External Genitalia & Perineum

The external genitalia and the skin and soft tissue of the perineum between the thighs, just in front of and around the anus.

The external genitalia are the outer sex organs.

In anal cancer, the target reaches the anal verge and the inguinal nodes are treated, so the genitalia and perineal skin sit close to high dose. Moist desquamation of the perineum and groin is one of the most painful acute effects and a common reason for treatment breaks, which protocols try hard to avoid. For low rectal tumors the same area can matter.

The NRG anal cancer template requires a "Genitals" structure — male or female external genitalia — and notes that consistent repositioning of male genitalia at simulation and treatment matters. It gives no further boundaries. Detailed boundaries from an open published source: verified source required; contour as your protocol describes.