Case Of Myxoid Liposarcoma Research Articles

Myxoid liposarcoma diagnosed on fine needle aspiration cytology: There is more to it than meets the eye.

Liposarcoma is a rare mesenchymal neoplasm commonly involving deep soft tissues and the retroperitoneum. Among the various types of liposarcoma, myxoid liposarcoma is the most frequently encountered in adolescents and young adults, with a predilection for lower extremities. Fine needle aspiration allows easy assessment and rapid on-site evaluation for distinguishing benign from malignant lipomatous lesions. Here, we present a case of myxoid liposarcoma in the calf region of a 19-year-old boy, diagnosed via fine needle aspiration cytology, and subsequently confirmed by histopathological examination after surgical resection. The intention behind this case report is to highlight the cytological features of myxoid liposarcoma and to improve understanding of this tumor entity, aiming to prevent misdiagnosis by inexperienced pathologists when evaluating cytology specimens.

Nuclear morphological atypia in biopsy accurately reflects the prognosis of myxoid liposarcoma.

Currently, it is difficult to predict the prognosis of myxoid liposarcoma (MLS) in biopsy specimens. In this study, we determined whether nuclear morphology may be used to predict the prognosis of MLS in primary biopsy specimens. Two pathologists evaluated nuclear morphology using the modified WHO/ISUP and Fuhrman grades. Survival analyses were performed by grouping nuclear high- and low-grades. We examined 53 MLS cases, which included 29 (54.7%) male and 24 (45.3%) female patients with a median age of 46years (interquartile range, 37 - 60). In total, 7 (13.2%) and 16 (30.2%) cases were assigned to the high nuclear grade group based on the modified WHO/ISUP and Fuhrman gradings, respectively. Survival analyses revealed a significantly worse disease-free survival in the high-grade group (hazard ratio (HR), 7.51; 95% confidence interval (CI), 2.67-21.1, p < 0.001 by the modified WHO/ISUP grading; HR, 4.45; 95% CI, 1.63-12.1, p = 0.001 by the modified Fuhrman grading). Moreover, the modified WHO/ISUP grade showed a significantly worse overall survival in the high-grade group (HR, 4.39; 95% CI, 1.04-18.6, p = 0.028), and the modified Fuhrman grade exhibited a similar, but not significant, trend. Our results indicate that nuclear morphology grading is a good predictor of patient prognosis at the time of biopsy in MLS. Even when cell density is sparse, treatment strategies should be carefully considered when individual tumor cells exhibit atypical nuclei.

Giant myxoid liposarcoma of the gluteal region: Resection and buttock reconstruction

Introduction: Soft tissue sarcoma in adult is a rare disease and it needs an interdisciplinary team management. Liposarcoma has mesenchymal origin and arises in the fatty tissue. Multiple histologic subtypes of liposarcoma are recognized, including myxoid liposarcoma. In two-third of the cases, this tumor occurs in the muscle while often demonstrating a misleading benign appearance. Myxoid liposarcoma occurs predominantly in the deep soft tissues of the extremities and has the ability of metastasis to a wide range of soft tissue. Surgery is the main therapeutic modality in treatment of soft tissue sarcoma. Case Report: Presenting an exceptional case of a 50-year-old woman with history of painless large swelling of size 28×21×13 cm in left gluteal region since one year with rapid progression over three months. It was diagnosed as low grade myxoid liposarcoma of the left buttock. This is the largest reported case of myxoid liposarcoma of extremities in Asia found in literature. The patient was treated with complete excision of tumor with gluteus maximum muscle. Gluteus minimus flap was elevated to cover the exposed sciatic nerve and buttock recontouring was done as single procedure for aesthetically favorable results followed by post-operative radiotherapy and necessary follow-ups. Conclusion: Buttock contouring done post-resection as single procedure is more acceptable by the patient and has got superior aesthetic results.

Prognostic value of nuclear morphometry in myxoid liposarcoma.

