Liposarcoma

We report a unique case of primary myxoid liposarcoma of the floor of the mouth, in which well-differentiated rhabdomyoblasts were present. The tumor was incompletely excised. The patient is free of recurrence or metastasis following a course of radiotherapy. Heterologous components found in a liposarcoma include leiomyosarcoma, rhabdomyosarcoma, chondrosarcoma, osteosarcoma, and angiosarcoma. These elements have been described mainly in dedifferentiated liposarcomas situated in the retroperitoneum. To our knowledge, there has been only one report of a liposarcoma with rhabdomyosarcomatous differentiation occurring in the absence of a dedifferentiated component. The clinical implications of the presence of heterologous components without dedifferentiation are unclear.

(Arch Pathol Lab Med. 1998;122:740-742)

Liposarcomas are common soft tissue sarcomas occurring most frequently in the thigh and retroperitoneum.1 One of the less common sites is the head and neck region. Recurrence is common, and the rate of metastasis is inversely related to the degree of histologic differentiation. Heterologous differentiation within liposarcomas is usually described in the dedifferentiated areas and is an uncommon occurrence. This may take the form of rhabdomyosarcoma, leiomyosarcoma, osteosarcoma, or angiosarcoma.2-5 We describe a case of myxoid liposarcoma of the floor of the mouth with rhabdomyoblasts occurring in the absence of a dedifferentiated component.

REPORT OF A CASE

A 34-year-old man presented with a swelling in the floor of the mouth of 3 months' duration. He had complained of difficulty in swallowing and an increase in the size of the mass over the last 3 months. On examination, a 5 x 5-cm firm mass was present in the left floor of the mouth with extension into the anterolateral neck. The hypopharynx and larynx were normal. There was no cervical or submental lymphadenopathy. The clinical impression was that of a ranula.

The routine hematologic and biochemical investigations were normal. At surgery after reflection of the mucosa, a large lipomatous tumor was excised. The excision was incomplete, and the patient received a course of radiotherapy. Six months after presentation, the patient is well and free of local recurrence or distant metastasis.

PATHOLOGIC FINDINGS

The excision specimen consisted of a well-circumscribed, partly encapsulated, dumb-bell-shaped mass, measuring 13 x 7 x 4 cm. Cut section showed a predominantly solid, pale yellow tumor with focal cystic areas. The tumor had a gelatinous appearance.

The specimen was fixed in 10% buffered formalin. The tumor was meticulously examined and generously sampled (30 sections). Selected slides were stained with antidesmin (monoclonal, D33; Signet Laboratories Inc, Dedham, Mass; 1 in 400), anti-muscle specific actin (monoclonal, HHF35; BioGenex Laboratories, San Ramon, Calif; prediluted), and anti-S100 protein (polyclonal, Signet; 1 in 1200), using the streptavidin-biotin detection system (Dako Corporation, Carpinteria, Calif) with diaminobenzidine as a chromogen (Liquid DAB, Dako). Antigen retrieval was achieved by microwave pretreatment in a 0.01mol/L sodium citrate solution (pH 6.0) at 85 deg C for 10 minutes. Phosphotungstic acid hematoxylin and Masson trichrome stains were also performed.

Histologic examination showed a tumor partly surrounded by a thin fibrous capsule. The tumor consisted predominantly of myxoid liposarcoma characterized by myxoid matrix and a delicate capillary network (Fig 1). A lipoblastic component was also present (Fig 2). There were also areas of well-differentiated (lipoma-like) liposarcoma, but a round cell component was not seen. A prominent feature, in sections taken from the center of the tumor, was the presence of numerous rhabdomyoblasts admixed with the myxoid component. These cells were mostly polygonal in shape and had eosinophilic cytoplasm. Also present were strap cells with cross striations. The cross striations were easily identified on phosphotungstic acid hematoxylin-and Masson trichrome-stained sections (Fig 3). The rhabdomyoblasts showed intense cytoplasmic staining for desmin and muscle specific actin. Cytogenetic analysis of the tumor was not performed.

COMMENT

Myxoid liposarcoma is the most common form of liposarcoma, accounting for approximately half of all liposarcomas.1 Myxoid liposarcomas are characterized histologically by myxoid matrix, a plexiform capillary pattern, and lipoblasts in different stages of differentiation. Furthermore, myxoid liposarcoma can be regarded as a distinct entity based on its cytogenetic and molecular profile, despite a wide variation in morphologic appearance. The t(12;16)(q13;p11) translocation involving the CHOP and FUS genes has been well documented in myxoid liposarcomas and recently in round cell liposarcomas.6,7 This common cytogenetic alteration supports the view that myxoid and round cell liposarcomas represent a histologic spectrum of a single entity.1,7-9 The presence of hypercellular areas composed of round cells or spindle cells in a myxoid liposarcoma is associated with more aggressive behavior than well-differentiated myxoid liposarcoma.7

Dedifferentiated liposarcomas account for less than 5% of all liposarcomas.1 This type of liposarcoma is characterized by the presence of a well-differentiated component together with a poorly differentiated component, which may be composed of areas histologically consistent with fibrosarcoma, malignant fibrous histiocytoma, or unclassified spindle cell sarcoma.10

Various heterologous elements have been identified in the dedifferentiated components of dedifferentiated liposarcomas. These include smooth muscle,2,5 striated muscle,2,4,5 vessels, and bone.2,5 Osseous and cartilaginous metaplasia has also been noted in myxoid and well-differentiated liposarcomas.1,10 Furthermore, heterologous elements, such as bone, cartilage, and smooth muscle, have been described in atypical lipomatous tumors that are not dedifferentiated10

