What are ALT Tumors?
Atypical lipomatous tumors (ALT) are biologically indolent, locally aggressive adipocytic neoplasms that are synonymous with well-differentiated liposarcoma (WDL), representing the same morphological and genetic entity but with site-dependent terminology. 1
Classification and Terminology
ALT and WDL are essentially the same tumor, as defined by the WHO classification. The distinction in terminology depends primarily on anatomical location 1:
- ALT is the preferred term for extremity tumors where complete surgical excision with margins is achievable
- WDL is more commonly applied to deep tumors in sites like the retroperitoneum where wide margin excision is unlikely and local recurrence is more common
Biological Behavior
These tumors are classified as "borderline tumours" rather than typical sarcomas 1. Their key characteristics include:
- Propensity for local recurrence but little to no capacity for metastatic spread 1
- Dedifferentiation into more aggressive disease is extremely rare in extremity locations 1
- In retroperitoneal locations, progressive dedifferentiation with each recurrence is more commonly observed 1
Diagnostic Features
Molecular Hallmark
The defining diagnostic test is fluorescence in-situ hybridization (FISH) demonstrating MDM-2 oncogene amplification 1. This molecular marker:
- Can be performed on percutaneous core needle biopsy
- Definitively differentiates ALT from benign lipomas
- Alters the surgical approach when positive 1
Imaging Characteristics
MRI with expert review can differentiate large, deep lipomas from ALT in up to 69% of cases 1. Suspicious features include:
- Nodularity
- Thick septations (>2mm)
- Stranding
- Relative size (>10cm)
- Location below deep fascia 2
However, MRI cannot reliably differentiate between deep lipomas and ALTs, and diagnostic uncertainty should be supplemented by core needle biopsy for MDM-2 amplification analysis 1.
Clinical Management
Surgical Approach
Complete en bloc resection preserving adjacent neurovascular structures but with no attempt to gain wide surgical margins will afford long-term local control 1. Key principles:
- Marginal resections as a complete en bloc specimen provide excellent rates of long-term local control, even if classified histopathologically as R1 1
- The goal is functional preservation rather than wide margins
- ALTs can be very large tumors abutting critical neurovascular structures and multiple muscle groups 1
Alternative Management
In older patients where surgery would be morbid or in those with significant comorbidities, radiological surveillance can be considered 1.
Adjuvant Therapy
Adjuvant radiotherapy may occasionally be considered in larger tumors or those where clear margins are difficult to achieve 1.
Follow-up
Following post-operative wound care, patients with ALT can be discharged to primary care with re-referral only if there is clinical suspicion of recurrence 1.
Prognosis
The prognosis is mostly excellent 1. These tumors:
- Remain localized
- Do not generally metastasize
- Have excellent long-term local control with appropriate surgical management
- Carry minimal risk of dedifferentiation in extremity locations
Critical Pitfall
The most important diagnostic pitfall is misdiagnosing ALT as a benign lipoma, which would lead to inadequate surgical planning and potentially suboptimal outcomes. Always obtain tissue diagnosis with MDM-2 testing for deep, large (>5cm), or imaging-suspicious lipomatous masses 1.