The impact of anatomic tumor location on inter-fraction tumor motion during lung stereotactic body radiation therapy (SBRT)

Narrow PTV margins and steep dose gradients underscore the importance of evaluating breathing-associated tumor motion for lung SBRT. The specific aim of this study was to determine the impact of anatomic tumor location on inter-fraction tumor motion. Forty-one patients underwent standard free-breathing 4DCT simulation and daily image-guidance 4DCTs during lung SBRT. Absolute tumor motion amplitude in the mediolateral (ML), anterior-posterior (AP), and superior-inferior (SI) directions was analyzed from 159 total 4DCT scans (simulation and daily pre-treatment). Overall, the inter-fraction tumor motion amplitude in the ML, AP, and SI directions was small (mean ≤2.5 mm). Similarly, while both upper lobe (UL) and lower lobe (LL) tumors exhibited limited inter-fraction motion in both the ML and AP directions (mean ≤2.2 mm), tumors in the LL had increased inter-fraction motion in the SI direction compared to UL tumors (mean 4.3±4.0 mm vs. 1.7±1.7 mm, p=0.008). Moreover, 28.6% (n=4) of LL tumors exhibited mean inter-fraction motion along the SI direction >5 mm (all of which resided in the supra-diaphragmatic basal segments of the LL). Mean inter-fraction tumor motion amplitude along the SI direction exceeded our PTV margins (an isotropic 5 mm expansion of the ITV) in 28.6% of LL tumors (all of which resided in the basal segments). These results suggest that typical ITV-to-PTV margins may be insufficient for a subset of LL lesions and that increased PTV margins, daily breathing motion re-assessment and/or adaptive re-planning may benefit patients with supra-diaphragmatic tumors in the LL.