According to the World Health Organization (WHO) classification standards, the definition of DDLPS is a bone and soft tissue tumor or ALT / WDLPS that has dedifferentiated into a different degree of sarcoma at the same time or before/after the development of ALT / WDLPS[2, 8]. Dedifferentiated areas usually consist of undifferentiated pleomorphic sarcoma or spindle cell sarcoma, with high to moderate cellularity and pleomorphism. In 10% of DDLPS cases, low-grade sarcoma components resembling fibromatosis or low-grade fibromyxoid sarcoma are apparent[4, 7]. In contrast to ALT / WDLPS, which has a relatively clear histological subtype, DDL represent a morphologically heterogeneous group. In the case described above, DDLPS was characterized by the coexistence of high- and low-grade dedifferentiated components. This manifestation is rare in clinical practice. We summarize the associated pathological features and MRI findings to deepen the awareness of this rare type of DDLPS.
DDLPS presents most commonly in middle-aged and older adults and affects both genders equally. The condition is extremely rare in children and adolescents. The retroperitoneum is the site most frequently affected, followed by the limb and spermatic cord / paratesticular area. Rarely affected sites include the chest cavity, mediastinum, and head and neck (such as the larynx or esophagus). Due to the large space for tumor growth in the posterior peritoneal area, ALT / WDLPS in this area can grow for a long time without causing symptoms. There is therefore a high risk (about 28%) that dedifferentiation will be observed at the time of diagnosis[4, 7].
The histology of DDLPS usually includes ALT / WDLPS components that have transformed into non-fatty tumor components, and the two components are usually clearly demarcated under the microscope. The most common histological type of ALT / WDLPS in DDLPS is lipomatous and sclerotic. As seen in our case report, the retroperitoneal mass was huge and contained many WDLPS components, the dedifferentiated components were characterized by the coexistence of low-grade and high-grade dedifferentiated components, the low-grade components accounted for only 10% of the dedifferentiated tumor, and were located in the periphery of the high-grade components. The boundary between the two components was identified in sections of the specimen. Pathologically, there was a sudden transition between high-grade and low-grade dedifferentiated components of the DDPLS, the transition between the two dedifferentiated portions like an abrupt line. The conversion to low-grade DDLPS starts from the periphery in high-grade DDLPS. We speculated that low-grade dedifferentiation may be a precursor to high-grade dedifferentiation, this manifestation has also been reported in other case report[9]. In the central region of a tumor with high-grade differentiation, large areas of ischemic necrosis appeared, due to poor tissue differentiation and rapid growth. In this case, the low-grade dedifferentiated tissues exhibited fibromatosis-like and inflammatory myofibroblastoid-like features.
In addition, in the differential diagnosis of fatty tumors other than ALT / WDLPS, immunohistochemical staining that is positive for p16, MDM2 and CDK4 has high sensitivity and specificity for the diagnosis of DDLPS. In this case, well-differentiated liposarcoma components and dedifferentiated liposarcoma components (including high-grade dedifferentiation and low-grade dedifferentiation) all diffusely express P16, MDM2, and CDK4. However, in the differential diagnosis of DDLPS and non-fat-derived tumors, the specificity of the above three markers is insufficient. At this time, the use of FISH to detect the amplification of the MDM2 gene is highly specific and sensitive for the diagnosis of DDLPS, especially when diagnosed with small biopsy specimens. The use of FISH is even more specific and sensitive in small biopsy specimens without typical WDLPS components or low-level dedifferentiation and in rare types of DDLPS[7, 10]. Amplification of the MDM2 gene is generally considered as the gold standard for the diagnosis of ALT / WDLPS and DDLPS.
The diagnosis of DDLPS requires the existence of two components in the tumor: lipogenic WDLPS and cellular nonlipogenic sarcoma. MRI can easily be used to identify fat-derived components in tumors through the use of fat-suppressed T2 images or short tau inversion recovery (STIR) imaging[11]. These approaches allow for the identification of WDLPS components in DDLPS, which is more helpful for diagnosis; however, in some cases, the WDLPS composition may go unnoticed. Because DDLPS is the conversion of WDLPS components to non-fat-derived tumor components, DDLPS lesions may lack signs of lipid characteristics on MRI. In addition, the ADC value of DDLPS lesions is low, indicating poor differentiation of tumor tissue. Dynamic contrast-enhancement of MRI can also be used to delineate the blood supply to the active area of the lesion, as well as the extent of necrosis or mucinous cystic changes in the lesion. Despite the existence of a clear transition between high- and low-grade differentiation components of DDLPS in the case described above, MRI failed to distinguish them.
In summary, for the diagnosis of liposarcoma, whether based on MRI or pathology, we should pay attention to typical fat components. MRI has some limitations when used for the preoperative diagnosis of liposarcoma in samples lacking fatty components or for the diagnosis of liposarcoma. The differential diagnosis for DDLPS is wide, and there are many diagnostic traps. Extensive sampling of the mass is recommended to avoid missing any component. Sampling should be performed in both non-fatty and fatty tissues. To avoid the misdiagnosis of DDLPS, it is sometimes necessary to perform immunohistochemistry (such as MDM2 gene amplification). The transition between high- and low-grade differentiation components of DDLPS characterized in this case report is an important aspect of rare pathological manifestations of DDLPS.