Intra-fractional uncertainties in cone-beam CT based image-guided radiotherapy (IGRT) of pulmonary tumors

PurposeIntra-fractional variability of tumor position and breathing motion was evaluated in cone-beam CT (CB-CT) based image-guided radiotherapy (IGRT) of pulmonary tumors.Materials and methodsTwenty-four patients (27 lesions: prim. NSCLC n = 6; metastases n = 21) were treated with stereotactic body radiotherapy (SBRT) (one to eight fractions). Prior to every treatment fraction (n = 66) and immediately after treatment a CB-CT was acquired. Patient motion, absolute drift and drift of the tumor relative to the bony anatomy were measured. Tumor motion was investigated based on the density distribution in the CB-CT.ResultsAbsolute intra-fractional drift (3D vector) of the tumor position was 2.8 mm ± 1.6 mm (mean ± SD), maximum 7.2 mm. Poor correlation between patient motion and absolute tumor drift was observed. Changes of the tumor position due to patient motion and due to drifts independently from the bony anatomy were of similar magnitude with 2.1 mm ± 1.4 mm and 2.3 mm ± 1.6 mm, respectively. No systematic increase or decrease of breathing motion was seen. The intra-fractional change of breathing motion was more than 2 mm and 3 mm in 39% and 16%, respectively.ConclusionIntra-fractional tumor position and breathing motion were stable. In IGRT of pulmonary tumors we suggest an ITV-to-PTV margin of 5 mm to compensate intra-fractional changes.