Surgical treatment including marginal resection of the two small masses followed by wide local resection of the huge tumor was performed on June 16, 1998. Because the first lesion was intracompartmental, the distal half of the involved trapezius muscle was resected en bloc with the underlying fascias and superficial halves of the muscle tissue of infraspinatus, supraspinatus, and rhomboideus. The skin including the needle biopsy tract also was resected en bloc. Macroscopic examination revealed that all three lesions were composed of mature lipomatous tissue. Careful observation of the cut surface of the first mass failed to detect any dedifferentiated foci. Histologic examination revealed that the huge tumor was composed of mature and benign looking lipocytes; however, some tumor cells contained slightly irregular cytoplasm and nuclei. Thick fibrous septa between lipomatous lobules, variation in the size and shape of the fat cells, and scattering of lipoblasts also were seen in several portions (Fig 4A). Mitotic activity was not evident, and there were no dedifferentiated foci. Based on these histologic findings, the diagnosis of well differentiated lipomalike liposarcoma was established. However, the second and third lesions were identified as intramuscular lipomas containing mature and benign lipocytes (Fig 4B). Careful observation failed to reveal lipoblasts or malignant cells in these lesions.
The postoperative course was uneventful. The patient was doing well without local recurrence of any masses 10 months after the surgery; however, a fourth small lesion identical to intramuscular lipoma was detected by MRI in the abdominal muscle at the last followup.
The diagnosis of the first large mass in the patient reported here was well differentiated lipomalike liposarcoma. However, it should be differentiated from other unusual fatty tumors, including spindle cell lipoma, pleomorphic lipoma, and atypical lipoma, in the current clinical setting. According to Enzinger and Weiss,5 spindle cell lipoma is a histologically distinct type of lipoma that is characterized by replacement of mature fat by collagen forming spindle cells. It occurs mainly in male patients between 45 and 65 years of age and is found chiefly in the regions of the posterior neck and shoulder. It manifests as a solitary, circumscribed or encapsulated, painless, firm nodule in the deep subcutis. Multiple lesions are rare but have been reported. The lesions average in size between 3 and 5 cm. Lipocytes are rare or absent, and the spindle cells dominate the histologic picture. Pleomorphic lipoma closely simulates a pleomorphic or sclerosing liposarcoma but most likely represents a benign, exceedingly pleomorphic variant of spindle cell lipoma. Pleomorphic lipoma also occurs as a circumscribed subcutaneous mass in the shoulder and neck region of men older than 45 years.5 All of the tumors in the current patient were located not in the subcutis but in the muscle tissue. Histologically, the lesions lacked the spindle cell pattern that is seen in spindle cell lipoma. Characteristic floretlike giant cells also were absent in the current lesions, although these cells are not specific of pleomorphic lipoma.5
Evans et al6 proposed the term atypical lipomas for all of the tumors formerly classified as well differentiated liposarcoma. Thus, atypical lipoma and well differentiated liposarcoma may be histologically indistinguishable. These authors additionally subclassified atypical lipomas into two categories: subcutaneous (atypical lipoma) and deep (atypical intramuscular lipoma). Atypical lipomas predominantly involved the neck and upper trunk, whereas the most common site of atypical intramuscular lipomas was the thigh.6 According to the subclassification, the huge mass seen in the patient in the current report may be not an atypical (subcutaneous) lipoma but an atypical intramuscular lipoma because of its location. However, Enzinger and Weiss5 suggested that the term "well differentiated liposarcoma" should be retained for all well differentiated liposarcomas that arise in deep soft tissues. That is because the biologic behavior of atypical (subcutaneous) lipomas differs from that of atypical intramuscular lipomas. The current authors agree with this opinion, and the final diagnosis of well differentiated liposarcoma was established.
Histologically, three subtypes of well differentiated liposarcoma can be distinguished: well differentiated lipomalike liposarcoma; well differentiated inflammatory liposarcoma; and well differentiated sclerosing liposarcoma.5 Well differentiated lipomalike liposarcoma is the most common type of well differentiated liposarcoma. It closely simulates lipoma, except for a scattering of lipoblasts with one or more lipid droplets in the cytoplasms, irregularly shaped cells with hyperchromatic nuclei, and lipocytes that have a slightly greater variation in size and shape than those of normal fat.5 These findings were observed in the large mass in the patient reported here. Complete excision with small margins may be adequate for the subcutaneous tumors (atypical lipomas), whereas a wider surgical margin is required for the treatment of well differentiated liposarcoma (atypical intramuscular lipomas) occurring in deep soft tissues. The latter may recur but never develop metastases.5 The rate of metastasis in liposarcoma is related closely to the degree of histologic differentiation.5 Well differentiated liposarcoma has the lowest metastatic activity, and a good prognosis is expected after appropriate wide local resection.
Lipoma is a common soft tissue tumor, whereas the intramuscular variant is relatively rare.1,3-5,8,9,11,12 Approximately 5% to 8% of patients with ordinary (subcutaneous) lipomas have multiple lesions.5 Multiple subcutaneous lipomas vary from a few to several hundred lesions and occur predominantly in the upper half of the body. Hyperlipidemia with high serum levels of cholesterol was reported in some patients5; however, the fat metabolism was normal in the patient of the current study. In addition, no previous study has reported multiple intramuscular lipomas to the authors' best knowledge. Although symmetric involvement of superficial lipoma is not an infrequent occurrence, the localization of one of the intramuscular lipomas and the liposarcoma was symmetric in the current case.
