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 den sity materials can result in a dose reduction along the central axis a nd near the beam edge for megavoltage photon beams. In this context, R adiation Therapy Oncology Group (RTOG) protocol #91-05 recommends the use of photon beams of energy 12 MV or less for nonsmall cell lung can cer therapy. This work presents data to support the use of higher ener gy photons for some clinical lung field setups. Methods and Materials: Beam profiles were obtained from films inserted into homogeneous (pol ystyrene) and heterogeneous (polystyrene and lung-equivalent material) phantoms and irradiated in both single-field and parallel-opposed set ups with 6 and 18 MV photon beams. Depth-dose curves were obtained wit h a parallel-plate ion chamber in the heterogeneous phantom to determi ne 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) a t the lung/polystyrene interface than single-field profiles. The diffe rence between 6 and 18 MV penumbra widths at the interface is also red uced when an opposed field is added. The central axis dose reduction a t the interface was negligible for single fields of a width of 8 cm or more. Conclusion: Our results show that for opposed fields, the diffe rence in penumbra degradation of the 6 and 18 MV photon beams is clini cally insignificant compared to daily setup errors and patient motion. The central axis dose reduction is also shown to be small. Our data s upport the use of higher energy beams to obtain lower peripheral dose maxima in some clinical geometries.