Diagnosis of Chondroid Lipoma by Fine-Needle Aspiration Biopsy

Chondroid lipoma is a recently described benign soft tissue tumor with a predilection for subcutaneous tissue, fascia, or skeletal muscle of the proximal extremities.1,2 Because it is a rare entity, lack of knowledge of this tumor may lead to erroneous diagnoses, such as extraskeletal chondrosarcoma or myxoid liposarcoma. Rare reports regarding the cytologic morphology and radiological images of this tumor have been published.3–5 Our review of the literature identified only 2 previously reported cases with cytologic description, one involving a “fake” fine-needle aspiration biopsy (FNAB) performed on a surgically excised specimen and another with an indeterminate diagnosis. To our knowledge, this case report represents the first correct diagnosis of chondroid lipoma by FNAB.

A 41-year-old white man presented with an 8-month history of a nonenlarging, painless soft tissue mass in the anterior midportion of his left leg. Past medical history was unremarkable. Physical examination showed a nontender, 6 × 5-cm, ovoid mass palpable within the anterior compartment of his left leg. The mass was firm and without overlying warmth. His neurovascular status distally was intact to light touch and manual motor testing. No other masses were identified.

Fine-needle aspiration biopsy performed on the lesion produced the diagnosis of chondroid lipoma. Based on this diagnosis, the tumor then was excised in a marginal fashion through the pseudocapsule surrounding the tumor. Gross inspection of the tumor showed it to be whitish-yellow with the consistency of firm adipose tissue. At the patient's 6-month follow-up, there was no evidence of recurrent tumor on clinical examination.

The magnetic resonance imaging examination demonstrated a soft tissue mass measuring 4.5 cm in the transverse dimension, 3.1 cm in the anteroposterior dimension, and 7.4 cm in the cephalocaudal dimension. The mass was identified at the midtibial level, interposed between the tibialis anterior and extensor digitorum longus muscles.

On the T1-weighted image, the mass was of similar signal intensity to the adjacent muscles with an area of decreased signal intensity centrally and an area of slightly increased signal intensity peripherally. On the inversion recovery images, the central area was bright and surrounded by a concentric circle of decreased signal intensity, which was in turn surrounded by another ring of increased signal intensity more peripherally (Figure 1). Increased signal intensity was also identified cephalad and caudad to the lesion in a spindle shape. The mass was heterogeneous with nonspecific signal characteristics, although this “target sign” appearance would typically suggest a peripheral nerve sheath tumor. A soft tissue sarcoma could not be totally excluded based on these radiological features.

Fine-needle aspiration biopsy was performed by a cytopathologist (Y.J.Y.) using a 22-gauge needle and 10-mL syringe attached to a syringe holder. Two passes were performed. Some smears were air-dried for Diff-Quik stain, while others were acidified alcohol fixed for Papanicolaou stain. A cell block was prepared with fragments retrieved from needle rinse (saline). The cell block was then fixed in formalin, embedded in paraffin, and stained with hematoxylin-eosin.

Examination of the smears revealed clusters of cohesive lipoblast-like cells with many small cytoplasmic vacuoles, centrally or eccentrically located small nuclei, and inconspicuous nucleoli. Clusters of mature adipocytes, spindle cells, and small vessels were also present. On the smears, the cells were in a predominately chondromyxoid and partially hyalinized matrix.

Sections from the cell block showed typical histologic features of a chondroid lipoma, including nests and cords of vacuolated lipoblast- or chondroblast-like cells admixed with mature adipocytes and embedded in a chondromyxoid matrix with an intertwined vascular network (Figure 2, A and B). No significant cellular atypia, necrosis, or mitotic figures were identified in either the smears or the cell block sections. Immunohistochemical stains performed on the cell block sections showed tumor cells to be focally positive for CD68 (Dako Corp, Carpinteria, Calif), S100 (Labvision Corp, Fremont, Calif), and pancytokeratin (Labvision Corp). Less than 1% of the cells were positive for Ki-67 (Dako Corp).

