Anus / Anal Canal

The last few centimeters of the bowel, running from the anorectal junction at the pelvic floor to the anal verge, surrounded by the internal and external sphincters. It lies directly behind the vagina and the posterior vulva.

The anus is the last part of the bowel; its sphincter muscles control the passing of stool.

In vulvar cancer the anus sits just behind the target, so it often receives a high dose — especially when the tumor reaches the posterior fourchette or perineum. Dose here is linked to soreness, urgency and incontinence. The NRG vulva template therefore lists the anus as its own organ, separate from the rectum, and notes that its limit may not be met without losing coverage when the tumor extends backward.

• The NRG anal cancer template describes the anal canal as running from the anal verge to the anorectal junction, with the internal and external sphincter muscles around it.
• Gay 2012's AnoRectum combines the anus and rectum, starting at the anal verge marked with a radiopaque marker at simulation.
• In the NRG vulva template the rectum starts 2 cm above the anal verge, so the anus covers the segment below it.

Contour the anus separately from the rectum when your protocol limits them separately, and check that the two contours meet without gaps or overlap.