The Pathology Report and Tissue Testing

When a piece of a fatty mass is removed—either through a small needle biopsy or a full surgery—it is sent to a pathologist. These doctors study the tissue under a microscope to confirm exactly what the mass is made of. For a lump in the armpit, the most critical job of the pathologist is to distinguish a standard, harmless lipoma from a more complex growth that might require closer monitoring.

Under the Microscope: Lipoma vs. ALT

To the naked eye, most fatty tumors look like yellowish blobs of grease. However, under the microscope, the pathologist looks for specific “cellular signatures” ^[1].

• Conventional Lipoma: This is the most common result. It consists of mature adipocytes (fat cells) that look exactly like normal body fat. The cells are uniform in size, and there is no “atypia”—meaning the cells don’t look weird or aggressive ^[1][2].
• Atypical Lipomatous Tumor (ALT): Also known as Well-Differentiated Liposarcoma (WDLPS), this is a “low-grade” tumor. They are closely related entities in the same biologic spectrum, but ALT is commonly used for surgically amenable peripheral tumors and WDLPS is often used for central or retroperitoneal disease. The pathologist looks for nuclear atypia—enlarged or darkened centers (nuclei) within the fat cells or the surrounding “scaffolding” (stroma) ^[1][3].

In the armpit, identifying an ALT is important because these tumors are locally aggressive. While they rarely spread to other parts of the body, they have a risk of local recurrence and dedifferentiation if they aren’t removed completely ^[3][4].

The “Gold Standard” Test: MDM2 FISH

Sometimes, the cells look so much like normal fat that a pathologist cannot be 100% sure just by looking. In these cases, they perform a molecular test called FISH (Fluorescence In Situ Hybridization) to look for MDM2 amplification ^[5][6].

Think of your DNA like a library. In a normal lipoma, there are two “books” (copies) of the MDM2 gene. In an ALT/WDLPS, the tumor has “amplified” this gene, creating dozens or hundreds of copies ^[5].

• Positive MDM2 Result: If the test shows a high MDM2-to-CEP12 ratio, it supports the diagnosis of an ALT/WDLPS. A ratio threshold (like 2.0 or higher) may be used by a particular laboratory to call amplification ^[7][8].
• Negative MDM2 Result: This suggests the mass is a benign lipoma. However, MDM2 results must be interpreted with morphology, assay criteria, and specimen quality; a negative result does not absolutely exclude an atypical lesion or sampling error ^[5][6].

Rely on the integrated pathology diagnosis, which combines molecular and physical features, rather than a single ratio. Other tests, like PLAG1 rearrangements, are sometimes used to identify specific subsets of adult lipomas ^[9][10].

Your Pathology Report Checklist

Depending on the specimen, diagnosis, and local practice, your report may include:

1. Specimen Type: Does it say “Core Biopsy” (a small sample) or “Excision” (the whole lump)?
2. Tumor Dimensions: The size of the mass. Distinguish the size of a core specimen from the total tumor size.
3. Histologic Diagnosis: The specific name of the tumor (e.g., “Conventional Lipoma,” “Spindle Cell Lipoma,” or “Atypical Lipomatous Tumor”).
4. Atypia and Mitoses: A description of whether the cells look unusual and how fast they are dividing (mitotic rate) ^[11][12].
5. MDM2 Status: For suspected or confirmed atypical or malignant tumors, whether MDM2 FISH testing was performed ^[6].
6. Margins: For an excised atypical tumor, the report may state if the edges are “clear” (negative) or “involved” (positive). Detailed margin expectations are reserved for suspected or confirmed atypical tumors ^[13][12].

If your results are “equivocal” (unclear), it is a good idea to ask if the slides can be reviewed by a specialist in soft tissue pathology ^[6][14].