Arytenoid sparing during irradiation of early stage vocal cord cancer

Optimal position of the posterior field border when irradiating early stage vocal cord cancer is controversial. Several experts recommend moving the posterior field border 5-15 mm anteriorly after 50-60 Gy to decrease the chance of arytenoid edema. This article will evaluate the effect of field position on arytenoid dose.

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 degree wedge filters using contours from an axial CT scan. Profiles with the posterior field border overlaying, 5, and 10 mm posterior to the posterior edge of the thyroid cartilage are presented.

With the posterior field border 10 mm posterior to the thyroid cartilage, the arytenoids are included in the 95% isodose volume. Reducing the field by 5 mm has no significant effect on the position of the 95 and 90% isodose lines relative to the arytenoids. A field reduction of 10 mm places the arytenoids in the beam penumbra and leaves approximately 10 mm between the 95% isodose line and the midpoint of the true vocal cord.

To achieve a significant dose differential between the arytenoids and the anterior portion of the vocal cord when using opposed lateral 6 MV photon beams the posterior field border must be at, or anterior to, the posterior edge of the thyroid cartilage. In view of the excellent results reported from institutions that include the arytenoids in 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 setup is very reproducible.