After en bloc tumor excision, the FNAB diagnosis was confirmed by complete histopathologic analysis of the entire excised specimen. Grossly, the tumor was well circumscribed, bright yellow at the periphery, mostly tan at the center, but intermixed with focally hemorrhagic areas attributed to the previous FNAB (Figure 2, C).

Chondroid lipoma is a peculiar benign tumor first reported by Meis and Enzinger in 1993.1 Subsequent studies have demonstrated this tumor either to be composed solely of white fat without true cartilage differentiation6 or to demonstrate both adipocytic and chondrocytic differentiation.2 Histomorphologically, chondroid lipoma is composed of mature lipocytes, lipoblast-like cells, and chondroblast-like cells embedded in a chondromyxoid matrix, closely mimicking myxoid liposarcoma and myxoid chondrosarcoma. Hence, chondroid lipoma is often initially misdiagnosed as 1 of these 2 latter entities.1 

In general, the risk of making an erroneous diagnosis based on a cytologic specimen can be even greater than with a core or open biopsy specimen because histologic architecture cannot normally be as well appreciated in the cytologic specimen as in the others. Nevertheless, in our experience and as illustrated in this case, histologic architecture can very often be appreciated in the cell block sections of FNAB specimens, even though by convention routine FNAB supplies only cytologic detail. A good cell block supplies not only histomorphology, but also can be utilized in ancillary studies, including immunochemical stains7 and molecular genetic analysis. In soft tissue pathology, determination of histologic differentiation can be critical in making a correct diagnosis, assigning a tumor grade, or both. In many cases, this histologic differentiation depends on cytomorphology, histomorphology, immunohistochemistry, and/or genetic analysis, emphasizing the need for cell block analysis in addition to cytomorphology on FNAB specimens for these tumors.7,8 

Caution must be exercised in the interpretation of lipoblast- or chondroblast-like cells in chondromyxoid matrix because similar findings may be seen in myxoid liposarcoma, extraskeletal myxoid chondrosarcoma, extraskeletal chondroma, and hibernoma. However, myxoid liposarcoma is characterized not only by lipoblasts and myxoid matrix similar to those seen in chondroid lipoma, but also by a plexiform capillary network not seen in chondroid lipoma. Vacuolated cells similar to those seen in chondroid lipoma may occasionally also be seen in extraskeletal myxoid chondrosarcoma, but typically in much less quantity than in chondroid lipoma. Necrosis and mitoses, when identified, would strongly favor sarcoma over chondroid lipoma, but these features may not be as readily detectable in a cytologic specimen as they are in a standard histologic specimen. Other less-challenging differential diagnostic considerations include benign tumors, such as extraskeletal chondroma and hibernoma. The presence of vacuolated lipoblast-like cells, for instance, is not a diagnostic feature of extraskeletal chondroma, and the presence of a chondromyxoid matrix is not a characteristic feature of hibernoma.

The appearance of chondroid lipoma on magnetic resonance imaging has been previously reported by Logan et al.5 In their report, the tumor was described as a well-circumscribed lesion with predominantly low signal intensity relative to muscle on T1-weighted image and with fine lacy strands of high signal intensity typical of fat. Their report also described a mildly lobulated margin with fine, low-signal-intensity septations on the T2-weighted images, suggestive of cartilage matrix. However, the tumor in our study did not show unequivocal signal or shape suggestive of fat or chondroid tissue. It did exhibit a target sign appearance resembling a neurogenic tumor. Therefore, it appears that chondroid lipoma may or may not show typical fat or chondroid signal characteristics on magnetic resonance images and that its features are in fact quite nonspecific.

In this case report, we describe the cytologic features of chondroid lipoma and emphasize the usefulness of a cell block in achieving an accurate FNAB diagnosis. In comparison to the previous single report,5 the magnetic resonance imaging signal of our case was nonspecific, but with an appearance more suggestive of a neurogenic tumor. The correlation between radiological features as well as clinical presentation and the cytohistomorphology of this case further demonstrates the importance of a harmonious and close working relationship among pathologists, radiologists, and surgeons in patient care.