Stereotactic body radiation therapy (SBRT) and respiratory gating in lung cancer: dosimetric and radiobiological considerations.

Abstract: The purpose of this study was to assess the impact of respiratory gating on tumor and normal tissue dosimetry in patients treated with SBRT for early stage non-small cell lung cancer (NSCLC). Twenty patients with stage I NSCLC were studied. Treatment planning was performed using four-dimensional computed tomography (4DCT) with free breathing (plan I), near end inhalation (plan II) and near end exhalation (plan III). The prescription dose was 60 Gy in 3 fractions. The tumor displacement was most pronounced for lower peripheral lesions (average 7.0 mm, range 4.1-14.3 mm) when compared to upper peripheral (average 2.4 mm, range 1.0-5.1 mm) or central lesions (average 2.9 mm, range 1.0-4.1 mm). There were no significant differences in tumor and normal tissue dosimetry among the three gated plans. Tumor location however, significantly influenced tumor doses because of the necessity of respecting normal tissue constraints of centrally located structures. For plans I, II and III, average doses to central lesions were lower as compared with peripheral lesions by 4.88 Gy, 8.24 Gy and 6.93 Gy for minimum PTV and 0.98, 1.65 and 0.87 Gy for mean PTV dose, respectively. As a result, the mean single fraction equivalent dose (SFED) values were also lower for central compared to peripheral lesions. In addition, central lesions resulted in higher mean doses for lung, esophagus and ipsilateral bronchus by 1.24, 1.93 and 7.75 Gy, respectively. These results indicate that while gating of SBRT treatments in patients with severe chronic obstructive pulmonary disease (COPD) alters dosimetry little because lung motion is so limited, tumor location is an important determinant.