Variation of lung density corrections in a typical patient population

The correction factors to allow for the excess transmission through the lung were worked out for 25 patients who underwent radiation treatment of lesions in thorax. The geometrical arrangements of the beams used for the treatments were reconstructed on the CT scans to identify the amount of lung that intervened. The lung transmission correction factors were worked out for four possible photon energies Cobalt-60, 6 MV, 10 MV, and 20 MV. Isocentric treatment was considered throughout and the isocenter was regarded as the point of dose prescription as is done in normal practice. Correction factors were based on published experimental data for situations that had the isocenter inside the lung. Ratio of TAR (or TMR) method and Batho's power law of TAR (or TMR) method were used if the isocenter was behind the lung. It was observed that the average increase in the total dose after lung corrections were 11.5 +/- 6%, 8.5 +/- 5%, 5.5 +/- 3%, and 4.0 +/- 2% at the energies of Cobalt-60, 6 MV, 10 MV, and 20 MV, respectively. This proves the importance of carrying out the lung transmission corrections to achieve a greater consistency in the doses delivered between groups of patients treated with different photon energies.