The uterus is held in place in the pelvis by four paired ligaments: broad ligaments (lateral from uterus to pelvic sidewall), round ligaments (anterolateral from uterus to pelvic sidewall, then through inguinal canal to connective tissue of labia majora), cardinal ligaments (lateral from cervix to pelvic sidewall), and uterosacral ligaments (posterior from cervix to sacrum, inserting into S1-S3). Cervical cancer can track along the uterosacral ligaments posteriorly into the presacral space.
The traditional lateral field borders for a 4-field beam arrangement for node-negative cervical cancer are 1 cm anterior to the pubic symphysis anteriorly and splitting the sacrum posteriorly. However, given the risk of presacral space involvement, many physicians no longer split the sacrum and instead include the entire sacrum in the lateral radiation fields. The superior border of the lateral fields for node-negative disease is at the bifurcation of the iliac vessels (bony landmark ~L4/L5) and the inferior border is the inferior obturator foramen or 3 cm below the most inferior extent of vaginal involvement. AP and PA fields have the same superior and inferior borders, with lateral borders 2 cm lateral to the pelvic brim. The treating physician must be cautious when shielding the rectum on lateral fields, particularly with advanced stage disease where the risk of presacral involvement is higher.