A diagnosis often considered in the presence of two or more heterologous components is a malignant mesenchymoma. Malignant mesenchymoma was originally defined as a sarcoma exhibiting two or more types of specialized differentiation in addition to an undifferentiated component.11 Others believe that this term should be avoided because many of the tumors in this category can be classified as triton tumors, dedifferentiated liposarcomas, and rhabdomyosarcomas or myxoid liposarcomas with cartilage formation.3

To the best of our knowledge, the presence of rhabdomyoblasts in a myxoid liposarcoma has been described only once previously.4 However, in the previously reported case a pleomorphic liposarcomatous component was also present. The rhabdomyosarcomatous component consisted of pleomorphic cells, which required immunohistochemistry to determine the striated muscle lineage of the cells. In our case, well-differentiated rhabdomyoblasts were easily detected on routine stains. We considered that the rhabdomyoblasts might represent nonneoplastic tissue trapped within the tumor. However, we feel that this is unlikely since rhabdomyoblasts were present mainly in the center of the lesion and not at the periphery (infiltrating edge), as one would expect with entrapped striated muscle. Furthermore, many desmin-positive strap cells were identified, and there was an intimate admixture of lipoblasts and rhabdomyoblasts. This may be due to a metaplastic process similar to that seen in well-differentiated and myxoid liposarcomas with cartilaginous and osseous foci. Alternatively, this may be an early sign of dedifferentiation3; however, there is no conclusive evidence that this occurs.

In our case, the possibility of a dedifferentiated liposarcoma was excluded by meticulous examination of the gross specimen and generous sampling of the tumor. Since liposarcomas are said to arise from primitive mesenchymal cells,1 we believe that the presence of rhabdomyoblasts indicates divergent differentiation rather than metaplasia.

The presence of rhabdomyoblasts in small biopsies of soft tissue tumors should be carefully interpreted. Its presence should neither be disregarded as entrapped skeletal muscle nor hastily interpreted as a rhabdomyosarcoma. The clinical relevance of the presence of rhabdomyoblasts in a liposarcoma is unclear. In the previously reported case of myxoid/pleomorphic liposarcoma with a rhabdomyosarcomatous component, the patient developed metastases and required radiotherapy.4 The biological behavior in this case was most probably determined by the pleomorphic component present in the tumor.

Although the clinical implications of the presence of heterologous components within non-"dedifferentiated" liposarcomas are unclear, there is some evidence to suggest that dedifferentiated liposarcomas, among the subgroup of pleomorphic sarcomas, may behave clinically in a less aggressive fashion.lz Similar findings were not observed in another study of dedifferentiated liposarcomas.5 As more liposarcomas with divergent differentiation are identified, the prognostic significance of this presentation will become clearer.

References

1. Enzinger FM, Weiss SW. Soft Tissue Tumors. 3rd ed. St Louis, Mo: Mosby; 1995:431466.

2. Tallini G, Erlandson RA, Brennan MF, Woodruff JM. Divergent myosarcomatous differentiation in retroperitoneal liposarcoma. Am J Surg Pathol. 1993;17: 546-556.

3. Evans HL, Khurana KK, Kemp BL, Ayala AG. Heterologous elements in the

dedifferentiated component of dedifferentiated liposarcoma. Am J Surg Pathol. 1994;18:1150-1157.

4. Shanks JH, Banerjee SS, Eyden BP. Focal rhabdomyosarcomatous differentiation in primary liposarcoma. J Clin Pathol. 1996;49:770-772.

5. Henricks WH, Chu YC, Goldblum JR, Weiss SW. Dedifferentiated liposarcoma: a clinicopathological analysis of 155 cases with a proposal for an expanded definition of dedifferentiation. Am J Surg Pathol.1997;21:271-281.

6. Knight JC, Renwick P), Dal Cin P, Van den Berghe H, Fletcher CDM. Translocation t(12;16)(q13;pl) in myxoid liposarcoma and round cell liposarcoma: molecular and cytogenetic analysis. Cancer Res. 1995;55:24-27.

7. Tallini G, Akerman M, Dal Cin P, et al. Combined morphologic and karyotypic study of 28 myxoid liposarcomas: implications for a revised morphologic typing (a report from the CHAMP group). Am J Surg Pathol.1996;20:1047-loss.

8. Azumi N, Curtis J, Kempson RL, Hendrickson MR. Atypical and malignant neoplasms showing lipomatous differentiation: a study of 111 cases. Am I Surg Pathol. 1987;11:161-183.

9. Kilpatrick SE, Doyon J, Choong PF, Sim FH, Nascimento AG. The clinicopathologic spectrum of myxoid and round cell liposarcoma: a study of 95 cases. Cancer. 1996;77:1450-1458.

10. Evans HL. Liposarcoma: a study of 55 cases with a reassessment of its classification. Am J Surg Pathol. 1979;3:507-523.

11. Stout AP. Mesenchymoma, the mixed tumor of mesenchymal derivatives. Ann Surg.1948;127:278-290.

12. McCormick D, Mentzel T, Beham A, Fletcher CDM. Dedifferentiated liposarcoma: clinicopathologic analysis of 32 cases suggesting a better prognostic subgroup among pleomorphic sarcomas. Am J Surg Pathol. 1994;18:1213-1223.

Accepted for publication February 12, 1998.

From the Department of Anatomical Pathology, University of Natal Medical School, Durban, South Africa.

Reprints: Dhirendra Govender, MRCPath, MMed, Department of Anatomical Pathology, University of Natal Medical School, Private Bag 7, Congella, 4013, Durban, South Africa.

Copyright College of American Pathologists Aug 1998
Provided by ProQuest Information and Learning Company. All rights Reserved