Sagittal magnetic resonance imaging in the design of lateral radiation treatment portals for patients with locally advanced squamous cancer of the cervix.

Twenty-five patients with FIGO clinical Stages IB-IVA squamous cancers of the uterine cervix underwent pelvic magnetic resonance imaging to assist in the design of radiation therapy portals. Magnetic resonance imaging was used primarily to define the treatment volume required to encompass the primary disease and its direct regional extensions, and only secondarily to assess the presence or absence of lymph node metastases. The sagittal scans revealed that use of "conventional" or "standard" lateral radiation portals would have resulted in a failure to encompass all gross cancer extensions (marginal miss) in 6 patients (24%). The beam edge of standard portals would have traversed tissue within 1 cm or less of gross cancer in an additional 8 patients (32%), increasing the risk of regional underdosage of subclinical disease extensions. Use of conventional lateral portals would have resulted in incomplete coverage of the uterine fundus in 15 of 24 patients (62.5%), of whom 3 had gross cancer extension to involve the uterine cavity or the myometrium of the lower uterine segment. Conventional lateral portal design, as described and illustrated in standard radiation oncology texts, may be suboptimal for a significant percentage of patients with locally advanced or bulky cervical cancer, and could be a contributing cause of failure to control pelvic disease. Design of lateral treatment portals should be based on imaging the morbid anatomy in the treatment position, rather than on an assumption of normal anatomic relationships.