ARYTENOID SPARING DURING IRRADIATION OF EARLY-STAGE VOCAL CORD CANCER

Purpose: Optimal position of the posterior field border when irradiati ng early stage vocal cord cancer:is controversial. Several experts rec ommend moving the posterior field border 5-15 mm anteriorly after 50-6 0 Gy to decrease the chance of arytenoid edema. This article will eval uate the effect of field position on arytenoid dose. Methods and Mater ials: 5 x 5 cm opposed lateral fields centered on the glottis were set up on a patient with typical anatomy. Isodose profiles were obtained with equally weighted 6 MV photon beams attenuated with 15 degrees wed ge filters using contours from an axial CT scan. Profiles with the pos terior field border overlaying, 5, and 10 mm posterior to the posterio r edge of the thyroid cartilage are presented. Results: With the poste rior field border 10 mm posterior to the thyroid cartilage, the aryten oids are included in the 95% isodose volume. Reducing the field by 5 m m has no significant effect oh the position Of the 95 and 90% isodose lines relative to the arytenoids. A field reduction of 10 mm places th e arytenoids in the beam penumbra and leaves approximately 10 mm betwe en the 95% isodose line and the midpoint of the true vocal cord. Concl usion: To achieve a significant dose differential between the arytenoi ds and the anterior portion of the vocal cord when using opposed later al 6 MV photon beams the posterior field border must be at, or anterio r to, the posterior edge of the thyroid cartilage. In view of the exce llent results reported from institutions that include the arytenoids i n the high-dose volume throughout treatment, it would seem appropriate to limit the use of arytenoid-sparing techniques to patients in whom there is no ambiguity about tumor location and in whom the treatment s etup is very reproducible.