# Impact of respiratory motion on dose to the airways in central and ultra-central lung stereotactic ablative body radiotherapy

## TL;DR

Respiratory motion can significantly increase radiation dose to lung airways during SABR, potentially exceeding safety limits in some patients.

## Contribution

The study shows that standard planning methods may mask dose violations in central lung tumors due to breathing motion.

## Key findings

- Airway D0.1 cm3 doses increased by up to 5 Gy when respiratory motion was considered.
- 28% of patients had dose differences exceeding 2 Gy due to breathing motion.
- A 5 mm PRV was inadequate in 5 of 18 patients when motion was accounted for.

## Abstract

•Accounting for respiratory motion caused airway D0.1 cm3 increases of up to 5 Gy.•D0.1 cm3 differences exceeded 2 Gy in 28% of the study cohort.•Tumours moving over 6 mm showed the largest dose discrepancies.•Standard planning approaches masked dose violations in 5 of 18 patients.

Accounting for respiratory motion caused airway D0.1 cm3 increases of up to 5 Gy.

D0.1 cm3 differences exceeded 2 Gy in 28% of the study cohort.

Tumours moving over 6 mm showed the largest dose discrepancies.

Standard planning approaches masked dose violations in 5 of 18 patients.

Stereotactic ablative body radiotherapy (SABR) achieves excellent local control in medically inoperable non-small cell lung cancer. However, treating central and ultra-central tumours carries significant risks to adjacent critical structures, notably the central airways. This study evaluated the suitability of average value intensity projection (AvIP) imaging for airway delineation and assessed the adequacy of a 5 mm planning risk volume (PRV), by quantifying the influence of respiratory motion on dose to the airways using four-dimensional (4D) dose accumulation.

Eighteen patients (11 ultra-central, 7 central) treated with 60 Gy in 8 fractions were retrospectively analysed. Clinical 4D computed tomography (CT) datasets were used to capture 10 respiratory phases. Central airway contours (trachea, main bronchi, bronchial tree) were delineated on the AvIP and deformably propagated across all phases. Dose was recalculated per phase and accumulated back into a single distribution using the deformable registration for comparison with the AvIP-based plan.

Maximum differences of 5 Gy were observed for the airway D0.1 cm3, with an average absolute difference of 0.95 ± 1.7 Gy. Notably, 28% of patients had differences >2 Gy, and 17% exceeded 3 Gy. The greatest dose discrepancies occurred in cases with tumours inferior to the first-order bronchi and with breathing motion >6 mm. Five patients’ plans exceeded dose–volume constraints when respiratory motion was accounted for.

Dose to the airways can be significantly underestimated when motion is not incorporated into treatment planning. A uniform 5 mm PRV may be inadequate in some central/ultra-central SABR cases. Patient-specific internal risk volumes merit investigation.