Alveolar soft-part sarcoma: can MRI help discriminating from other soft-tissue tumors? A study of the French sarcoma...

To investigate the imaging features of alveolar soft-part sarcomas (ASPS) on pre-treatment MRI in order to identify relevant criteria to distinguish ASPS from other soft-tissue tumors.

A series of 25 patients (mean age, 18.5 years old) with histologically proven ASPS from five French comprehensive cancer centers was compared to a control cohort of 292 patients with various histologically proven benign and malignant soft-tissue tumors representative of the 10-year long activity of one center. All had a baseline MRI with contrast-agent administration. Two radiologists independently reviewed the MRIs. Features assessing location, size, signal, architecture, periphery, and vascularization were reported. Their association with the histological diagnosis of ASPS was evaluated with chi-square or Fisher's test. Their prevalence, sensitivity, specificity, odds ratio, and reproducibility were calculated.

Eight MRI features were significantly associated with ASPS: deep location (p < 0.001), high signal intensities on T1-weighted imaging (p < 0.001), central area of necrosis (p = 0.001), absence of fibrotic component (p = 0.003), infiltrative growth pattern (p = 0.003), absence of tail sign (p = 0.001), presence of intra- and peritumoral flow-voids (p < 0.001), and number of flow-voids ≥ 5 (p < 0.001). Twenty out of the 25 (80%) ASPS showed at least 7 of these 8 features compared to only four out of 292 (1.4%) tumors of the control cohort (1 benign vascular tumor, 1 solitary fibrous tumor, 2 high-grade soft-tissue sarcomas). The five ASPS with less than 7 out of 8 features measured less than 40 mm.

The striking histological uniformity of ASPS translates into imaging. However, ASPS may be misdiagnosed as benign tumors or pseudo-tumors, notably intramuscular benign vascular tumors or vascular malformations.

• ASPS are rare aggressive mesenchymal tumors displaying recurrent MRI features highly reminiscent of the diagnosis. • Deep-seated tumors presenting with mainly high signal intensity on T1-weighted imaging, an absence of fibrotic component, ill-defined margins without aponeurotic extension, and more than five central and peripheral flow-voids are very likely to be ASPS. • ASPS may be misdiagnosed as intramuscular benign vascular tumor or vascular malformation, which occur in the same age group.