DOS 513 - Week 2 Discussion
Initial Post: IMRT for Vulvar Cancer
The vulva comprises the external female genitalia including the labia majora and labia minora, the clitoris, and the area encompassed by these, called the vestibule.1 The vestibule usually contains the urethral meatus. Vulvar cancer is usually found in the form of a lump or mass, and is frequently located in the labia majora.
When contouring for vulvar cancer treatment, the first thing that needs to be determined is the extent of disease. The progression of vulvar cancer through the lymphatic system is predictable,1 so it is important to identify which, if any, lymph nodes will be included in the target volumes. Lymphatic involvement usually starts in the inguinal nodes, then the deep femoral nodes, and then the pelvic nodes. In some cases, the mass may be surgically removed prior to radiation, or less frequently, after radiation. Once the extent of disease and the area of suspected tissue infiltration is determined, the gross tumor volume (GTV), clinical target volume (CTV), and planning target volume (PTV) are drawn. SCCA Proton Therapy has not treated a vulvar cancer case yet, so I consulted with Patty Sponseller, MS, CMD, RT(R)(T) (Oral communication, November 26, 2014) from the University of Washington for advice. Besides the target volumes, several other contours will be needed:
- external genitalia (labia, clitoris)
- large bowel
- small bowel
- femoral heads
While Uschold & Anderson recommend an AP-PA field arrangement,1 Patty said the UW group avoids this practice because of significant morbidity in the skin, especially around the gluteal fold. Patty's preferred approach (Oral communication, November 26, 2014) is an IMRT plan, which will allow some of the beam dose to come in from angles other than AP and PA. By approaching the target(s) from multiple angles, each beam can be of lower intensity, and it is possible to create a more conforming dose distribution that can spare dose to sensitive structures such as the ovaries, bladder, and rectum, not to mention the delicate skin of the genitalia. The UW group does not separately contour the uterus or cervix, because they have very high radiation tolerances1 and their constraints would not affect beam optimization. If lymph nodes are involved, an IMRT plan can concentrate dose in those areas without globally irradiating the pelvis at high levels.
The reason that many of the listed organs at risk are contoured is so that they can be protected in the course of beam pattern optimization. The small bowel is more radiosensitive than the large bowel, with a tolerance of 45 Gy, and the ovaries are the most radiosensitive anatomy, with doses as low as 4-5 Gy producing a high likelihood of permanent cessation of menses.1
It is important to realize that there are several techniques available to treat any region of the body, and each should be evaluated relative to the planning and delivery resources available to the radiation therapy staff at each clinic, as well as the specific needs of each patient.
- Uschold GM, Anderson JE. Gynecological tumors. In: Principle and Practice of Radiation Therapy. 3rd ed. St. Louis, MO: Mosby; 2010.