picture.

Discrete mass in younger patient

Delicate stroma + ducts

Fibroadenoma pattern

Interpret in clinical context

Slide 40 · Phyllodes tumor: The related fibroepithelial neoplasm

Phyllodes tumor also arises from intralobular breast stroma.

It is much less common than fibroadenoma.

It retains a biphasic tumor pattern but displays different stromal behavior.

Epithelial-lined stromal projections

Slide 41 · How uncommon is phyllodes tumor?

Phyllodes tumors account for approximately 2.5% of fibroepithelial lesions.

The reported proportion refers to fibroepithelial lesions, not all breast masses.

Fibroadenoma therefore dominates the everyday benign fibroepithelial differential.

How to read the proportion

Phyllodes: ~2.5%

Denominator: Fibroepithelial lesions

Much less common than FA

Slide 42 · Age helps, but cannot establish the diagnosis

Most phyllodes tumors present in the sixth decade.

This is approximately 10–20 years later than the fibroadenoma peak.

A patient’s age is contextual evidence, not a histologic grading criterion.

Typical age at presentation

Fibroadenoma: 20s–30s

Phyllodes: Sixth decade

Overlap is possible

Morphology decides

Slide 43 · Phyllodes tumor: How it presents

Most are detected as palpable masses.

Some are detected by mammography.

Sizes extend from a few centimeters to massive tumors occupying the breast.

Circumscribed stromal nodule

Slide 44 · Why the tumor is called “phyllodes”

Proliferating stromal nodules form bulbous projections.

Each projection is covered by epithelial tissue.

The resulting architecture resembles leaves (“phyllodes”).

Epithelial-lined stromal projections

Slide 45 · Projections may extend into cystic spaces

Bulbous stromal fronds can project into a cystic cavity.

The epithelial covering outlines the leaf-like spaces.

This architecture is a growth pattern, not a stand-alone malignancy marker.

Epithelial-lined stromal projections

Slide 46 · Leaf-like architecture is not synonymous with malignancy

Large fibroadenomas can occasionally show a phyllodes-like growth pattern.

Phyllodes tumors themselves include benign, borderline, and malignant categories.

Grade depends on stromal histologic characteristics rather than leaf shape alone.

Epithelial-lined stromal projections

Slide 47 · Molecular overlap supports related pathogenesis

Most phyllodes tumors harbor MED12 and RARA alterations.

These are also common in fibroadenomas.

The shared profile is consistent with related intralobular stromal biology.

Recurrent stromal alterations

MED12

RARA

Transcriptional regulation

Mechanism remains incompletely explained

Slide 48 · Additional gene mutations in phyllodes tumor

Phyllodes tumors are more likely to harbor additional mutations.

Examples reported by Robbins include TERT, TP53, and RB.

The source presents these as comparative associations, not definitive grade-specific tests.

Recurrent stromal alterations

MED12

RARA

Phyllodes also: TERT / TP53 / RB

Associations; Not diagnostic cutoffs

Slide 49 · Phyllodes is graded in three categories

Benign, borderline, and malignant are the recognized categories.

Histologic evaluation focuses on stromal features.

The grades describe different likelihoods of recurrence and metastatic behavior.

Benign

Borderline

Malignant

Greater stromal atypia/activity →

Slide 50 · Benign phyllodes: Similarities to fibroadenoma

Benign phyllodes resembles fibroadenoma.

Relative stromal cellularity and mitotic activity are increased.

Cytologically benign tumors can nonetheless recur locally.

Benign

Borderline

Malignant

Greater stromal atypia/activity →

Slide 51 · Borderline phyllodes: An intermediate phenotype

Borderline tumors show more stromal atypia.

Increased stromal cellularity and mitotic activity distinguish them from benign lesions.

This category reflects a histologic assessment—not inevitable progression to malignancy.

Benign

Borderline

Malignant

Greater stromal atypia/activity →

Slide 52 · Malignant phyllodes: Infiltrative stroma

Malignant tumors may be widely infiltrative.

Marked stromal overgrowth may leave few residual epithelial elements.

In advanced examples, separating the lesion from sarcoma can be difficult.

Epithelial-lined stromal projections

Slide 53 · Stromal assessment determines category

The features named include cellularity, atypia, mitotic activity, and infiltration.

Stromal overgrowth is especially relevant in malignant tumors.

The supplied excerpt does not specify numerical mitotic cutoffs or comprehensive modern grading criteria.

Benign

Borderline

Malignant

Greater stromal atypia/activity →

Slide 54 · A practical fibroadenoma–phyllodes distinction

Fibroadenoma: Delicate stroma; Usually lower cellularity.

Phyllodes: Greater stromal cellularity and proliferative activity.

Leaf-like architecture alone cannot resolve the differential.

Fibroadenoma

Phyllodes

More delicate stroma

Greater cellularity

Slide 55 · Benign grade does not guarantee no recurrence

Up to 75% of phyllodes tumors have benign cytologic features.

These can occasionally recur locally.

The source reports no metastases in the benign group.

