Why does malignant tumor call cancer

Why does malignant tumor call cancer If inscribe Cancer is malignant tumor, is not malignant tumor it is cancer. For example sarcoma also is malignant tumor, but it does not call cancer, a general designation that malignant tumor is the tumor that has malign operation just Epithelial malignant tumor calls cancer. Referenced material: Http://www.weipinkang.com.cn/