PURPOSEThis study was initiated to determine if the location of a small air cavity would modify the underdosing that occurs at the air tissue interface. Several authors have measured the lack of electronic equilibrium for most megavoltage X-ray beams including Co-60. In the treatment of cancer of the larynx and paranasal sinuses with 4 and 6 MV x-rays. Often the cavity depth can be less than one centimeter. The cavity effect may influence the local control of these tumors.METHODSTo evaluate this effect a cavity 2.5 cm in width, 3 cm in depth and 15 cm long was located at depths of 0.5, 1.0, 1.5 and 2.5 cm in a polystyrene phantom and irradiated. The re-build-up of the dose at the interface was determined for Co-60, 4, 6 and 10 MV x-rays using a parallel plate ionization chamber, the field sizes evaluated were 10 10, 10 5 and 5 5 cm at the interface. In addition a polystyrene phantom was designed to simulate the neck at the level of the vocal cord including a small air cavity. This phantom was irradiated with conventional right and left lateral wedged fields using radiochromic film to evaluate the dose under treatment conditions.RESULTSThe results indicate that for Co-60 irradiation the depth of the cavity did not modify the re-build up of dose beyond the air cavity and that the relative dose at the surface for the same field size remained the same. The dose increased with cavity depth for the smallest field by 6% for 4 MV x-rays. The increase in dose for the 5 5 cm field at the interface for 6 MV and 10 MV x-rays was 10% and 25% respectively. The modification in dose at the interface due to the size of the radiation field increased as the field size increased and the length of the side walls irradiated. The comparison of the dose profiles from the opposed lateral fields across the small air cavity in the neck phantom for 6 MV x-rays and Co-60 treatment indicated a superficial dose of 54% and 66% respectively at the interface of the cavity but increasing rapidly to the planned dose in less then 0.3 cm for 6 MV and 0.2 cm for Co-60. The dose from the anterior surface of the neck to the cavity at one cm depth varied from the surface to a maximum dose of 9% lower then the planned dose for 6 MV x-rays and for the Co-60 beam the dose was only 3% less than the planned dose.DISCUSSIONIn the treatment of disease in or near air cavities with medium and high energy x-ray beams, the air cavity effect needs to be considered carefully. Most commercial treatment planning computers do not consider the effects disequilibruim due to the presence air cavities and will not predict the dose accurately. This study indicates that the use of bolus above an air cavity will increase the dose at the interface beyond the cavity.