LARYNX MOTION ASSOCIATED WITH SWALLOWING DURING RADIATION-THERAPY

Purpose: A basis is presented for predicting the reduction in radiation dose to the larynx attributable to swallowing during radiation therapy treatment. Methods and Materials: Laryngeal movement associated with swallowing can occur during radiation therapy even when the patient's head is immobilized. Data on the extent and timing of laryngeal motions and the frequency of swallowing were used to predict the effect such motion would have on accuracy of radiation dose to the larynx. Results: In a nontracheostomized adult the larynx elevates approximately 2 cm during a swallow and moves anteriorly less than 1 cm. The normal frequency of swallowing in the supine position is once every 1-2 min. During therapy, the likelihood of a swallow occurring during an irradiation interval depended on the duration of the interval. For irradiation intervals less than 2 min long the ratio of number of swallows to number of intervals was 0.27. For irradiation intervals between 2-3 min long the ratio was 1.76. Based on conservative estimates of radiation field dimensions, larynx motion, and incidence of swallowing, the reduction in radiation dose attributable to swallowing during treatment would be approximately 0.5%. Conclusion: With small fields the total dose is only decreased by 0.5% with swallowing, so the change in the total dose is insignificant.