According to Genes & Development, a heterogeneous mass in biology is a tumor with both normal cells and neoplastic cells, which are cells of abnormal growth tissue. Heterogeneous masses are called solid tumors and can be cancerous.
Dr. Barry T. Kahn from HealthTap explains that heterogeneous masses can be malignant or benign. Depending on the size, heterogeneous masses need to be removed and examined for malignancy. Some heterogeneous masses on the ovaries resolve on their own.
Wikipedia goes on to explain that the heterogeneity of cancerous masses makes it difficult to treat cancer because the cells have different forms, gene expressions, motility and metabolism. For example, some of the cells in tumors are cancer stem cells, which can in turn form more tumors. Moreover, there are differences in cancer stem cells. These differences in cancer cells occur in part because of mutations, gene instability, impaired DNA and outside or environmental influences. All of these different and varying cells make the tumor itself heterogeneous and difficult to treat. Drug administration, one treatment option for cancer, does not kill all the different kinds of cancerous cells in a tumor.
Cancers that have heterogeneous masses include breast cancer, prostate cancer, colon cancer, brain cancer, myeloma and leukemia, Wikipedia states.