A benign smooth muscle neoplasm arising from the mediastium. It is characterized by the presence of spindle cells with cigar-shaped nuclei, interlacing fascicles, and a whorled pattern. The disease is mediastinum leiomyoma (MONDO_0003284). Also known as: leiomyoma of mediastinum, leiomyoma of the mediastinum, mediastinal leiomyoma.