Extraskeletal osteosarcomas (ESOS) are rare soft-tissue sarcomas (STS), that represent 2–5% of all osteosarcomas and < 1% of all STS [1], [2], [3]. They mostly occur in patients in their sixth decade and are exceptional before 30 years, contrarily to osseous osteosarcomas [1,4]. According to the World Health Organization (WHO) classification of tumors, ESOS typically appear as progressively growing mineralized malignant soft-tissue tumors, characterized by neoplastic cells producing an osteoid and/or chondroid matrix, without bone or periosteum attachment [1]. ESOS demonstrates a wide morphologic spectrum and the neoplastic bone amount and distribution differ from tumor to tumor and even between different areas in a same tumor [1].

Most studies involving ESOS consisted in clinical and pathologic retrospective series including depiction of patients’ outcome and impact of STS-type or osseous osteosarcoma-type treatments. They showed that ESOS were mostly high-grade neoplasms with 5-year overall survival (OS) ranging between 47% and 61% for locally-advanced ESOS, and 37% for metastatic ESOS [5], [6], [7], [8], [9], [10]. Moreover, metastatic relapses were reported in 30–65% of patients, almost all within the first two years and in the lungs [5], [6], [7], [8], [9], [10].

So far, the imaging features of ESOS have been mainly described through case reports, single-center case series and pictorial reviews, which stressed their ubiquitous natures (either in visceral or extremities soft-tissues) and their predominat deep location in the lower limbs [4,11,12]. Researchers also showed a rather frequent presence of necrotic and hemorrhagic areas in ESOS and a non-systematic mineralization on imaging [4,11,12]. Altogether, the radiological presentation of ESOS on computed tomography (CT) and conventional magnetic resonance imaging (MRI) seems to lack specificity. Yet, recently-described MRI features from the sarcoma literature (i.e., tumor heterogeneity, tail sign, MRI growth pattern, peritumoral edema and enhancement) [13], [14], [15], [16], [17], and quantitative and functional imaging such as diffusion-weighted imaging (DWI), dynamic contrast enhanced (DCE) MRI or 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) could improve the characterization of ESOS [18]. Furthermore, no studies have ever investigated the prognostic value of radiological findings in patients with ESOS.

The purpose of this study was to analyze the imaging features of ESOS on CT and MRI and to investigate their associations with OS using uni- and multivariable survival analyses.