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A 64-year-old man presented with a subcutaneous mass on the back of his neck, which had been slowly enlarging over the course of several years. The mass was excised and consisted of a 12.0 x 3.8 x 5.5 cm relatively well-circumscribed yellowish-gray tumor.
Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2007, Case 12 and is a spindle cell lipoma.
Criteria for Diagnosis and Comments:
Spindle cell lipoma is a variant of conventional lipoma that is characterized by varying amounts of mature adipose tissue and cytologically bland spindled cells. The spindle cells are uniform in appearance and are typically arranged in parallel bundles, which may exhibit nuclear palisading. Prominent bands of thick “rope-like” collagen are a characteristic feature. Mast cells may also be conspicuous. Mitoses should be absent or exceedingly rare. Some cases may exhibit prominent myxoid change in the stroma, as is demonstrated in several of the slides in this case. The background vasculature is typically inconspicuous although some cases are reported with a prominent pattern. The spindle cells are characteristically strongly positive for CD34 but negative for actin and desmin. S-100 may stain background adipocytes but the spindle cells are negative.
Spindle cell lipoma classically occurs as an asymptomatic subcutaneous mass of the posterior neck, upper back or shoulder region. Occasionally, tumors may involve the forehead, scalp or upper arm but lower extremity involvement is very rare. There is an overwhelming male predominance and the median age of presentation is 55. The tumor grows slowly and most cases have been present for several years prior to excision. Complete excision is generally curative although local recurrence may occur.
Grossly, spindle cell lipoma is a circumscribed lesion. Depending on the proportions of adipose tissue and spindle cells, the cut surface will vary from fatty to grayish-white. A gelatinous consistency may be present in tumors with a prominent myxoid stroma.
Histologically, spindle cell lipoma forms a spectrum with so-called pleomorphic lipoma. In addition to the features described above, there may be a component of round cells with hyperchromatic nuclei or multinucleate cells with radially arranged nuclei (“floret cells”).
Spindle cell lipoma is frequently hypodiploid. The most common abnormalities are monosomy of chromosome 13 or chromosome 16.
The main entities in the differential diagnosis of spindle cell lipoma are the spindle cell variant of well-differentiated liposarcoma, myxoid liposarcoma, dermatofibrosarcoma protuberans (DFSP), fibromatosis, nodular fasciitis and benign neural tumors such as schwannoma and neurofibroma.
Well-differentiated liposarcoma may contain a component of spindled cells, thus creating histologic overlap with spindle cell lipoma. Such tumors, termed the spindle cell variant of well-differentiated liposarcoma, contain a component of bland spindle cells that have a neural-type appearance. Atypical lipocytes and lipoblasts should be identifiable, and the rope like collagen seen in spindle cell lipoma should not be present. Spindle cell lipomas with a prominent myxoid background may resemble myxoid liposarcoma. The plexiform vascular network seen in myxoid liposarcoma is absent in most cases of spindle cell lipoma, and lipoblasts should not be present. CD34 staining may occasionally be present in liposarcoma and should not be used as the sole differentiating feature.
DFSP is comprised of spindle cells arranged in a striking storiform arrangement. This pattern should not be present to any significant degree in spindle cell lipoma. DFSP is also typically mitotically active and has an infiltrative interface with the adjacent tissue, in contrast to spindle cell lipoma. CD34 is positive in both tumors and therefore does not have a discriminating role.
Fibromatosis may occur as a superficial form, typically involving the palms or soles, or as a deep seated “desmoid type” variety. Both are characterized by relatively uniform slender spindled cells set in a collagenous background. The mitotic rate is variable and the cells are typically arranged in sweeping fascicles. Unlike spindle cell lipoma, fibromatosis is poorly circumscribed and the cells show irregular infiltration of adjacent skeletal muscle or adipose tissue at the periphery. Fibromatosis is also typically negative for CD34.
Nodular fasciitis is a mass-forming fibroproliferative process which, like spindle cell lipoma, tends to be superficially located and may frequently involve the head and neck, although almost any site may be affected. Nodular fasciitis is characterized by plump, spindled cells which are classically described as having a “tissue-culture” appearance. The process is highly cellular but typically should contain hypocellular myxoid zones. Extravasated red blood cells are also usually present. Collagen deposition is not a typical finding but may be present and occasionally exhibit a hyalinized keloid-like appearance. Mitotic activity is variable and may be quite high, in contrast to spindle cell lipoma. Nodular fasciitis typically arises rapidly and generally has an infiltrative border, which is also in contrast to spindle cell lipoma. Nodular fasciitis is also typically CD34 negative and actin positive.
Benign neural tumors such as neurofibroma and schwannoma may also enter the differential diagnosis. Immunohistochemical studies readily distinguish these two entities from spindle cell lipoma, as both are positive for neural markers such as S-100.