Myxoid liposarcoma (MLS) accounts for 20%-30% of liposarcoma and the round cell component (RCC) is believed to be a specific poor prognostic factor. However, the RCC assessment criteria are vaguely defined and, therefore, are inconsistently employed by pathologists. In this study, we modified and applied two established grading systems to evaluate nuclear atypia (namely, the World Health Organization/International Society of Urological Pathology and the Fuhrman grading in renal cell carcinoma) in 64 MLS cases. Detailed software-based assessments of the morphology and the cellularity were performed. DNA mutation analysis, comprehensive mRNA expression analysis, and immunohistochemistry were also performed. Our findings revealed that the high-nuclear-grade group according to the modified Fuhrman grading system exhibited a significantly poor disease-free survival (hazard ratio: 4.43; 95% confidence interval: 0.9-22.6; p=0.047). On the other hand, the cellularity was significantly higher in the modified Fuhrman high-grade group (p=0.010 at the percentage of the hypercellular area; p=0.003 at the maximum cell density) but did not qualify per se as a poor prognostic factor in the survival analyses. Furthermore, the modified Fuhrman high-grade group significantly expressed the cell cycle-related genes (such as FOXM1, PLK1, and CDK1). In conclusion, our analyses suggest that an evaluation focusing on nuclear morphology (rather than on cellular density) can be more reliable in predicting the MLS prognosis.

A cryptic EWSR1::DDIT3 fusion in myxoid liposarcoma: Potential pitfalls with FISH and cytogenetics.

Myxoid liposarcoma (MLS) is molecularly characterized by fusions involving the DDIT3 gene in chromosome band 12q13; the fusion partner is FUS in band 16p11 in 90-95% of the cases and EWSR1 in band 22q12 in the remaining 5-10%. Hence, molecular studies, often fluorescence in situ hybridization (FISH) for DDIT3 rearrangement, are useful for establishing a correct diagnosis. Although all MLS tumors should have DDIT3 fusions, it is important to be aware of reasons for potential false-negative results. We here present a case of MLS that was negative for FISH for DDIT3, that showed an unexpected t(11;22) at G-banding, but that displayed a characteristic EWSR1::DDIT3 fusion at RNA-sequencing. The results suggest that neoplasia-associated fusions that, due to the transcriptional orientations of the two genes involved, cannot arise through only two double-strand breaks are more likely to be associated with negative FISH-findings and unexpected karyotypes.

Myxoid liposarcoma of small bowel presenting with intussusception and obstruction: A case report

Gastrointestinal liposarcomas are extremely rare with the most common reported morphological subtype being dedifferentiated liposarcoma and well-differentiated liposarcoma. These tumors are rarely diagnosed preoperatively and diagnosis is only confirmed on histopathological analysis. Treatment of gastrointestinal liposarcomas consists of surgical excision with widely negative margins followed by post-operative irradiation and close follow-up. We report an exceedingly rare case of myxoid liposarcoma of the small bowel (ileum) presenting with an unusual presentation with intussusception and intestinal obstruction. A 42-year-old male presented to the emergency department with features of intestinal obstruction. Contrast-enhanced computed tomography abdomen revealed ileo-ileal intussusception with an endoluminal soft-tissue lesion at the leading edge. The patient was taken for surgical intervention and the involved segment of the bowel along with the lesion was resected and re-anastomosis done. Histological sections of the mass along with immunohistochemistry suggested the pathological diagnosis of myxoid liposarcoma.

Superficial Myxoid Liposarcoma of the Thigh

Myxoid liposarcoma is the most common form of myxoid sarcoma. Their primary and superficial localizations are very rare compared to the deeper forms. We report the case of myxoid liposarcoma in a 57-year-old patient. The clinical picture marked by painless swelling of the right thigh had evolved over a period of about ten years. The diagnosis was guided by ultrasound and magnetic resonance imaging. A pathological examination of the surgical specimen after removal of the tumor helped to clarify the diagnosis. We discuss the clinical presentation and therapeutic management.

A Rare Case of Myxoid Liposarcoma Combined With Papillary Carcinoma in the Same Thyroid Gland

A Rare Case of Myxoid Liposarcoma Combined With Papillary Carcinoma in the Same Thyroid Gland

Expression of CTAG1B clone EPR13780 versus DDIT3 gene rearrangement distinguishes myxoid liposarcoma from its mimics with detection of novel DDIT3 gene copy number variations

