An investigation into lung correction for tangential breast irradiation

A solid water breast phantom with a lung equivalent material was constructed to investigate interface effects of 6 MV tangential photon fields. The phantom was built so that Thermo-luminescence Dosimeters (TLD) could be used for comparison to a pencil beam and Equivalent Tissue-Air-Ratio (ETAR) algorithms. The investigation highlighted the limitations in both of these algorithms under clinically anatomical conditions. At the lung/tissue interface both algorithms showed errors between 3-12% depending on the position of the TLD. In tissue greater than 1 cm from the interface region the errors on average were within +/- 5%. Dose comparisons in lung tissue showed similar results to the interface region. These results indicate that normalisation of Reference Dose in the interface region for tangential breast treatments cause over or under estimates of dose.