Background and purpose: Dose calculations in trachea HDR brachytherapy treatment planning systems are greatly based on TG-43 protocol in which, all materials including air inside trachea are treated the same as water. The aim of this study was to survey the effect of air on dose calculations of Flexiplan treatment planning system in trachea HDR brachytherapy.

Materials and methods: To evaluate the effect of air inhomogeneity, a neck-equivalent plexiglass cylindrical phantom was used. Dose measurement was carried out by EDR2 film. Treatment planning and irradiation were performed using Flexiplan software and Flexitron brachytherapy system, respectively.

Results: The results showed that considering the air inside trachea as water increases the absorbed dose by 12% which can lead to increment of patient dose.

Conclusion: A significant difference was seen between dosimetry results in the two conditions. Therefore, taking the air similar to water has a considerable effect on dose calculations of trachea HDR brachytherapy and accuracy of treatment plan performed.