Carcinoid tumor is a rare type of tumor that grows slowly. Malignant tumors are cancers that often continue to grow and may spread to other areas of the body. Benign (noncancerous) tumors tend to grow more slowly and don’t spread.Carcinoid tumors are somewhere between malignant and benign tumors.arcinoid Tumor. Carcinoid tumor is a rare type of tumor that grows slowly. Malignant tumors are cancers that often continue to grow and may spread to other areas of the body. Benign (noncancerous) tumors tend to grow more slowly and don’t spread. Carcinoid tumors are somewhere between malignant and benign tumors.