Discussion
Liposarcomas are the second most common soft-tissue sarcoma, there are four subtypes of liposarcoma confirmed by WHO [1]. MLS accounts for about 5% of all soft tissue sarcomas and 15-20% of all liposarcomas. It is usually located in deep soft tissue of the limbs and more than half of the cases are seen in the thigh muscles [2]. MLS is a translocation-related sarcoma and is characterized by translocation with FUS-DDIT3 or EWSR1-DDIT3 fusion, as a result of the t (12;22) or t (12;16) balanced translocation on the 12q13.3 locus [3]. The fusion protein produced by these fusion genes acts as deregulated and activated transcriptional factors, it stimulates the proliferation of tumor cells [2-4]. Estourgie et al. [5] reported that 55% of MLS patients with metastatic disease had extrapulmonary metastases. MLS usually metastasizes to the retroperitoneum, abdominal wall, abdominal cavity, and bone, but cardiac metastases are very rare [5-7]. To date, only about 30 or so cases of cardiac metastases in MLS patients have been reported, including pericardial [8]. It has been reported in the literatures that the tendency to metastatic spread in extrapulmonary sites was attributed to an affinity for adipose tissues. The number of pericardial adipocytes is small compared to that within the soft tissues of the extremities, retroperitoneum, etc, suggest that MLS transfer is not predicated on the enrichment of the tissue with adipose tissue cells. The time interval between primary lesion and cardiac metastasis is relatively long, ranging from 1 to 25 years [8]. This patient had a short delay between the first postoperative recurrence of the tumor and cardiac metastasis. Only 10% of patients with cardiac metastasis have been reported to show some symptoms [9]. Cardiac metastases from liposarcoma often present as congestive heart failure or a heart murmur, these signs and symptoms are associated with impaired myocardial contraction or pericardial tamponade due to tumor invasion of the myocardium [3]. The patient in this case presented clinically with chest tightness and wheezing and lower extremity edema, and also had a recurrence of MLS in the left groin and left popliteal fossa, which made it difficult to identify the possibility of cardiac metastasis in the patient. Early diagnosis of cardiac metastases is not adequate based on the patient’s physical examination, US and CT alone.
On histopathological examination, the MLS can be seen under light microscopy to consist of uniform, round or ovoid, small lipogenic cells with an interstitium rich in fine venous structures [2]. The cytoplasm and nucleus of some tumor cells are pushed to one side by the contained lipid droplets and appear as imprinted adipoblasts. Some tumors have round cell areas that represent histologic progression to high-grade tumors. Histologically low-grade MLS has a better prognosis, with a 5-year survival rate of 90%, and high-grade MLS with a round cell component >5% has a relatively low 5-year survival rate of 60% [10-11]. The fatty component of MLS is its special sign, and histologically it is predominantly mucinous with minimal fat content; therefore, this case shows a hypodense mass of indeterminate nature with well-defined borders on US and CT, which was not easily distinguishable from a common mucinous cyst. The high soft tissue contrast and resolution of MRI scans, supplemented by fat suppression techniques to detect a small fat component in the lesion, is key to the diagnosis. The case shows isointensity on T1WI, slightly mixed hyperintensity on T2WI, and significantly inhomogeneous high signal on fat-saturated T2WI, with speckled low signal seen locally. It is evidence of the presence of adipose tissue nodules within them, pathologically areas of high adipoblast aggregation, provides an important imaging basis for the diagnosis of MLS. Delayed enhancement scan shows a strip of flocculent soft tissue density shadow with heterogeneous mild enhancement within the lesion. In contrast, true cystic masses are only mildly reinforced in the peripheral cyst wall and are generally not reinforced internally, thus distinguishing them from pericardial cysts. Myocardial thickening on CT images in this case, but no infiltrative tumor growth was found on CMR, and the lesion was clearly demarcated from the myocardium. The clinical symptoms of chest tightness and wheezing in this patient were related to compression of the myocardium or surrounding great vessels caused by the expansive growth of the tumor, which corresponded to a decrease in left ventricular end-diastolic volume and end-systolic volume on CMR. The whole-body MRI and PET-CT are the most reliable modalities for surveillance of all likely sites of extrapulmonary metastases. In this case, the pericardial metastasis of MLS has low uptake on PET.
Surgery is the mainstay of treatment for limited MLS, wide local excision of the tumor with a minimum margin of 3 cm in consideration of their tendency to occur locally [12]. When a cardiac metastasis is found incidentally, it is usually incurable, this may be because the poor control of the primary lesion and systemic dissemination that make excision ineffective [7-8]. For patients with locally advanced or metastatic MLS, systemic therapy is usually used. MLS is radiosensitive, whether preoperative, intraoperative or postoperative radiotherapy [13].