Liposarcoma is an extremely rare primary bone sarcoma. We report a case of primary pleomorphic liposarcoma that arose in an 18 year old male in the metaphysis of the left tibia. Plain radiographs showed a partly sclerotic lesion and MR imaging a heterogeneous tumour predominantly isointense on T1- and high-signal on T2-weighted sequences with focal areas of increased T1 signal that suppressed with fat saturation. PET/CT showed marked FDG uptake (SUV = 17.1) in the primary tumour as well as a metastasis in the right distal femur and multiple small pulmonary metastases. Histologically, the tumour was a pleomorphic liposarcoma containing large tumour cells with vacuolated cytoplasm and hyperchromatic pleomorphic nuclei as well as numerous lipoblasts and scattered brown fat-like cells. Tumour cells strongly expressed FABP4/aP2, a marker of adipocyte differentiation, and UCP1, a marker of brown fat, but not S100. The case was treated with neoadjuvant MAP chemotherapy, resulting in extensive (> 95%) necrosis in the primary tumour and almost complete resolution of the femoral and pulmonary metastases. Pleomorphic liposarcoma can present as a sclerotic primary malignant bone tumour; markers of adipose differentiation are useful in histological diagnosis and neoadjuvant MAP chemotherapy results in significant tumor necrosis.

Uncoupling protein 1 (UCP1) is a mitochondral protein transporter that uncouples electron transport from ATP production. UCP1 is highly expressed in brown adipose tissue (BAT), including hibernomas, but its expression in other adipose tumours is uncertain. UCP1 has also been found in other tissues (e.g. smooth muscle) but whether it is expressed in non-adipose benign and malignant soft tissue tumours is unknown. Immunohistochemical staining of normal (axillary) BAT and subcutaneous/abdominal white adipose tissue (WAT) as well as a wide range of benign and malignant primary soft tissue tumours (n = 171) was performed using a rabbit polyclonal antibody to UCP1. BAT and hibernomas were also stained by immunohistochemistry with monoclonal and polyclonal antibodies to adipose/non-adipose tumour markers in order to characterise the immunophenotype of BAT cells. UCP1 was strongly expressed in the cytoplasm of brown fat cells in BAT and hibernomas, both of which also expressed aP2, S100, CD31, vimentin and calponin. UCP1 was not expressed in WAT or other adipose tumours with the exception a few tumour cells in pleomorphic liposarcoma. UCP1 was variably expressed by tumour cells in a few non-adipose sarcomas including leiomyosarcoma, rhabdomyosarcoma, alveolar soft part sarcoma, synovial sarcoma and clear cell sarcoma. UCP1 is strongly expressed in BAT but not WAT and is found in all hibernomas and a few pleomorphic liposarcomas but not in other adipose tumours. UCP1 expression in a few non-adipose soft tissue sarcomas may possibly reflect origin of tumour cells from a common mesenchymal stem cell precursor and/or developmental pathway.

Background/Objectives: Extremity leiomyosarcoma is an uncommon soft tissue sarcoma with a more favorable prognosis than other anatomical subtypes. Risk stratification currently rests on tumor grade, size, and metastatic status. Vascular invasion is an established predictor of metastasis in mixed soft tissue sarcoma cohorts but has seldom been formally examined in extremity leiomyosarcoma. We assessed its prognostic value in a single-center cohort. Methods: We retrospectively studied 62 adults who underwent resection of primary localized extremity leiomyosarcoma at a tertiary sarcoma center between January 2015 and February 2025. Vascular invasion was reported contemporaneously at original diagnostic reporting by specialist sarcoma histopathologists. Overall (OS), disease-specific (DSS), local recurrence-free (LRFS), and metastasis-free (MFS) survival were estimated by the Kaplan–Meier method and compared using log-rank tests. Death from another cause was treated as a competing event for disease-specific death and distant metastasis in a sensitivity analysis. Multivariable Cox models were refitted using Firth’s penalized likelihood, with discrimination corrected for optimism by bootstrapping. Results: Median follow-up was 59.5 months. Five-year OS, DSS, and MFS were 71.0%, 80.2%, and 74.6%. Vascular invasion was reported in 11 of 62 patients (17.7%) and was associated with inferior OS (p = 0.002), DSS (p < 0.001), and MFS (p < 0.001), but not LRFS (p = 0.631). Adjusted for grade, it was associated with inferior OS (HR 3.89, 95% CI 1.35–11.26), DSS (HR 5.48, 95% CI 1.46–20.58), and MFS (HR 5.63, 95% CI 1.69–18.80). The association persisted on separate adjustment for tumor size, depth, and neoadjuvant radiotherapy. Conclusions: Vascular invasion was associated with inferior survival in localized extremity leiomyosarcoma. With 17 deaths and 14 metastatic events, these are exploratory estimates, but they support documenting vascular invasion explicitly, as present or absent, in routine histopathological reporting so that its role can be evaluated at scale.