The treatment planning for tumors located in the vicinity of the air-tissue interface in the case of head and neck cancer involves the assessment of dose reduction due to transient electronic disequilibrium. To avoid underexposure of the tumor a precise method of calculation of the dose perturbation incorporating a valid physical model of particle transport in inhomogeneous media, stereometrical representation of the patient’s organ, and clinically feasible computational system is required. A Monte Carlo (MC) PENELOPE code satisfying all three requirements was used to simulate the dose distribution in an accurately reconstructed phantom model based on CT scans of the neck of an actual patient. The results of simulations show good agreement with known measurements of the underdose in the water phantom with a parallel-plane air channel. The ratio of the dose in heterogeneous phantom to the dose in a homogeneous medium at the same location was chosen to measure the underdose. In the case of a single megavoltage photon beam, this ratio, evaluated at a distance of 0.25 mm from the air-tissue interface, can decrease to as low as 40% depending on the size of the air cavity.
For the parallel-opposed beams the ratio is higher and does not go lower than 90% for the typical air cavity sizes in the neck. The results of this work show that the small size fields and large inhomogeneities result in a large volume of underdose, while the actual setup used for the treatment of mucosa causes less significant decrease in underdosage.