Are atypical lipomatous tumors (ALT) and well-differentiated liposarcomas the same or different in terms of management and prognosis?

ALT and Well-Differentiated Liposarcoma: Same Entity, Different Names

Atypical lipomatous tumor (ALT) and well-differentiated liposarcoma (WDL) are the exact same disease—they are synonymous terms describing identical tumors both morphologically and genetically, with the naming convention based solely on anatomic location and surgical resectability. 1, 2, 3

Nomenclature Explained

The dual terminology exists purely for clinical communication purposes:

• "ALT" is used for tumors in the extremities and trunk where complete surgical resection is typically achievable 3
• "Well-differentiated liposarcoma" is used for retroperitoneal and deep-seated locations where complete resection is often impossible 4, 3
• Both harbor identical genetic aberrations: high-level amplifications in chromosome 12q13-15 region, including MDM2 and CDK4 oncogenes 3
• Both show the same histologic features: mature adipose tissue with fibrous septation and variable nuclear atypia 3

Clinical Behavior: Location Determines Outcome

The critical distinction is not biological but anatomical:

Extremity/Trunk ALT

• Local recurrence rate after marginal excision: 11.9% 5
• Local recurrence rate after wide excision: 3.3% 5
• Recurrences are almost always amenable to re-resection 5
• Dedifferentiation risk: 1-4% 6, 7
• Metastasis: exceptionally rare (0.1%) 5

Retroperitoneal WDL

• Significantly worse outcomes due to inability to achieve complete resection 4, 3
• Higher local recurrence rates with each recurrence increasing dedifferentiation risk 8
• Progressive dedifferentiation can occur with inadequate excisions 8

Diagnostic Approach

Imaging Characteristics

MRI features suggesting ALT/WDL rather than benign lipoma include: 1, 9

• Nodularity and thick septations
• Contrast enhancement (81.2% of ALT vs 18.8% of lipomas)
• Size >5 cm
• Deep-seated location
• Increased intratumoral vascularity

However, MRI can definitively differentiate benign lipoma from ALT in only 69% of cases 1, 8

Definitive Diagnosis

• Core needle biopsy with MDM-2 amplification testing by fluorescence in-situ hybridization is mandatory when suspicion exists 1, 2, 8
• This molecular test definitively distinguishes benign lipoma from ALT/WDL and fundamentally alters surgical planning 8
• Core biopsy has a false-negative rate of 6.67% for detecting dedifferentiation 7

Management Algorithm

Surgical Treatment

Complete en-bloc marginal excision is the standard treatment for extremity/trunk ALT, even if histopathologically R1, offering excellent long-term local control 1, 5

• Marginal excision results in 11.9% local recurrence vs 3.3% for wide excision 5
• The slightly higher recurrence rate with marginal excision is acceptable because recurrences are successfully re-resected 5
• Surgery should be performed by a surgeon trained in sarcoma management 1

Referral Criteria to Sarcoma Center

Mandatory referral for: 8

• Deep-seated masses or any mass >5 cm
• Retroperitoneal or intra-abdominal location
• Atypical MRI features (nodularity, thick septations)
• Diagnostic uncertainty between lipoma and ALT

Special Populations

• Elderly patients with significant comorbidities: radiological surveillance is acceptable even for larger lesions if surgery would be highly morbid 1, 2

Follow-Up Strategy

Long-term surveillance is essential because recurrences can develop as late as 140 months after initial treatment 6

Practical approach: 4

• Every 3-4 months for first 2-3 years
• Every 6 months until year 5
• Annually thereafter
• Focus on local recurrence detection and chest imaging for metastases

Critical Pitfalls to Avoid

• Do not assume core biopsy showing ALT excludes dedifferentiation: 7.3% of tumors biopsied as ALT show DDLS on final pathology 7
• Do not perform piecemeal removal: incomplete excision increases recurrence risk and potential for dedifferentiation 2
• Do not discharge patients after 5 years: recurrences occur beyond 10 years 6
• Do not ignore rapid growth in a known ALT: this mandates re-biopsy to exclude dedifferentiation 2