The involvement of well differentiated intramuscular liposarcoma in association with multiple intramuscular lipomas is unusual. There is no report in which intramuscular liposarcoma was associated with multiple intramuscular lipomas. A literature review also showed two cases of liposarcoma of an extremity associated with multiple subcutaneous lipomas.14 In addition, in rare cases, retroperitoneal liposarcoma has been associated with one or more lipomas at other sites.2,7 These authors considered that the occurrence of multiple lipomas in association with liposarcoma was attributable to the reaction of the adipose tissue as a whole to unknown tumor producing stimuli14 or common derangement of adipose tissue.2
The authors of the current study speculate there are three theoretical explanations for the pathogenesis of the current case. One is that the liposarcoma was the result of malignant transformation from one of the preexisting multiple intramuscular lipomas. The second is that the coexistence of benign and malignant lipomatous tumors was coincidental. The final explanation is that the two small masses were not intramuscular lipomas but metastatic lesions without marked malignant lipoblasts from the huge and longstanding liposarcoma. The first theory is the most likely explanation for this patient because the symmetric localization of the first (liposarcoma) and second (intramuscular lipoma) masses suggest that the coexistence of these lesions was not coincidental. In addition, well differentiated liposarcoma is a locally aggressive low grade malignant tumor with little metastatic activity.
Malignant transformation in a preexisting lipoma is rare, and only a few cases have been reported previously.13,15 Some of these were considered to be benign lipomas associated with focal myxoid, spindle cell, and pleomorphic areas. Others were well differentiated liposarcomas containing a lipomalike portion and a poorly dedifferentiated area (dedifferentiated liposarcoma).5 There were three lipomatous lesions around the shoulder joints in the current patient. One was well differentiated liposarcoma, and the others were intramuscular lipomas. Trauma is reported to be one of the possible etiologic factors in ordinary lipomas and in liposarcomas.5 The patient reported here was a construction laborer who received repetitive minor trauma at the shoulder and scapular regions. There may be a relationship between the patient's occupation and involvement of multiple lipomatous tumors around the shoulder. The authors speculate that the multiple intramuscular lipomas initially developed because of repetitive minor trauma or another unknown cause. The symmetric localization of the intramuscular lipoma and the liposarcoma suggests that one of the benign lesions may have transformed secondarily into a malignant tumor, but definitive evidence is lacking.
The authors thank Masahiro Nabae, MD, and Seiji Hiraoka, MD, for their contribution to this work.
1. Austin RM, Mack GR, Townsend CM, Lack EE: Infiltrating (intramuscular) lipomas and angiolipomas. A clinicopathologic study of six cases. Arch Surg 115:281-284, 1980.
2. Barkhof F, Melkert P, Meyer S, Blomjous CEM: Derangement of adipose tissue: A case report of multicentric retroperitoneal liposarcoma, retroperitoneal lipomatosis and multiple subcutaneous lipomas. Eur J Surg Oncol 17:547-550, 1991.
3. Bjerregaard P, Hagen K, Daugaard S, Kofoed H: Intramuscular lipoma of the lower limb. Long-term follow-up after local resection. J Bone Joint Surg 71B:812-815, 1989.
4. Dionne GP, Seemayer TA: Infiltrating lipomas and angiolipomas revisited. Cancer 33:732-738, 1974.
5. Enzinger FM, Weiss SW: Soft Tissue Tumors. Ed 3. St Louis, Mosby 381-466, 1995.
6. Evans HL, Soule EH, Winkelmann RK: Atypical lipoma, atypical intramuscular lipoma, and well differentiated retroperitoneal liposarcoma. A reappraisal of 30 cases formerly classified as well differentiated liposarcoma. Cancer 43:574-584, 1979.
7. Farbman AA: Retroperitoneal fatty tumors. Report of a case and collective review of the literature from 1937 to 1947. Arch Surg 60:343-362, 1950.
8. Fletcher CD, Martin-Bates E: Intramuscular and intermuscular lipoma: Neglected diagnoses. Histopathology 12:275-287, 1988.
9. Greenberg SD, Isensee C, Gonzalez-Angulo A, Wallace SA: Infiltrating lipomas of the thigh. Am J Clin Pathol 39:66-72, 1963.
10. Jelinek JS, Kransdorf MJ, Shmookler BM, Aboulafia AJ, Malawer MM: Liposarcoma of the extremities: MR and CT findings in the histologic subtypes. Radiology 186:455-459, 1993.
11. Kindblom LG, Angervall L, Stener B, Wickbom I: Intermuscular and intramuscular lipomas and hibernomas. A clinical, roentgenologic histologic, and prognostic study of 46 cases. Cancer 33:754-762, 1974.
12. Matsumoto K, Hukuda S, Ishizawa M, Chano T, Okabe H: MRI findings in intramuscular lipomas. Skeletal Radiol 28:145-152, 1999.
13. Sampson CC, Saunders EH, Green WE, Laurey JR: Liposarcoma developing in a lipoma. Arch Pathol 69:506-510, 1960.
14. Starkloff GB, Saxton JA Jr, Johnson RE: Liposarcoma of an extremity associated with multiple subcutaneous lipomas. Report of two cases treated by amputation. Ann Surg 133:261-266, 1951.
© 2000 Lippincott Williams & Wilkins, Inc.
15. Sternberg SS: Liposarcoma arising within a subcutaneous lipoma. Cancer 5:975-978, 1952.