Fibrolipoma of the Ring Finger: MR Imaging and Histological Correlation

Discussion

Benign lipomatous tumors are classified into various subtypes based on their clinical, histological, or cytogenetic features (2). Fibrolipoma is one of the less common histological subtypes of lipoma. It has a prominent fibrous component in addition to mature adipose tissue element. Marginal excision is curative and local recurrence is rare. Although fibrolipoma may develop in virtually any region of the body, there is, to the best of our knowledge, no English literature describing the occurrence of this tumor in the finger.

MRI is the most commonly used modality in the evaluation of lipomatous tumors. Ordinary lipomas have generally been described as showing homogeneous signal intensity identical to subcutaneous fat in all MR pulse sequences, with complete loss of signal after fat suppression (3). No enhancement is usually observed following intravenous contrast administration (4). In the current case, however, mild inhomogeneous enhancement was found in nonadipose (fibrous) areas. Therefore, we could not entirely rule out the possibility of a well-differentiated liposarcoma. Kajihara et al. (5) also noted the existence of non-adipose areas and heterogeneous enhancement in fibrolipoma. It may be difficult to distinguish fibrolipoma from well-differentiated liposarcoma by enhancement with gadolinium alone.

The differential diagnosis for fibrolipoma includes sclerotic lipoma, spindle cell lipoma, neural fibrolipoma, and well-differentiated liposarcoma.

Sclerotic lipoma, first described by Zelger et al. (6) in 1997, is a rare but distinctive fibroma-like variant of lipoma. It occurs in all age groups but more often in young to middle-aged adults, with a slight male predominance. Sclerotic lipoma usually presents as a slow-growing, painless mass in the finger (7). Histological analysis shows a dominant fibrous or fibrosclerotic matrix that contains scattered mature adipocytes. The fatty component comprises between 5% and 50% of the mass. Immunohistochemically, the non-lipogenic tumor cells stains for CD99 and variably for CD34, S-100 protein, and smooth muscle actin (7). In the current case, we eliminated the possibility of a sclerotic lipoma because the fatty component of the lesion prevailed (approximately 80%) and immunostaining for CD99 and smooth muscle actin was negative.

Spindle cell lipoma typically presents as a slow-growing, painless, firm nodule in the subcutaneous tissue of the posterior neck, shoulder, and back. It rarely arises in the finger. There is a marked male predilection between 40 and 70 years of age (8). The MRI appearance is not pathognomonic and displays a spectrum of features (9). Histologically, spindle cell lipoma consists of a relative equal mixture of spindle-shaped cells and mature adipocytes. Immunohistochemically, the spindle-shaped cells are strongly positive for CD34 but negative for smooth muscle actin or S-100 protein (10, 11). The current case seems to be different from spindle cell lipoma because a strong and diffuse pattern of CD34 staining was not demonstrated.

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Figure 1.

Axial magnetic resonance images of fibrolipoma at the proximal phalanx level of the right ring finger. T1- (A) and T2- (B) weighted sequences show that the radial component (*) has intermediate-to-high signal intensity and the ulnar component has high signal intensity. Contrast-enhanced fat-suppressed T1-weighted sequence (C) demonstrates a mild enhancement of the radial component (*).

Neural fibrolipoma, also known as fibrolipomatous hamartoma of nerve, is a rare tumor-like lipomatous process that almost exclusively involves the median nerve (and its digital branches) in the hand. It is associated with macrodactylia in about one-third of cases (12). Most cases occur within the first three decades of life. Neural fibrolipoma presents as a slow-growing, sometimes painful, soft lump. The MRI feature is virtually pathognomonic and reveals a coaxial cable-like appearance on axial planes and a spaghetti-like appearance on coronal planes on both T1- and T2-weighted sequences (13). Histologically, neural fibrolipoma consists of fibrofatty tissue that grows along epineurium and perineurium and surrounds and infiltrates the nerve trunk. In the current case, we were able to eliminate the possibility of a neural fibrolipoma on the basis of gross and microscopic features.

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Figure 2.

A: Gross appearance of the tumor. B: The tumor is composed of mature adipocytes and collagenous fibrous tissue. C: A recognizable non-adipose component consists of spindle-shaped cells without atypia in a sclerotic collagenous stroma.

Well-differentiated liposarcoma is the most common form of liposarcoma encountered in late adult life. It usually presents as a slow-growing, painless mass in the lower extremity, especially the thigh. On MRI, well-differentiated liposarcoma typically demonstrates a largely lipomatous mass representing over 75% of the lesion and non-adipose components in thick septa (> 2 mm) of irregular aspect or nodular foci (3, 4, 14). The thick septa show moderate or marked enhancement after intravenous gadolinium administration (4). Histologically, well-differentiated liposarcoma consists of mature adipocytes with significant variation in cell size and hyperchromatic nuclei. Immunostaining for MDM2, CDK4, and p16 has recently been shown to be a highly sensitive and specific means of identifying and separating well-differentiated liposarcoma from benign lipomatous tumors (15). In the current case, MDM2 and CDK4 were not expressed, strongly suggesting that the tumor was a benign lipomatous tumor.

In summary, as far as we are aware of, we have reported the first case of fibrolipoma without nerve involvement in the finger. Clinicians should consider fibrolipoma as a possible diagnosis for a lipomatous lesion in the finger, thereby avoiding unnecessarily aggressive treatment.