Most cases of pleomorphic lipoma are benign [1, 4, 6], and malignant transformation of pleomorphic lipoma is very rare. Pleomorphic lipoma usually occurs in the hands, feet, thighs, shoulders, arms, knees, and buttocks [7, 8]. Lipomas are termed giant when they are larger than 5 cm, and 75% of cases are symptomatic . Pleomorphic lipoma accounts for 1.5% of all lipomas . In our case, the diameter of the pleomorphic lipoma was more than 10 cm. A typical clinical presentation of pleomorphic lipoma is important for diagnosis. Pleomorphic lipoma usually presents as a painless soft tissue lesion. Large pleomorphic lipoma is characterized by swelling and expansion, which might cause pain when it compresses a nerve. In contrast, small pleomorphic lipomas are usually asymptomatic [11, 12]. In this case, pleomorphic lipoma occurred in the buttock and had relatively specific symptoms, such as walking inconvenience and defecation difficulty.
Pleomorphic lipoma is very difficult to diagnose, and is not easy to distinguish from highly specialized liposarcoma . The diagnosis of pleomorphic lipoma is mainly based on CT, magnetic resonance imaging (MRI), and histopathology. The imaging findings of pleomorphic lipoma are variable because of the proportion of fat to non-fat components [13, 14]. On CT, pleomorphic lipoma is a relatively clear and inhomogeneous low-density mass . Pleomorphic lipoma usually has clear boundaries and sometimes has local calcifications . Occasionally, it reveals a mildly lobulated, irregular soft tissue mass . On MRI, pleomorphic lipoma usually presents with a heterogeneous hypointense signal on T1 and a heterogeneous hyperintense signal on T2 [13, 15]. Contrast-enhanced MRI indicated heterogeneous enhancement that corresponded to a hypointense signal on T1-weighted images . Heterogeneous enhancement within the tumor in CT findings might be a sign of malignant transformation of pleomorphic lipoma . In our case, although there was no clear sign of infiltration on MRI, the surrounding area of the lesion seemed to be blurred, which is a suspicious radiological finding . At the same time, some imaging features have been proven to increase the risk of malignant transformation of pleomorphic lipoma, such as a thick septum, a size larger than 10 cm, a nodule or mass like area, and less than 75% macroscopic fat composition . In this case, we reported a large mass with lumpy and mixed low to medium density shadows on CT. The wall of the mass was thick, with soft tissue density. The boundary between the left side of the mass and the anus was not clear. The pre-operative biopsy histopathology only found a large amount of adipose and fusiform fibrous tissues within the tumor and the CT findings revealed low to medium density shadows; therefore, we did not suspect malignant transformation. However, the post-surgical histological and immunohistological examinations indicated malignant transformation of lipoma. As a result, according to this case, mixed low to medium density shadows and an unclear boundary on CT may be an indicator for the diagnosis of malignant transformation of pleomorphic lipoma. However, this needs to be further confirmed by subsequent studies.
Preoperative pathology may be effective; however, if the tissue is not large enough, it may be misdiagnosed. In this case, the preoperative pathology showed that the mass was mesenchymal tumor. However, the postoperative pathology indicated that the tumor was a pleomorphic lipoma with malignant transformation. The preoperative diagnosis was not clear. Frequently, there is no significant difference in imaging findings between malignant transformation and benign lipoma. Preoperative pathology did not indicate malignancy, possible because the tumor volume was too large and part of the tissue necrosis reduced the accuracy of pathological diagnosis. Preoperative pathology through biopsy is sometimes imprecise because atypical cells may be concentrated and easily missed . Histological and immunohistochemical examinations are the “gold standard” for the diagnosis of pleomorphic lipoma. Pleomorphic lipoma is usually characterized by multinucleated giant cells and rough collagen fibers . Unique giant cells with overlapping nuclei and edge arrangement could also be found in pleomorphic liposarcoma [20, 21]. In immunohistochemistry, MDM2 and CDK4 are positive, which are helpful to distinguish benign lipoma from other lipomas [13, 20, 22]. CD34, B-cell lymphoma-2 (BCL-2), and CD99 are always strongly positive, while S-100 protein and signal transducer and activator of transcription 6 (STAT6) are usually negative [11, 14, 23, 24]. However, in a few cases, S-100 and desmin are positive in pleomorphic lipoma . In this case, the immunohistochemistry showed positive staining for CD34, CD99, CDK4, MDM2, P16, and S-100.
The malignant transformation of pleomorphic lipoma is very rare in the clinic. In our opinion, if the margin of the pleomorphic lipoma is malignant, a further wider resection is needed. In addition, pleomorphic lipoma with malignant transformation might require further postsurgical treatment, such as local radiology. However, the patient in this case did not receive any treatment after surgery. The definitive prognosis of pleomorphic lipoma with malignant transformation has not been reported. As a result, close follow-up is necessary. A second surgery or other treatment methods might be needed if any tumor recurrence is found.
Complete resection of the tumor is vital to treat giant pleomorphic lipoma. Postoperative recurrence of pleomorphic lipoma is rare . It is important to be familiar with the association between the lipoma and nearby organs before surgery . In this case, the giant pleomorphic lipoma was located near the anus. The operation required the maximum preservation of anal function and complete resection of the lesion. The incision was large, and the postoperative recovery period was relatively long. The patient received active dressing changes and was placed on a low slag diet to prevent infection of the incision.