MR imaging of clear cell sarcoma (malignant melanoma of the soft parts): amulticenter correlative MRI-pathology study of 21 cases and literature review

Objective. To evaluate MR imaging and pathology findings in order to define the characteristic features of clear cell sarcoma of the soft tissues (mal ignant melanoma of the soft parts). Design and patients. MR examinations of 21 patients with histologically pro ven clear cell sarcoma of the musculoskeletal system were retrospectively r eviewed and assessed for shape, homogeneity, delineation, signal intensitie s on T1- and T2-weighted images, contrast enhancement, relationship with ad jacent fascia or tendon, secondary bone involvement, and intratumoral necro sis. In 19 cases the pathology findings were available for review and for a comparative MR-pathology study. Results. On T1-weighted images, lesions were isointense (n=3), hypointense (n=7) or slightly hyperintense to muscle (n=11). Immunohistochemical examin ation was performed in 17 patients. All 17 specimens showed positivity for HMB-45 antibody. In nine of 11 lesions with slightly increased signal inten sity on T1-weighted images, a correlative MR imaging-pathology study was po ssible. All nine were positive to HMB-45 antibody. Conclusions. Clear cell sarcoma of the musculoskeletal system often has a b enign-looking appearance on MR images. In up to 52% of patients, this lesio n with melanocytic differentiation has slightly increased signal intensity on T1-weighted images compared with muscle. As the presence of this relativ e higher signal intensity on T1-weighted images is rather specific for tumo rs displaying melanocytic differentiation, radiologists should familiarize themselves with this rare entity and include it in their differential diagn osis when confronted with a well-defined, homogeneous, strongly enhancing m ass with slightly higher signal intensity compared with muscle on native T1 -weighted images.