Other forms: cancerously
Cancerous describes things that are related to a disease in which abnormal cells grow and spread very quickly. Biopsies are usually done to determine if a suspicious lump or mass is cancerous or not.
Anything cancerous contains or is distinguished by cancer, like a cancerous tumor or cancerous tissue that has to be treated with radiation. You can also use the word figuratively, for destructive things that seem to multiply and spread the way cancer does: "The cancerous rumor was all over the school by the next morning, when everyone was whispering about it." The Greek root of cancerous and cancer is karkinos, which means both "tumor" and "crab."