Is it true that canceris only caused by carcinogens?

No: While long term exposure to carcinogens especially in water can be associated with cancer, most known malignancies are secondary to viruses. HPV virus induces squamous Ca, Hepatitis virus is associated with hepatoma of liver, helicobactor of stomach Is a major cause of stomach cancer and polyoma with colon cancer. EBV induces telomerase which keeps chromosomes long and adds longevity to tumors