**7. Radiation therapy treatment techniques**

For external-beam RT, the use of CT-based treatment planning and conformal blocking is considered the standard of care (EBRT). MRI is the best imaging modality for patients with advanced malignancies for evaluating soft tissue and parametrial involvement. PET imaging is effective in individuals who have not been surgically staged to assist in defining the nodal volume of coverage and may be helpful postoperatively to confirm the excision of suspicious nodes.

To reduce treatment setup errors, CT simulation should be performed with the patient in a supine position and a specialized immobilization device. Patients with cancer covering the distal one-half of the vagina (or vaginal primary) should get bilateral inguinal RT, with CT simulation conducted in the "frog-leg" posture to avoid skin fold toxicity. Scans with a slice thickness of ≤3 mm should be acquired. The bladder and rectal filling level seen during simulation should ideally match that found with daily treatments. Consider two scans for the bladder full and empty to create an internal target volume (ITV). Fuse with MRI/PET imaging (if available) to define tumor extent. Treatment with a full bladder can shift the bowel from the treatment field and enhance bowel dosimetry; however, treatment with an empty bladder may be more repeatable and minimizes the absolute fluctuation in bladder volume. To simulate an empty rectum, bowel preparation with an enema might be employed. Because the patient's pelvic vasculature acts as a reference for lymph node placement, intravenous contrast simulation is advised unless medically contraindicated. Implantation of fiducial markers prior to CT simulation or placement of radiopaque markers in the vaginal apex and introitus during simulation are two techniques for increasing target volume identification. Using PO contrast could also help delineate a bowel bag. Consider marking the lower portion of pathology if there is a vaginal extension.

In all settings, effort must be taken to encompass all pelvis regions at risk for pathology. EBRT is delivered using multiple conformal fields or intensity-modulated volumetric techniques, such as IMRT/volumetric-modulated arc therapy (VMAT)/ tomotherapy. Typically, IMRT is used for most post-operative whole pelvis irradiation or extended field RT when inguinal and/or para-aortal nods are treated. Most ongoing clinical trials only utilize IMRT as the standard of EBRT.

For conformal RT, particularly IMRT, the gross target volume (GTV), clinical target volume (CTV), planning target volume (PTV), organs at risk (OARs), internal organ motion, and dose-volume histogram (DVH) have been established. The volume of EBRT should include the gross disease (if present), the parametria, the uterosacral ligaments, a sufficient vaginal margin from the gross disease (at least 3 cm), the presacral lymph nodes, and any additional at-risk nodal volumes. For patients with negative surgical or radiologic imaging of the lymph nodes, the radiation volume should encompass the whole external iliac, internal iliac, obturator, and presacral nodal basins. For individuals thought to be at a greater risk of lymph node involvement (e.g., bulkier tumors; suspected or confirmed lymph nodes localized to the low true pelvis), the radiation dose should be raised to include the common iliacs. In individuals with common iliac and/or para-aortic nodal involvement, pelvic and paraaortic radiation up to the level of the renal vessels is indicated (or even more cephalad