Stromal phenotype / grade

Excision margin status

Local recurrence assessment

Margin predicts recurrence after excision

Slide 56 · Margins influence local recurrence

Borderline and malignant tumors show higher recurrence rates.

Margin status is the most important predictor of local recurrence after excision.

This connects the infiltrative stromal interface to surgical-pathology assessment.

Stromal phenotype / grade

Excision margin status

Local recurrence assessment

Margin predicts recurrence after excision

Slide 57 · Lymphatic spread is uncommon

Lymphatic spread is rare irrespective of grade.

Axillary lymph node dissection is contraindicated.

Nodal behavior differs from typical epithelial breast carcinoma.

Malignant stromal tumor

Bloodstream

Distant stromal deposit

Slide 58 · Malignant phyllodes can spread hematogenously

Malignant phyllodes tumors are uncommon.

About one-third of malignant lesions give rise to distant hematogenous metastases.

The bloodstream—not the axillary lymphatic route—is the key metastatic pathway described.

Malignant stromal tumor

Bloodstream

Distant stromal deposit

Slide 59 · Metastases contain only neoplastic stroma

Epithelial elements accompany the primary biphasic tumor.

Distant metastatic deposits contain the stromal neoplastic component alone.

This observation reinforces which component is biologically malignant.

Neoplastic stromal cells

Accompanying epithelial ducts

Slide 60 · Integrating grade, margins, and spread

Grade describes stromal histologic aggressiveness.

Margin status predicts local recurrence after excision.

Distant metastasis is associated with the uncommon malignant lesions.

Three clinical domains

Grade: Stromal aggressiveness

Margin: Local recurrence

Hematogenous metastasis

Nodal spread: Rare

Slide 61 · Phyllodes tumor: Case reasoning

Consider a large, fast-growing fibroepithelial mass in an older patient.

Search histology for leaf-like stromal fronds and stromal hypercellularity.

Then assess atypia, mitotic activity, infiltration, and stromal overgrowth.

Large mass in older patient

Stromal fronds + cellularity

Phyllodes: Assess grade

Morphologic diagnosis

Slide 62 · Fibroadenoma versus phyllodes: Synthesis

Both are neoplastic-stromal, biphasic lesions of intralobular origin.

Morphologic differences are most informative in the stromal compartment.

Phyllodes grading predicts clinically important variation in behavior.

Fibroadenoma

Phyllodes

More delicate stroma

Greater cellularity

Slide 63 · An origin-based diagnostic summary

Intralobular stromal tumors preserve epithelial association at the primary site.

Interlobular tumors lack an accompanying epithelial component.

Within intralobular tumors, stromal cytology and growth behavior separate entities.

Lobular unit + intralobular stroma

Interlobular stroma surrounds the unit

Slide 64 · Case 1: A young woman with a breast nodule

A 25-year-old has a discrete, well-circumscribed breast mass.

Histology shows bland myxoid stroma and compressed epithelial-lined clefts.

Which biphasic stromal lesion is most consistent with these findings?

Clinical presentation

Architectural examination

Stromal assessment

Use all levels of evidence

Slide 65 · Case 1: Interpret the stromal architecture

The pattern is consistent with fibroadenoma.

Compressed, cleft-like ducts represent an intracanalicular pattern.

Age, circumscription, and delicate stroma support the interpretation.

Dominant stromal features

Architecture interpreted

Clinicopathologic concordance

No single feature is sufficient

Slide 66 · Case 2: An older patient with a large mass

A woman in her sixth decade presents with a large breast mass.

Sections show epithelial-covered leaf-like projections and cellular stroma.

Which tumor family and histologic features should be assessed next?

Clinical presentation

Architectural examination

Stromal assessment

Use all levels of evidence

Slide 67 · Case 2: Grade the stromal component

The findings raise phyllodes tumor as the fibroepithelial lesion.

Assess cellularity, atypia, mitotic activity, infiltration, and stromal overgrowth.

Do not infer malignancy from leaf-like architecture or size alone.

Dominant stromal features

Architecture interpreted

Clinicopathologic concordance

No single feature is sufficient

Slide 68 · Essential pathology distinctions

Fibroadenoma is the dominant benign intralobular stromal tumor.

Phyllodes tumor shares stromal origin and recurrent alterations but varies in grade.

Local recurrence, lymphatic spread, and hematogenous metastasis are distinct outcomes.

Core facts

Origin: Intralobular

Tumor component: Stromal

Grading: Phyllodes

Metastasis: Malignant stromal cells

Slide 69 · Reference and source limitations

Primary source: Robbins, Cotran & Kumar, Pathologic Basis of Disease, 11th ed. (2026), Chapter 23, pp. 967–969.

The supplied Markdown extraction interleaves adjacent columns; relevant continuations were reconciled by section context.

Original textbook figure images/chapter PDF were not supplied; microscopy is taught with editable schematics, not synthetic specimens.

Methods and attribution

Source: Robbins 11th ed.

Breast chapter: pp. 967–969

No textbook images embedded

Diagrams are authored PPT shapes