ABSTRACT Myxoid liposarcoma (MLPS) has different patterns that are often difficult to distinguish from other soft tissue lesions. MLPS is characterized by a reciprocal translocation involving the DNA Damage Inducible Transcript 3 gene (DDIT3) that can be detected using fluorescent in situ hybridization (FISH). Recently, the marker for cancer testis antigen 1b (CTAG1B) was found to be expressed in MLPS. The aim of the present study was to assess the potential use immunohistochemistry (IHC) for CTAG1B expression and DDIT3 rearrangement to diagnose MLPS and distinguish it from similar lesions. Out of 29 cases including MLPS and its mimics, CTAG1B was expressed in 92.86% of cases of MLPS and 20% of its mimics. DDIT3 rearrangement was 100% sensitive and 92.86% specific in distinguishing MLPS from its mimics. The DDIT3 rearrangement was found to be more sensitive but less specific than cytoplasmic expression of CTAG1B marker. DDIT3 polysomy and amplification were detected in some cases. Therefore, both CTAG1B expression and FISH for DDIT3 gene can be used to distinguish MLPS from similar tumors. The use of both immunohistochemistry for CTAG1B in addition to DDIT3 gene rearrangement detection by FISH was more specific than using either of them alone. However, the DDIT3 gene rearrangement alone was the most sensitive test for distinguishing MLPS from its mimics.

A Case of Myxoid Liposarcoma of Neck

Liposarcoma is a very rare malignant tumor affecting the head and neck area. Since it is an invasive and life-threatening disease, active treatments is necessary. However, differential it from lipoma, its corresponding benign counterpart, may be difficult. Herein, we report a rare case of posterior neck myxoid liposarcoma in a 72-year-old man who was initially misdiagnosed with lipoma, including a literature review of differential diagnosis between lipoma and liposarcoma.

Expression of Platelet Derived Growth Factor a, Its Receptor, and Integrin Subunit Alpha V in Feline Injection-Site Sarcomas

Abstract Injection-site sarcomas are considered an important entity in veterinary oncology, especially in cats. The current study investigated the immunohistochemical expression of platelet-derived growth factor A (PDGFA), platelet-derived growth factor receptor-α (PDGFR-α), and integrin alpha subunit v in feline injection-site sarcomas (FISS). A total of 14 paraffin-embedded tissue samples previously diagnosed as soft tissue sarcomas were selected from the departmental archive and sectioned at 5µm using a microtome. Tissue sections were stained with Hematoxylin &amp; Eosin, Alcian blue-Periodic Acid Schiff (AB-PAS), Masson’s Trichrome, and immunohistochemically for PDGFA, PDGR-α, and integrin alpha v. The majority of the tumors were fibrosarcomas (n=13/14), except one case of myxoid liposarcoma. The tumors occurred at sites commonly used for injections or vaccine administration. Histologically, tumors were characterized by peripheral infiltration of mononuclear cells mainly lymphocytes, variable sized central necrotic areas, and many multinucleated tumor giant cells. AB-PAS staining revealed the presence of myxoid or mucinous areas in (8/14) tumors while trichrome staining demonstrated a variable amount of collagenous stroma in 12/14 tumors. Vimentin immunoreactivity was observed in all the tumors while smooth muscle actin and muscle actin staining was noticed in four and two cases, respectively. PDGFA and PDGFR-α immunoexpression was observed in all 14 cases while integrin alpha v in 13/14 cases. The results of the current study indicate that fibrosarcoma is the major morphologic phenotype of FISS. The PDGFA, its receptor, and integrin alpha v immunoexpression are increased in these tumors in cats which elucidates their role in the pathogenesis of FISS.

Molecular profiling of soft-tissue sarcomas with FoundationOne® Heme identifies potential targets for sarcoma therapy: a single-centre experience.

Background:Molecular diagnosis has become an established tool in the characterisation of adult soft-tissue sarcomas (STS). FoundationOne® Heme analyses somatic gene alterations in sarcomas via DNA and RNA-hotspot sequencing of tumour-associated genes.Methods:We evaluated FoundationOne® Heme testing in 81 localised STS including 35 translocation-associated and 46 complex-karyotyped cases from a single institution.Results:Although FoundationOne® Heme achieved broad patient coverage and identified at least five genetic alterations in each sample, the sensitivity for fusion detection was rather low, at 42.4%. Nevertheless, potential targets for STS treatment were detected using the FoundationOne® Heme assay: complex-karyotyped sarcomas frequently displayed copy-number alterations of common tumour-suppressor genes, particularly deletions in TP53, NF1, ATRX, and CDKN2A. A subset of myxofibrosarcomas (MFS) was amplified for HGF (n = 3) and MET (n = 1). PIK3CA was mutated in 7/15 cases of myxoid liposarcoma (MLS; 46.7%). Epigenetic regulators (e.g. MLL2 and MLL3) were frequently mutated.Conclusions:In summary, FoundationOne® Heme detected a broad range of genetic alterations and potential therapeutic targets in STS (e.g. HGF/MET in a subset of MFS, or PIK3CA in MLS). The assay’s sensitivity for fusion detection was low in our sample and needs to be re-evaluated in a larger cohort.

