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The impact of anatomic tumor location on inter-fraction tumor motion during lung stereotactic body radiation therapy (SBRT)
Katelyn M. Atkins, Yiyi Chen, David A. Elliot, Tulsee S. Doshi, Sadja Ognjenovic, Arjun S. Vachhani, Monica Kishore, Steven L. Primack, Martin Fuss, Mark E. Deffebach, Charlotte Dai Kubicky and James A. Tanyi

Purpose: 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.

Methods and Materials: 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).

Results: 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 interfraction 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).

Conclusions: 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.

Keywords: SBRT, 4DCT, lung, intra-fraction, inter-fraction, tumor motion

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