Light micrograph of a section of human heart muscle tissue showing a cardiac sarcoma. A cardiac sarcoma, otherwise known as a heart tumour, is a rare type of primary malignant (cancerous) tumour that occurs in the heart. These are growths in the heart muscle or its adjacent structures. A primary cardiac tumour is one that originates in the heart, but is extremely rare accounting for about a quarter percent of cases. A secondary cardiac tumour starts somewhere else in the body and then spreads to the heart. Fortunately, the majority of diagnosed cardiac tumours are benign (non-cancerous).