DDIT3 Immunohistochemistry Is a Useful Tool for the Diagnosis of Myxoid Liposarcoma.

Myxoid liposarcoma is a malignant adipogenic neoplasm characterized by prominent arborizing capillaries, occasional lipoblasts, and primitive-appearing spindle cells in a myxoid background. A recurrent translocation in myxoid liposarcoma results in an oncoprotein consisting of full-length DDIT3 (CHOP) fused to an N-terminal segment of either FUS (TLS) or, less often, EWSR1. Here, we explore the diagnostic significance of DDIT3 expression in myxoid liposarcoma using a mouse monoclonal antibody recognizing an epitope in the N-terminal region. Studying a total of 300 tumors, we find diffuse, moderate-to-strong nuclear-localized anti-DDIT3 immunoreactivity in all 46 cases of myxoid liposarcoma representing 36 unique tumors, including 6 cases with high-grade (round cell) morphology. DDIT3 immunohistochemistry also highlighted a distinctive vasculocentric growth pattern in 7 myxoid liposarcomas treated with neoadjuvant radiation. In contrast, the vast majority of other examined lipomatous and myxoid neoplasms exhibited no DDIT3 expression; limited, weak immunoreactivity in <10% of cells was infrequently observed in dedifferentiated liposarcoma (6/39, 15%), solitary fibrous tumor (3/12, 25%), pleomorphic liposarcoma (1/15, 7%), and high-grade myxofibrosarcoma (2/17, 12%). Although this minimal DDIT3 expression did not correlate with DDIT3 amplification or myxoid liposarcoma-like morphology in dedifferentiated liposarcoma, there was evidence among sarcomas (excluding myxoid liposarcoma) of a relationship between expression and exposure to neoadjuvant radiation or cytotoxic chemotherapy. The constellation of findings indicates that DDIT3 immunohistochemistry may have utility in the evaluation of myxoid and lipomatous neoplasms to support the diagnosis of myxoid liposarcoma.

Sonographic Appearances of Liposarcoma: Correlations with Pathologic Subtypes

Liposarcoma is the second most common malignant soft-tissue tumor. This entity is pathologically categorized into 4 subtypes: well-differentiated, myxoid, dedifferentiated and pleomorphic. Although features on magnetic resonance imaging and computed tomography for these 4 subtypes have been reported quite precisely, those on sonography have not been fully investigated. The present study was therefore aimed at clarifying the sonographic appearances of each liposarcoma subtype and assessing correlations with histopathology. The study population was made up of 35 cases, including 21 cases of well-differentiated liposarcoma, 6 cases of myxoid liposarcoma, 6 cases of dedifferentiated liposarcoma and 2 cases of pleomorphic liposarcoma. Compared with the other subtypes, well-differentiated liposarcoma was characterized by the high frequency of the following findings: isoechogenicity, tiny hyperechoic lines and hypovascularity (p < 0.01, in each). Myxoid liposarcomas were characterized by low echogenicity, intermingled with anechoic areas and moderate vascularity (p < 0.01, in each). Dedifferentiated liposarcomas showed a specific biphasic pattern of hyperechoic and hypoechoic areas and hypervascularity (p < 0.01, in each). Pleomorphic liposarcomas showed a specific gyrus-like mixture of hyperechoic and hypoechoic areas (p < 0.01). In conclusion, the present study revealed different characteristics of sonographic appearance among the 4 histopathologic subtypes of liposarcoma.

The role of 18F-FDG PET/CT in soft tissue sarcoma.

Soft tissue sarcomas (STS) are highly fluorine-18-fluorodeoxyglucose (F-FDG)-avid tumours. PET seems to be effective for the assessment of the extent of disease. However, the use of PET to stratify STS into different risk histotypes still remains controversial. Our aim was to evaluate F-FDG uptake in different STS types and to assess the prognostic value of the maximum standardized uptake value (SUVmax). We reviewed 50 adult patients with primary high-grade STS of the extremities with a preoperative PET. Overall survival and local recurrence were analysed. The mean SUVmax was 12.9 (range: 2.2-33.4). All cases of myxoid liposarcoma and all cases of synovial sarcoma had SUVmax of less than 10.3. A better overall survival and local recurrence were observed in patients with SUVmax of less than 10.3 (P=0.005 and 0.046, respectively). SUVmax seems to be specific among different STS histotypes. PET does not seem to be useful in myxoid liposarcoma as well as synovial sarcoma as these tumours seem to have a low uptake of glucose. SUVmax might also be included as a prognostic factor.

