Room: Exhibit Hall
Purpose: For patients treated with IMRT for lung cancers, investigate the impact of intra-fractional motion on dose distributions.
Methods: Twenty patients who received IMRT for localized and locally advanced lung cancer were selected for this study. For each patient four dimensional CT image comprised of ten 3D images set was acquired. Treatment planning was performed on the average CT. Dose distributions were re-calculated for each of the ten 3D images. The ten dose distributions were then combined using deformable image registration and mapped on the planning CT image for comparison.
Results: Intra-fractional motion reduced PTV coverage for all patients. The PTV volume covered by the prescription isodose, and D98 were reduced by 3.4% / 12% and by 3.1 Gy / 8.6 Gy (median/maximum). V20 for the lung increased for most patients, by 1.6% / 7.9%, the mean dose changes where in the range of Â±0.7 Gy. Changes to the maximum dose to the cord were within Â±0.9 Gy. The maximum dose to the esophagus was reduced for most patients, by 0.74 Gy / 2.1 Gy, so did the mean dose and V30 for the heart, by 0.22 Gy / 1.3 Gy and by 2.5% / 23%. The median volume where dose differences exceeded 3% / 3 mm was 6.2 %, volumes where differences exceeded Â±5 Gy were ~200 cmÂ³. Significant volumes where patients received doses by more than 5 Gy higher than planned were found in the normal lung (5 patients), mediastinum (2), chest wall (1). There was no apparent correlation between the extent of tumor motion and dose differences.
Conclusion: Neglecting intra-fractional motion results in a slightly reduced PTV coverage. We did not observe significant dose underestimation for organs at risk. A measure of intra-fractional anatomical changes is needed to identify patients at risk of PTV underdosage.