A spindle cell lipoma is a noncancerous tumor made of mature fat cells, long, thin cells called spindle cells, and thick bands of collagen. It is a type of lipoma, and it does not spread to other parts of the body.
Spindle cell lipoma usually develops just under the skin of the back of the neck, the upper back, or the shoulder. It occurs mostly in men between about 45 and 70 years old. Removing the lump is normally curative.
This tumor is closely related to pleomorphic lipoma. The two share the same genetic change and are considered part of one group, so your report may describe features of both. They are not the same diagnosis, and pleomorphic lipoma also contains large cells with several nuclei.
This article explains how a spindle cell lipoma is diagnosed and what the immunohistochemistry results and margins in your pathology report mean.
A spindle cell lipoma usually appears as a firm or rubbery lump under the skin that grows slowly over months or years. Most are painless and are noticed because of their size or position.
A large lump on the back or neck can cause discomfort from pressure, especially when lying down or leaning against a chair. These tumors rarely cause any other symptoms.
Spindle cell lipoma is caused by the loss of a small part of chromosome 13, in a region known as 13q14. This region contains a gene called RB1, which normally helps control cell growth. When one copy is lost, the cells that make up this tumor begin to grow.
This change happens in the tumor cells during a person’s lifetime. It is not inherited, and it does not carry the risks associated with the inherited condition that affects the same gene in the eye. The same change is found in several other noncancerous tumors, including pleomorphic lipoma and cellular angiofibroma.
A pathologist diagnoses spindle cell lipoma after examining tumor tissue under a microscope. The tissue may come from a biopsy or from surgery that removes the whole lump.
Under the microscope, a spindle cell lipoma contains three parts in varying proportions. These are mature fat cells that look like normal fat, bland spindle cells arranged in short bundles, and thick, rope-like bands of collagen. The collagen is a very useful clue for this diagnosis, and your report may describe it as ropey collagen.
The stroma, the supporting tissue that holds the tumor together, often looks bright pink under the microscope and may contain gel-like myxoid areas. Some spindle cell lipomas contain very little fat, and these can be harder to recognize, which is one reason additional tests are performed.
Immunohistochemistry is a test that uses antibodies to show which proteins cells make. For a spindle cell lipoma, it confirms the diagnosis and rules out other spindle cell tumors. Your report may include some of the following:
Some fatty tumors that can look similar have extra copies of a gene called MDM2, which is called amplification. The most important of these is atypical lipomatous tumor, which can come back after surgery.
Normal cells and the cells in a spindle cell lipoma have two copies of MDM2. Your pathologist may order fluorescence in situ hybridization (FISH) to count these copies. A normal result supports the diagnosis of a spindle cell lipoma.
Because a spindle cell lipoma contains fat, spindle cells, and collagen, it can resemble several other tumors. The tests above are used to tell them apart:
Tumor size is the greatest dimension of the spindle cell lipoma, measured in centimeters (cm). Most are small, and size does not change how this tumor behaves.
A margin is the edge of tissue cut by the surgeon. A negative margin means no tumor cells are seen at the cut edge, and a positive margin means tumor cells reach it. For a spindle cell lipoma, a positive margin does not carry the meaning it does for cancer. It slightly raises the chance that the lump comes back, and further surgery is usually not needed.
The outlook is excellent. A spindle cell lipoma is noncancerous, does not spread, and does not turn into cancer. Simple removal is curative for almost everyone, and the tumor rarely comes back even when a margin is positive.
Routine follow-up imaging is not needed. Your doctor may ask you to return if a new lump appears or if the area changes.
Most spindle cell lipomas are removed in a minor procedure, often under local freezing. Doctors usually recommend removal when the diagnosis is uncertain, the lump is large or growing, or it causes discomfort.
If the whole lump was already removed when the diagnosis was made, no further treatment is needed. If only a biopsy was done and the tumor is small and comfortable, watching it is a reasonable option.