Evaluation of a reproducible breath hold technique for the SABR treatment of lower lobe lung tumours

Aim Deep inspiration breath hold (DIBH) is a method of motion management used in stereotactic ablative body radiotherapy (SABR) for lung tumours. An external gating block marker can be used as a tumour motion surrogate, however, inter-fraction gross target volume (GTV) displacement within DIBH occurs. This study measured this displacement during a reproducible breath hold regime. Additionally, factors such as position of the gating block marker were analysed. Methods and Materials 121 cone beam computed tomography scans (CBCTs) from 22 patients who received DIBH SABR were retrospectively evaluated and the magnitude of inter-fraction GTV displacement was calculated for each fraction. This data was analysed to assess if any correlation existed between tumour displacement and variation in the gating block marker position on the patient, the amplitude of BH at CT, the amplitude of BH at treatment and the tumour location. The measured tumour displacement was applied to the original planning CT to evaluate the dosimetric effect on surrounding organs at risk (OARs) using cumulative dose volume histograms (DVHs). Results BH amplitude was reproducible within 0.13 cm ± 0.1 cm (mean ± standard deviation). The magnitude of tumour displacement within BH ranged from 0 to 1.52 cm (0.41 cm ± 0.28 cm). Displacement in the superior-inferior (SI), anterior-posterior (AP) and left-right (LR) planes were 0.31cm ± 0.26 cm, 0.16 cm ± 0.18 cm and 0.07 cm ± 0.12 cm respectively. No statistically significant correlation was detected between tumour displacement within DIBH and the factors investigated. The range of variation in OAR dose was -7.0Gy to +3.6Gy with one statistically significant increase in OAR dose observed (oesophagus mean dose increasing by 0.16Gy). Findings Reproducible BH was achievable across a range of patients. Inter-fraction GTV displacement measured 0.41 cm ± 0.28 cm. Due to this low level of motion, the correction of soft tissue moves did not adversely affect OAR dose.