Pathological significance of NY-ESO-1 expression in the diagnosis of myxoid liposarcoma

Objective: To detect the expression of New York esophageal squamous cell carcinoma antigen 1 (NY-ESO-1) in common types of mesenchymal myxoid tumors, and to investigate its significance in the diagnosis and differential diagnosis of myxoid liposarcoma. Methods: A total of 43 formalin-fixed paraffin-embedded samples of mesenchymal myxoid tumors from the Affiliated Hospital of Qingdao University and Qingdao Municipal Hospital ranging between 2010 and 2017 were selected. NY-ESO-1 expression was detected by immunohistochemical staining. DDIT3 gene status was detected by fluorescence in situ hybridization (FISH). NY-ESO-1 mRNA was detected by reverse transcription-PCR (RT-PCR). Results: Histopathology and FISH results confirmed that there were 11 cases of myxoid liposarcoma and 32 other types (including 7 cases of well-differentiated liposarcoma, 1 dedifferentiated liposarcoma, 3 lipomas, 2 lipoblastomas and 19 non-adipocytic tumors). Immunohistochemical staining showed that the positive expression propotion of NY-ESO-1 in myxoid liposarcoma was 11/11, and the positive location was the cytoplasm and nucleus of lipoblast cells. The expression intensity is higher in regions with round cell differentiation. Among the 32 cases of other mesenchymal myxoid tumors, only one well-differentiated liposarcoma showed positive immunoreactivity for NY-ESO-1. RT-PCR confirmed that 7 cases of myxoid liposarcoma (7/11) and one well-differentiated liposarcoma (1/7) had NY-ESO-1 mRNA expression. Conclusions: NY-ESO-1 is positively expressed in myxoid liposarcoma. It can be served as a useful marker for the diagnosis and differential diagnosis of myxoid liposarcoma.

Preoperative evaluation of the efficacy of radio-hyperthermo-chemotherapy for soft tissue sarcoma in a case series.

PurposeRadio-hyperthermo-chemo (RHC) therapy, which combines radiotherapy, hyperthermia, and chemotherapy, for malignant soft tissue tumors has been introduced with the aim of decreasing the possibility of local recurrence after surgery. To avoid unnecessary neoadjuvant therapy and to plan the appropriate surgical treatment, surveillance of RHC therapeutic efficacy during treatment is necessary. In this study, we determined the optimal response criteria to evaluate the efficacy of RHC by comparing preoperative images before and after RHC with pathological evaluation of necrosis in the resected tumor.Patients and methodsFrom 2004 to 2014, 20 patients were enrolled into this study. Needle biopsy revealed 6 cases of myxoid liposarcoma, 6 cases of undifferentiated pleomorphic sarcoma, 4 cases of myxofibrosarcoma, and 4 cases of synovial sarcoma. Based on the Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 or modified RECIST, we calculated the responses to RHC therapy by comparing pre- and post-RHC therapy images. In addition, resected specimens underwent pathological analysis to evaluate response based on tumor necrosis. The correlation between assessment based on preoperative images and resected tumors were evaluated by the Spearman’s rank-order correlation coefficient.ResultFrom the surgical specimens, pathological assessment of necrosis in resected tumor were assessed as less than 50% (2 cases), 50–90% (9 cases), 90–99% (6 cases), and total necrosis (3 cases). Use of the RECIST 1.1 underestimated good responders as stable disease (SD) or progressive disease (PD) in 5 out of 15 cases; on the other hand, use of the modified RECIST did not underestimate the pathological assessment of necrosis. The correlations between responses based on preoperative images and those based on histological assessments were 0.23 (RECIST 1.1) and 0.76 (modified RECIST).ConclusionBecause pathological responses can be underestimated using the RECIST 1.1, the modified RECIST, which take into consideration tumor viability, as assessed by contrast MRI, should also be considered when evaluating the efficacy of RHC.

Myxoid liposarcoma with cartilaginous differentiation showing DDIT3 rearrangement.

