Proton Versus Photon Therapy in Anal Squamous Cell Carcinoma

This study is comparing two types of radiation for treating anal cancer: proton therapy and standard photon (X-ray) radiation. Both are given with chemotherapy, but proton therapy may cause fewer side effects by delivering radiation more precisely to the tumor and less to surrounding tissue. **You may be eligible if:** - You are 18 or older - You have confirmed, previously untreated squamous cell carcinoma of the anal canal - Your tumor is larger than 4 cm or has spread to nearby lymph nodes, but has not spread to distant organs - You are in good general health (WHO/ECOG performance status 0-1) - You are able to understand the study and provide written consent **You may NOT be eligible if:** - Your cancer is on the perianal skin only, without involving the anal canal itself - You have another active cancer (with some exceptions for long-term cancer survivors) - You have had previous pelvic radiation, surgery, or chemotherapy that would interfere with the planned treatment - You are pregnant or breastfeeding - You have a metal implant (like a hip prosthesis) that could affect radiation delivery - Your life expectancy is less than 2 years due to other illness Talk to your doctor to see if this trial is right for you.