This study explores how a special procedure, called ganglion impar neurolysis, can help ease pain caused by radiation treatment for anal or perianal skin cancer. This type of cancer is located in the anal area and hasn't spread to other body parts. The treatment usually combines chemotherapy (using drugs to kill cancer cells) and radiation therapy (using high-energy rays to kill cancer cells). These treatments can cause side effects, like pain, which may interrupt the treatment. Ganglion impar neurolysis is a nerve block where medicine is injected near the tailbone to numb nerves, reducing pain. This study checks if this procedure can reduce pain and help patients continue their treatment without breaks.

• Participants will have the procedure during the fourth week of their combined treatment.
• Follow-ups will happen during the last week of radiation therapy and 3-6 months after treatment ends.
• Participants must be over 18, with localized anal or perianal cancer, and experiencing pain scores of 5 or higher.