Cardiac rhabdomyoma, light micrograph. Cardiac rhabdomyoma is a benign (non-cancerous) tumour of cardiac myocytes (muscle cells). It is the most common congenital cardiac neoplasm. Many cases occur in the setting of tuberous sclerosis complex - an autosomal dominant neurocutaneous disorder. It is also associated with a number of congenital heart diseases, including Ebstein anomaly, hypoplastic left heart syndrome and transposition of great vessels. Microscopically, it is composed of large, glycogen-filled, vacuolated cells, also known as spider cells.