Evaluation of underdosage in the external photon beam radiotherapy of glottic carcinoma: Monte Carlo study.

PURPOSE: Underdosage in the human larynx may be the true factor behind the decrease in local control rates. PATIENTS AND METHODS: To evaluate underdosage with Monte Carlo a CT-based geometrical model of the patient's neck (mathematical neck) was created. Dose was calculated for a pair of 6 Me V parallel-opposed photon beams modulated with 15 degree steel wedges. RESULTS: At least 5% of volume of 3.5 cm(3) hypothetical tumor near the air wall of the larynx receives less than 86% of the maximum tumor dose. The same volume received less than 91% of the maximum tumor dose when the mathematical neck had no air cavities. CONCLUSIONS: We conclude the significant underdosage at the air-tissue interface in the larynx occurs in traditional radiotherapy treatments, especially in the glottic part of the larynx.