The dosimetric differences in calculating lung SBRT plans on different image data sets: Comparison of the free breathing scan to both the average intensity projection scan and to the sum of calculations on each respiratory phase of the 4DCT scan.

The purpose of this study is to evaluate the dosimetric differences to the lung internal target volume (ITV) in stereotactic body radiation therapy (SBRT) when calculated on the free breathing (FB) scan in comparison to calculations on the average intensity projection (AIP) scan as well as the sum of dose calculated on each of the treated respiratory phases. The clinical treatment plan data for 16 SBRT lung patients were retrospectively chosen for this study, 5 of which were 30% to 70% respiratory phase gated. All patients had ITV contours and fixed monitor units from the treatment approved plan copied over to each scan on which calculations were to be made. The results of this study yielded 6 out of 16 patients with greater than or equal to 2% difference in ITV maximum dose (D0) and 2 of 16 patients with greater than or equal to 2% difference in ITV minimum dose (D100). The range of ITV dose differences for these 8 patients was 2% to 4.7% with no patients exceeding a 5% difference in D0 or D100. None of the patients had greater than or equal to 2% difference in ITV mean dose (Dmean). Sixty-three percent of patients with greater than 2% ITV dose difference in any of the categories reviewed were those patients with greater than 1 cm gross tumor motion. This study concluded that in order to reduce uncertainty in dose to the ITV, tumor motion should be assessed at simulation and limited to below 1 cm in any direction if possible.