This study focuses on reducing pain from radiation therapy in anal or perianal skin cancer. When treating this cancer, doctors use a combination of chemotherapy and radiation (called CRT). One side effect of CRT is significant pain. To help with this, the study will use a technique called ganglion impar neurolysis, which is a nerve block procedure. This involves injecting medicine near the tailbone to numb specific nerves and reduce pain.

• **Duration:** The study takes place during week 4 of CRT and includes follow-ups at the end of radiation therapy and 3-6 months later.
• **Eligibility:** Adults over 18 with localized anal or perianal cancer undergoing CRT, and experiencing significant pain.
• **Participation:** You must be willing to sign a consent form and complete questionnaires.

The primary goal is to see if this nerve block can reduce unscheduled breaks from CRT to 3 days or fewer. Participants will be observed for any side effects and asked about their pain levels and overall experience.