Myxoid liposarcoma (MLPS) is the second most common histologic subtype of liposarcoma. However, cartilaginous differentiation within MLPS is an extremely rare phenomenon, with only 7 cases of MLPS with cartilaginous differentiation reported to date. The majority of MLPS cases show the t(12;16)(q13;p11) translocation, resulting in the fused in sarcoma-DNA damage-inducible transcript 3 (FUS-DDIT3) fusion gene. This fusion gene as a hallmark of MLPS is very useful for differential diagnosis from other soft tissue sarcomas, and the associated protein, FUS-DDIT3, performs an important role in the phenotypic selection of targeted multipotent mesenchymal cells during oncogenesis. In this report, a case of MLPS with cartilaginous differentiation that occurred in the thigh of a 44-year-old woman is described. Histopathologically, the tumor was composed of a typical myxoid liposarcoma area and a mature hyaline cartilaginous area. Using fluorescence in situ hybridization analysis, rearrangement of the DDIT3 gene was detected in not only the liposarcomatous area but also in the chondrocytes of the cartilaginous area. Based on these findings, the cartilaginous differentiation area appears to be partially associated with oncogenesis through the specific fusion gene FUS-DDIT3.

Prognostic factors and outcome of Liposarcoma patients: a retrospective evaluation over 15\xa0years

BackgroundSoft tissue sarcomas are rare entities with over 50 histological subtypes. Liposarcoma (LS) is the most common neoplasm in this group; it is a complex neoplasm that is divided into different histological subtypes. Different therapy options, such as surgical resection, radiation, and chemotherapy, are available. Depending on the subtype, location, status of the resection margins and metastatic status, different therapy options are used. Therefore, the aim of this study was to determine the prognostic factors influencing the survival of patients affected by LS with consideration for the grading, histological subtype, state of the resection margin, size, location, metastases and local recurrence in a retrospective, single-centre analysis over 15 years.MethodsWe included 133 patients (male/female = 67/66) in this study. We recorded the histologic subtype, grade, TNM classification, localization, biopsy technique, tumour margins, number of operations, complications, radiation and dose, chemotherapy, survival, recrudescence, metastases and follow-up. Survivorship analysis was performed.ResultsWe detected 56 (43%; 95%-CI 34.6–51.6%) atypical LS cases, 21 (16.2%; 95%-CI 9.8–22.5) dedifferentiated LS cases, 40 (30.8%; 95%-CI 22.8–38.7) myxoid LS cases and 12 (9.2%; 95%-CI 4.3–14.2) pleomorphic LS cases. G1 was the most common grade, which was followed by G3. Negative margins (R0) were detected in 67 cases (53.6%; 95%-CI 44.9–62.3) after surgical resection. Local recurrence was detected in 23.6% of cases. The presence of metastases and dedifferentiated LS subtype as well as negative margins, grade and tumour size are significant prognostic factors of the survival rates (p < 0.015).ConclusionGrading, LS subtype, negative margins after surgery, metastases and tumour size are independently associated with disease-specific survival, and patients with local recurrence had lower survival rates. We hope our investigation may facilitate a further prospective study and clinical decision-making in LS.

Primary Retroperitoneal Myxoid Liposarcomas.

Myxoid liposarcomas (MLSs) are genetically defined by the presence of DDIT3 gene fusions and most commonly arise in the extremities of young adults. Whether MLSs develop primarily in the retroperitoneum is controversial, and a recent retrospective study found no molecularly confirmed examples. Because MLSs tend to metastasize to deep soft tissues, purported examples of primary retroperitoneal lesions might represent distant metastasis, most commonly from extremities. In addition, well-differentiated or dedifferentiated liposarcomas, which are characterized by MDM2 amplifications, may exhibit prominent myxoid changes and mimic MLSs. Here, we document 5 cases of MLSs that originated in the retroperitoneum that were identified through critical clinicopathologic reevaluation. These cases accounted for 2.3% of 214 primary retroperitoneal liposarcomas and 3.2% of 156 MLSs in our database. They occurred in 3 men and 2 women with a median age of 32 years. All tumors were localized to the retroperitoneum at presentation, and no patient developed extra-abdominal recurrences during the clinical course (median, 50 mo). All 5 cases exhibited at least focal classic histologic findings. All harbored DDIT3 gene rearrangements, and none harbored MDM2 amplifications according to fluorescence in situ hybridization. This study demonstrates that primary MLSs can occur in the retroperitoneum, albeit rarely, and can be accurately diagnosed through combined clinicopathologic and molecular analysis.