Definition/General

Introduction:
-Borderline phyllodes tumors are biphasic fibroepithelial tumors of the breast with features intermediate between benign and malignant phyllodes tumors
-They account for about 15-25% of all phyllodes tumors.
Origin: They arise from the intralobular stroma of the breast.
Classification:
-Phyllodes tumors are classified as benign, borderline, or malignant
-Borderline tumors have a low risk of metastasis but a significant risk of local recurrence.
Epidemiology: They are most common in women in their 40s.

Clinical Features

Presentation:
-Typically presents as a painless, firm, mobile, palpable breast mass
-They can grow rapidly.
Symptoms: A painless, large breast lump is the most common symptom.
Risk Factors: The exact cause is unknown.
Screening:
-On mammography, they appear as large, well-circumscribed, lobulated masses
-Ultrasound shows a solid mass, sometimes with cystic spaces.

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Gross Description

Appearance:
-A large, well-circumscribed, lobulated mass
-The cut surface has a characteristic leaf-like (phyllodes) appearance.
Characteristics: Borderline tumors may show some areas of necrosis or hemorrhage.
Size Location:
-Can occur anywhere in the breast
-Size is highly variable.
Multifocality: Multifocality is rare.

Microscopic Description

Histological Features:
-Biphasic tumor with a benign epithelial component and a cellular stromal component
-The features are intermediate between benign and malignant: moderate stromal cellularity, moderate stromal atypia, moderate mitotic activity (5-9 per 10 high power fields), and pushing borders
-Stromal overgrowth may be present.
Cellular Characteristics: The stromal cells are spindle-shaped with moderate atypia.
Architectural Patterns: The characteristic leaf-like architecture is prominent.
Grading Criteria:
-The diagnosis of borderline phyllodes tumor is based on the combination of moderate stromal hypercellularity, moderate atypia, moderate mitotic rate, and pushing margins
-Stromal overgrowth can be present.

Immunohistochemistry

Positive Markers: The stromal cells are positive for vimentin and CD34.
Negative Markers: The stromal cells are negative for cytokeratins.
Diagnostic Utility: IHC is not usually necessary for diagnosis but can be used to differentiate from other spindle cell lesions.
Molecular Subtypes: Not applicable.

Molecular/Genetic

Genetic Mutations: Mutations in MED12 and TERT promoter are common.
Molecular Markers: No specific molecular markers are routinely used for diagnosis.
Prognostic Significance: Borderline phyllodes tumors have a high risk of local recurrence (around 20-30%) but a low risk of distant metastasis (<5%).
Therapeutic Targets: Treatment is wide local excision with clear margins to reduce the risk of local recurrence.

Differential Diagnosis

Similar Entities:
-Benign phyllodes tumor
-Malignant phyllodes tumor
-Cellular fibroadenoma.
Distinguishing Features: The distinction between benign, borderline, and malignant phyllodes tumors is based on the degree of stromal cellularity, atypia, mitotic activity, and the nature of the borders.
Diagnostic Challenges: The distinction between the different grades of phyllodes tumors can be subjective and requires careful evaluation of all histological parameters.
Rare Variants: Not applicable.

Sample Pathology Report

Template Format

Sample Pathology Report

Complete Report: This is an example of how the final pathology report should be structured for this condition.

Specimen Information

[specimen type], measuring [size] cm in greatest dimension

Diagnosis

[diagnosis name]

Classification

Classification: [classification system] [grade/type]

Histological Features

Shows [architectural pattern] with [nuclear features] and [mitotic activity]

Size and Extent

Size: [X] cm, extent: [local/regional/metastatic]

Margins

Margins are [involved/uninvolved] with closest margin [X] mm

Lymphovascular Invasion

Lymphovascular invasion: [present/absent]

Lymph Node Status

Lymph nodes: [X] positive out of [X] examined

Special Studies

IHC: [marker]: [result]

Molecular: [test]: [result]

[other study]: [result]

Prognostic Factors

Prognostic factors: [list factors]

Final Diagnosis

Final diagnosis: [complete diagnosis]