Myxomas are the most common type of primary heart tumor. The tumor is derived from multipotential mesenchymal cells and may cause a ball valve-type obstruction.About 75% of myxomas occur in the left atrium of the heart, usually beginning in the wall that divides the two upper chambers of the heart.n atrial myxoma is a benign tumor of the heart, commonly found within the left and right atria on the interatrial septum.