Giant Pleomorphic Lipoma With Malignant Transformation in the Buttock: a Rare Case Report and Literature Review


 Background: Pleomorphic lipoma is a benign tumor, which is often located in the neck, shoulder, and other subcutaneous tissues. Pleomorphic lipoma with malignant transformation in the buttock is rarely reported.Case Presentation: A 53-year-old male attended to our department with a large mass in his buttock that had been present for about one year, leading to difficulty with walking and defecation. Computed tomography (CT) showed a 10. 2 × 8. 3 cm mass with lumpy and mixed low to medium density shadows in the right side of the lower part of the tailbone. Color Doppler ultrasound showed that the skin and subcutaneous tissues of the right buttock were thickened with lymphedema. Pre-operative pathology revealed a large amount of adipose and fusiform fibrous tissues with obvious blood vessels. The patient underwent complete resection of the mass in the right buttock. Postoperative pathology indicated that the tumor was pleomorphic lipoma with malignant transformation. The patient recovered well and was discharged from the hospital at day 12 post-surgery. Follow up at 6 months did not find any symptoms of tumor recurrence.Conclusions: Pleomorphic lipoma with malignant transformation is rare. CT, magnetic resonance imaging (MRI) and histopathology are the main methods used to diagnose pleomorphic lipoma. Complete resection of the tumor is vital to treat giant pleomorphic lipoma successfully.

A 53-year-old male attended to our department with a large mass in the buttock that had been present for about one year, leading to di culties with walking and defecation. In the past two months, the mass had become obviously swollen and the defecation di culty was aggravated gradually; however, no mucopurulent bloody stool was found.
The patient received heart stent surgery one year ago. He denied any other history of hypertension, diabetes, blood transfusions, or drug and food allergies. A 15 × 10 cm mass was observed on the right side of the hips. The mass was tough, with poor mobility and no tenderness. Compression by the mass had moved the anus to the left and upper direction.
Biochemical examination indicated normal levels of carcinoembryonic antigen (CEA), carbohydrate antigen 19 − 9 (CA 19 − 9), alpha fetoprotein (AFP), total leukocytes, and hemoglobin. Abdominal computed tomography (CT) scan showed a 10. 2 × 8. 3 cm mass with lumpy and mixed low to medium density shadows in the right side of the lower part of the tailbone (Fig. 1A). 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. Multiple enlarged lymph nodes were observed in the bilateral inguinal region. Color Doppler ultrasound showed that the skin and subcutaneous tissues of the right buttock were thickened with lymphedema. An irregular mass of 15.3 × 7.4 × 11.1 cm was observed (Fig. 1B). Under the guidance of ultrasound, the mass was punctured and biopsied for pathology. The preoperative pathology found a large amount of adipose and fusiform brous tissues with obvious blood vessels. The mass was considered to be a mesenchymal tumor.
The patient underwent complete resection of the mass in the right buttock. The skin was incised longitudinally ( Fig. 2A). During surgery, we observed that the capsule of the tumor was intact. The upper margin of the tumor was closely related to the levator ani muscle. The tumor was separated completely along the skin ( Fig. 2B and C). A drainage tube was retained in the residual cavity, which was then sutured and xed. The resected tumor was sent for intraoperative rapid pathological examination and was diagnosed as lipoma. The operation was performed smoothly and the patient received antiin ammatory treatment.
Mild edema of the incision occurred after surgery (Fig. 2D). Active dressing changes were performed to eliminate the edema. The patient recovered well and was discharged on day 12 after the surgery. During a follow-up time of 6 months, we did not observe any signs of tumor recurrence.

Discussion And Conclusions
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 [9]. Pleomorphic lipoma accounts for 1.5% of all lipomas [10]. 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 speci c symptoms, such as walking inconvenience and defecation di culty.
Pleomorphic lipoma is very di cult to diagnose, and is not easy to distinguish from highly specialized liposarcoma [10]. The diagnosis of pleomorphic lipoma is mainly based on CT, magnetic resonance imaging (MRI), and histopathology. The imaging ndings 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 [15]. Pleomorphic lipoma usually has clear boundaries and sometimes has local calci cations [12]. Occasionally, it reveals a mildly lobulated, irregular soft tissue mass [16]. 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 [13]. Heterogeneous enhancement within the tumor in CT ndings might be a sign of malignant transformation of pleomorphic lipoma [17]. In our case, although there was no clear sign of in ltration on MRI, the surrounding area of the lesion seemed to be blurred, which is a suspicious radiological nding [6]. 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 [18]. 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 brous tissues within the tumor and the CT ndings 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 con rmed 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 signi cant difference in imaging ndings 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 [19]. 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 bers [6].
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 de nitive 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 [10]. It is important to be familiar with the association between the lipoma and nearby organs before surgery [4]. 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.