Radiological Case: Well-differentiated liposarcoma of the groin

Well-differentiated liposarcoma of the groin

Findings

CT revealed a fatty lobulated mass with suggestions of internal septations, a characteristic not typically seen in simple lipomas (Figure1). Skeletal scintigraphy revealed focal increased uptake in the left groin, a finding suggestive of malignancy (Figure 2). MRI showed a predominantly fatty mass that clearly contained curvilinear septations, particularly at the tumor's inferior aspect (Figures 3 through 5). Enhancement was minimal and peripheral. The many suspicious clinical and imaging features of this tumor led to biopsy, which revealed histologic findings consistent with a high-grade liposarcoma (Figures 6 and 7).

Discussion

Liposarcomas account for 16% to 18% of malignant soft tissue tumors and are the second most common soft tissue sarcoma behind malignant fibrous histiocytoma. Liposarcomas are most prevalent in the fifth and sixth decades¹and are the most common soft tissue sarcomas ofthe lower extremities between the ages of 26 and 45.²In 42% of cases, they occur in the trunk (often in the retroperitoneum), 41% in a lower extremity (favoring the thigh), 11% in an upper extremity, and 6% in the head and neck region.³These tumors, which are known to occur in the deep soft tissues, are of primitive mesenchymal cell origin, differentiating into mature adipose tissue. Patients usually present with a large painless mass.

The histology of the mass is vitally important to treatment. The prognosis, risk of recurrence, and risk of metastasis are closely related to the histologic type and degree of differentiation. Although CT and MRI may help to differentiate between atypical lipomas, myxoid liposarcomas, pleomorphic round-cell liposarcomas, and dedifferentiated liposarcomas, histologic examination remains the diagnostic gold standard and aids in preoperative planning. In general, liposarcomas are well circumscribed and multilobular. The above- mentioned morphologic subtypes are differentiated based on the presence of gross fat and histologic findings. The low- and intermediate-grade tumors, displaying extensive gross fat or myxoid material, are atypical lipomas and myxoid subtypes. The high-grade tumors, which contain very minimalor no gross fat, but rather a very cellular appearance, are the pleomorphic, round cell, and dedifferentiated liposarcomas.²

Atypical lipomas are also referred to as well-differentiated liposarcomas and are believed to be low-grade tumors. Fat usually constitutes >75% of the volume in these tumors. This subtype is known to resemble a benign lipoma on CT and MRI; however, liposarcomas may show enhancement of septations and of intratumoral nodular densities on gadolinium-enhanced, T1-weighted (T1W), fat-suppressed MRI.^3-5Due to the difficulty in distinguishing the different subtypes by imaging, a thorough pathologic examination is oftentimes necessary. Histologically, these tumors show lipoblasts (irregularly shaped cells with hyperchromatic nuclei) and lipocytes.⁴Although these tumors are known to locally recur if marginally excised, metastasis does not occur. It is possible, however, for these tumors to dedifferentiate further into a more aggressive form, with the possibility of metastasis.

Myxoid liposarcomas are low-to-intermediate-grade tumors that account for 50% of all liposarcomas. Fat makes up <25% of the tumorvolume. Half of these tumors show no imaging evidence of fat. Mucoid material often fills cystic areas of the tumor. T1-weighted MRIshows a relatively homogeneous material that is isointense to muscle, while T2-weighted (T2W) MRI shows a homogeneous high-signal-intensity lobular mass containing linear septa. Contrast-enhanced MRI shows heterogeneous enhancement. Histologically, these tumors are composed of proliferating lipoblasts, a plexiform capillary pattern, and a myxoid matrix.⁴

Pleomorphic and round-cell liposarcomas appear similar radiologically. Both are high-grade tumors that frequently metastasize. Both are largely cellular, and little-to-no fat is seen in most cases. MRI usually shows a heterogeneous pattern. The signal intensity on T1W images islow, while the signal intensity is high on T2W images. The enhancement of these tumors is heterogeneous with necrotic components that are more distinguishable after contrast administration.^3,4 Histologically, these tumors show cellular pleomorphism, bizarre giant cells, hemorrhage, and necrosis. Without the presence of characteristic lipoblasts, these tumors may be difficult to differentiate from malignant fibroushistiocytoma.

Dedifferentiated liposarcomas, more common to the retroperitoneum because of their association with large tumors, are bimorphic in appearance. These tumors have a well-differentiated liposarcoma component and a high-grade, nonlipogenic sarcoma component.³

CONCLUSION

Liposarcomas are tumors of mesenchymal origin that differentiate into adipose tissue. Generally, the grade of malignancy increases with tumor heterogeneity, cellularity, and degree of contrast enhancement. The treatment of liposarcomas almost always involves surgical resection with wide margins. Local adjuvant radiation and/or chemotherapy may also be a part of the treatment plan.

1. Kransdorf MJ, Murphey MD. Imaging of Soft Tissue Tumors. Philadelphia, PA: Saunders; 1997;79-94.
2. Ramsdell MG, Chew FS, Keel SB. Myxoid liposarcoma of the thigh. AJR Am J Roentgenol. 1998;170:1242.
3. Munk PL, Lee MJ, Janzen DL, et al. Lipoma and liposarcoma: Evaluation using CT and MR imaging. AJR Am J Roentgenol. 1997;169:589-594.
4. Arkun R, Memis A, Akalin T, et al. Liposarcoma of soft tissue: MRI findings with pathologic correlation. Skeletal Radiol. 1997;26:167-172.
5. Yang YJ, Damron TA, Cohen H, Hojnowski L. Distinction of well-differentiated iposarcoma from lipoma in two patients with multiple well-differentiated fatty masses. Skeletal Radiol. 2001;30:584-589.