Interbreath-hold reproducibility of lung tumour position and reduction of the internal target volume using a voluntary breath-hold method with spirometer during stereotactic radiotherapy for lung tumours

The purpose of this study was to evaluate the interbreath-hold reproducibility of the tumour (gross tumour volume, GTV) position and relative reduction of the internal target volume (ITV) using a voluntary breath-hold method with a spirometer in a clinical setting of stereotactic radiotherapy (SRT) for lung tumours 11 patients with 14 lung tumours were enrolled in this study. CT scans were performed once at the free breathing phase and five times at the breath holding phase before the first treatment day. Patients held their breath at the end-expiration phase under spirometer-based monitoring. All GTVs were delineated by a physician and the GTV centroid was calculated automatically. To evaluate the interbreath-hold reproducibility of the tumour position, we measured the distance of three dimensions (craniocaudal, CC; left–right, LR; anteroposterior, AP) and vectors between the GTV centroid and bony landmark. The reproducibility was defined as the average of the differences between the GTV centroid and bony landmark from the second to fifth CT scans with regard to that from the first CT scans. We also evaluated the relative reduction of ITV between the free breathing and breath-holding phase. The interbreath-hold reproducibility of the tumour position was 1.3±1.3 mm, 1.4±1.8 mm, 2.1±1.6 mm and 3.3±2.2 mm in CC, LR and AP directions and vectors, respectively. ITV at the breath-holding phase was significantly smaller than that at the free breathing phase (P<0.01). In conclusion, the voluntary breath-hold method with a spirometer is feasible, with relatively good reproducibility of the tumour position for SRT in the clinical setting.