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.