Definition/General
Introduction:
Colorectal lipoma is a benign adipose tissue tumor composed of mature fat cells
It is the second most common benign submucosal tumor of the colon after adenomatous polyps
Represents 4-5% of all colonic polyps
Shows excellent prognosis with complete excision.
Origin:
Arises from mature adipocytes in the submucosal layer
Can originate from subserosal fat
May develop from intramural fat
Shows hamartomatous growth pattern
Demonstrates slow growth over time.
Classification:
Classified as submucosal lipoma (most common)
Subserosal lipoma
Intramural lipoma
Pedunculated lipoma
Sessile lipoma
WHO classification: Benign adipose tumor.
Epidemiology:
Peak incidence in 5th-7th decades
Slight female predominance
More common in right colon (cecum, ascending)
Common entity in Indian population
Often asymptomatic
May reach large size before detection.
Clinical Features
Presentation:
Asymptomatic in most cases (70-80%)
Change in bowel habits
Abdominal pain or cramping
Rectal bleeding (if ulcerated)
Bowel obstruction in large lesions
Intussusception (lead point)
Palpable mass.
Symptoms:
Intermittent abdominal pain (30% cases)
Alternating constipation and diarrhea
Rectal bleeding (10-15%)
Mucus discharge
Feeling of incomplete evacuation
Nausea and vomiting (if obstructed)
Weight loss uncommon.
Risk Factors:
Age >40 years
Obesity
Metabolic syndrome
Familial lipomatosis (rare)
Gardner syndrome (multiple lipomas)
No clear environmental factors
Female gender (slight predilection).
Screening:
Colonoscopy for symptomatic patients
CT scan shows fat density (-50 to -150 HU)
MRI shows fat signal characteristics
Endoscopic ultrasound for submucosal lesions
Biopsy may show only normal mucosa.
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Gross Description
Appearance:
Well-circumscribed yellow mass
Soft, fatty consistency
Lobulated surface
Encapsulated appearance
Cut surface shows yellow fat
No necrosis or hemorrhage.
Characteristics:
Smooth overlying mucosa
Pedunculated or sessile morphology
Compressible consistency
Homogeneous cut surface
May show ulceration if large
Clear demarcation from surrounding tissue.
Size Location:
Size ranges from 0.5-20 cm
Cecum most common (40%)
Ascending colon (25%)
Transverse colon (15%)
Right-sided predominance
Submucosal location typical.
Multifocality:
Usually solitary (95% cases)
Multiple lipomas in 5% cases
Familial lipomatosis - multiple GI lipomas
Gardner syndrome association
No associated adenomas typically.
Microscopic Description
Histological Features:
Mature adipocytes with uniform appearance
Single peripheral nucleus
Large cytoplasmic fat vacuole
Thin fibrous septa between fat lobules
Intact overlying mucosa
No atypia.
Cellular Characteristics:
Large, uniform fat cells
Eccentric, small nuclei
Clear cytoplasm (fat dissolved)
Thin cell membranes
No pleomorphism
No mitotic activity.
Architectural Patterns:
Lobular architecture
Fibrous septa dividing fat lobules
Blood vessels in septa
No capsule typically
Pushing margins
Intact muscularis mucosae above.
Grading Criteria:
No grading system for benign lipomas
Cellular uniformity important
Absence of lipoblasts
No myxoid change
No sclerosis typically.
Immunohistochemistry
Positive Markers:
S-100 positive (weak)
Adipophilin positive
Vimentin positive
FABP4 positive (specific for adipocytes)
Perilipin positive
CEBPα positive.
Negative Markers:
Cytokeratins negative
EMA negative
Smooth muscle actin negative
Desmin negative
CD34 negative (except vessels)
c-KIT negative.
Diagnostic Utility:
Usually not required for diagnosis
Morphology sufficient in most cases
S-100 weak positivity helps
FABP4 specific for fat cells
Rules out other tumors.
Molecular Subtypes:
No molecular subtypes
Brown fat markers (UCP1) negative
White adipocyte markers positive
Ki-67 very low (<1%)
No specific molecular markers required.
Molecular/Genetic
Genetic Mutations:
No specific mutations identified
12q13-15 rearrangements in some lipomas
HMGA2 rearrangements occasionally
Different from soft tissue lipomas
MDM2 amplification absent.
Molecular Markers:
Adipogenesis markers (PPARγ, CEBPs)
Lipid metabolism genes
Low proliferation markers
No oncogene activation
Tumor suppressor genes intact.
Prognostic Significance:
Excellent prognosis
No malignant potential
Rare recurrence after complete excision
Large size may cause complications
Complete excision curative.
Therapeutic Targets:
Complete surgical excision
Endoscopic resection for pedunculated lesions
Segmental resection for large lesions
Enucleation possible for well-defined lesions
No medical therapy required.
Differential Diagnosis
Similar Entities:
Liposarcoma - pleomorphic cells, lipoblasts, atypia
Hibernoma - brown fat, multivacuolar cells
Angiolipoma - vascular component prominent
Spindle cell lipoma - spindle cells admixed
Fibrolipoma - fibrous tissue mixed.
Distinguishing Features:
Liposarcoma: cellular atypia, lipoblasts, MDM2 amplification
Hibernoma: granular eosinophilic cytoplasm, multivacuolar
Angiolipoma: prominent vascular component
Spindle cell lipoma: CD34+ spindle cells
Fibrolipoma: mature fibrous tissue admixed.
Diagnostic Challenges:
Sampling issues in small biopsies
Fat necrosis may cause confusion
Distinguishing from normal fat
Excluding well-differentiated liposarcoma
Clinical correlation important.
Rare Variants:
Fibrolipoma - fibrous tissue mixed with fat
Myxolipoma - myxoid stroma present
Chondrolipoma - cartilaginous differentiation
Osteolipoma - osseous metaplasia
Angiolipoma - prominent vasculature.
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
Colorectal polypectomy/resection specimen containing a [size] cm submucosal mass
Diagnosis
Colorectal Lipoma
Classification
Benign adipose tissue tumor
Histological Features
Shows mature adipocytes with uniform morphology. No cellular atypia, lipoblasts, or increased mitotic activity.
Size and Extent
Size: [X] cm, confined to submucosa
Margins
Margins: [involved/uninvolved]
Special Studies
Immunohistochemistry not required
No molecular testing indicated
Morphology diagnostic
Final Diagnosis
Colorectal lipoma, benign