Impact of tumor size and location on lung dose difference between stereotactic body radiation therapy techniques for non-small cell lung cancer

Background To evaluate the lung dose differences between three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiation therapy (IMRT) techniques for lung stereotactic body radiation therapy (SBRT) and the correlations with tumor characteristics, such as size and location. Methods Dosimetric comparisons between the two SBRT techniques in high- and low- to intermediate-dose regions were retrospectively performed using four planning indices and lung-dose parameters in 31 lung tumors. The magnitude of differences in these parameters was analyzed with relation to the planning target volume (PTV) and location-related parameters. Results The absolute differences between the two techniques in lung-dose parameters were small in both ipsilateral and bilateral lungs. The dosimetric differences were mainly correlated with the PTV rather than location-related parameters, with positive and negative correlations with the high-dose and intermediate-dose parameters, respectively. The distances from the ipsilateral lung centroid to the PTV center were not correlated with the differences in any of the lung-dose parameters. Additionally, the negative correlations with the MLD and V20 differences disappeared after applying a more rapid dose fall-off in the IMRT plans for tumors with small PTVs of <= 15 cc. Conclusions Lung dose differences between the 3D-CRT and IMRT techniques for lung SBRT were mainly correlated with the PTV rather than location-related parameters. Together with the dosimetric benefit in high-dose lung regions of IMRT for larger tumors, the relative increases in the MLD and V20 for small-sized tumors could be reduced by applying a more rapid dose fall-off outside the PTV.