Anal swelling can be caused by bowel cancer or by other rectal-anal tube diseases. It is not possible to make a definitive diagnosis based on one symptom alone.
Anal swelling is a symptom caused by various intestinal or extra-intestinal factors that irritate the rectal canal. Bleeding or secretion of active substances from rectal cancer can irritate the mucous membrane of rectoanal tube and cause anal swelling.
Besides bowel cancer, many other diseases can also cause anal distension, such as internal hemorrhoids, anal fistula, perianal abscess, pelvic abscess, pelvic acute appendicitis, female pelvic inflammatory disease, etc. Therefore, anal distension does not necessarily mean that one is suffering from bowel cancer.
It is recommended to go to the hospital as soon as possible to find out the cause of anal swelling and treat it accordingly.