Comparison of dose homogeneity effects due to electron equilibrium loss in lung for 6 MV and 18 MV photons

Purpose: Loss of electronic equilibrium within and adjacent to low density materials can result in a dose reduction along the central axis and near the beam edge for megavoltage photon beams. In this context, Radiation Therapy Oncology Group (RTOG) protocol #91-05 recommends the use of photon beams of energy 12 MV or less for nonsmall cell lung cancer therapy. This work presents data to support the use of higher energy photons for some clinical lung field setups. Methods and Materials: Beam profiles were obtained from films inserted into homogeneous (polystyrene) and heterogeneous (polystyrene and lung-equivalent material) phantoms and irradiated in both single-field and parallel-opposed setups with 6 and 18 MV photon beams. Depth-dose curves were obtained with a parallel-plate ion chamber in the heterogeneous phantom to determine the range of field sizes over which the dose reduction at the lung/polystyrene interface becomes clinically significant. Results: Opposed field profiles show less degradation in the penumbra (50-90% width) at the lung/polystyrene interface than single-field profiles. The difference between 6 and 18 MV penumbra widths at the interface is also reduced when an opposed field is added. The central axis dose reduction at the interface was negligible for single fields of a width of 8 cm or more. Conclusion: Our results show that for opposed fields, the difference in penumbra degradation of the 6 and 18 MV photon beams is clinically insignificant compared to daily setup errors and patient motion. The central axis dose reduction is also shown to be small. Our data support the use of higher energy beams to obtain lower peripheral dose maxima in some clinical geometries.