Session Item

Stereotactic body radiotherapy (SBRT) for lung tumors is often performed during free-breathing, irradiating an internal target volume (ITV) incorporating all motion. If respiratory tumor motion is significant (e.g. ≥15mm) this can result in large target volumes and irradiation of more lung, increasing the chance of toxicity. In addition the tumor boundaries can be less clear on a 3D CBCT-scan, especially when the target has a low density, which makes image registration for setup more difficult. Deep inspiration breath hold (DIBH) may reduce the irradiated lung volume, butDIBH is not feasible for all lung SBRT patients and the tumor position can vary between breath holds. Therefore we implemented free-breathing expiration gating in our department. The free breathing removes any burden associated with breath-hold and generally, the tumor moves considerably less in the 50% expiration phases than in the 50% inspiration phases, reducing the target volume. We evaluated the treatment time, ITV and residual motion of expiration gating in lung SBRT patients.

Expiration gating was performed in 10 lung SBRT patients treated on a TrueBeam® linac. Patients were treated with VMAT to a total dose of 30-60Gy in 1-8 fractions. Typically, the 30-70% phases were used for treatment but this was patient dependent. At the LINAC, amplitude gating was performed based on the motion of an external marker block (RPM) after translating the phases to RPM amplitude thresholds. Positional set-up was performed using a gated CBCT prior to the 1^st arc, and 1-2 more between arcs. During CBCT acquisition, online tumor position monitoring, using non-clinical software (RTR), consisting of template matching of each kV image followed by triangulation, was used to confirm the motion of the tumor in the gating window. ITV size was compared to that for a non-gated treatment. The times for imaging and delivery are reported. 

Longitudinal tumor motion on the 10-phase (0-90%) 4DCT was 16-31mm (mean 18±6mm).  For the selected expiration gating phases the longitudinal tumor motion reduced to 1-7 mm (mean 4±4mm). The ITV on all phases was 1.2-65cm³, mean=15±20cm³. For the expiration gating phases the ITV was 0.7-50cm³ (mean 10±15): a mean reduction of 38%. The total time from first set-up imaging to the end of the last arc was 6-66 minutes (mean 19.4±10.8). 80% of the treatments required 24 minutes or less. RTR tracking showed that motion during gated CBCT acquisition was in good agreement with motion during the gating phases on the planning CT, and RTR positional verification corresponded with the average CBCT shifts. 

Expiration gating has been successfully applied for lung SBRT. It achieved a clinically relevant reduction in ITV size and longitudinal tumor motion compared with non-gated free breathing. Average treatment time was clinically acceptable and the expiration phase led to a reproducible tumor position.