Colonoscopy can differentiate between radiation proctitis and rectal cancer by the scope of lesion, shape of lesion, and extent of lesion. 1. Scope of lesions: radiorectitis has a larger scope of lesions, while rectal cancer has a smaller and more limited scope of lesions. 2. Shape of lesion: radiological proctitis presents flaky redness and swelling, and the shape of the lesion is more regular, while rectal cancer mainly manifests as bulging, punctate and localized ulcerative lesions, which are irregular in shape and have unclear boundary with the surrounding. 3. Degree of lesion: radiological proctitis only has ulceration, redness, swelling and mild erosion of mucosal surface, while rectal cancer is usually accompanied by crater-like ulcers, brittle and easy to bleed. When it is not easy to distinguish between the two under colonoscopy, it is recommended to cut out the tissue for immunohistochemistry to clarify the nature of lesions under pathological staining.