Repeated deep-inspiration breath-hold CT scans at planning underestimate the actual motion between breath-holds at treatment for lung cancer and lymphoma patients.

/objective Deep-inspiration breath-hold (DIBH) during radiotherapy may reduce dose to the lungs and heart compared to treatment in free breathing. However, intra-fractional target shifts between several breath-holds may decrease target coverage. We compared target shifts between four DIBHs at the planning-CT session with those measured on CBCT-scans obtained pre- and post-DIBH treatments. Material/methods Twenty-nine lung cancer and nine lymphoma patients were treated in DIBH. An external gating block was used as surrogate for the DIBH-level with a window of 2mm. Four DIBH CT-scans were acquired: one for planning (CT) and three additional (CT) to assess the intra-DIBH target shifts at scanning by registration to CT. During treatment, pre-treatment (CBCT) and post-treatment (CBCT) scans were acquired. For each pair of CBCT, the target intra-DIBH shift was determined. For lung cancer, tumour (GTV-T) and lymph nodes (GTV-N) were analysed separately. Group mean (GM), systematic and random errors, and GM for the absolute maximum shifts (GM) were calculated for the shifts between CT and between CBCT. Results For GTV-T, GM was larger at CBCT than CT in all directions. GM in cranio-caudal direction was 3.3mm (CT)and 6.1mm (CBCT). The standard deviations of the shifts in the left-right and cranio-caudal directions were larger at CBCT than CT. For GTV-N and CTV, no difference was found in GM or SD. Conclusion Intra-DIBH shifts at planning-CT session are generally smaller than intra-DIBH shifts observed at CBCT and therefore underestimate the intra-fractional DIBH uncertainty during treatment. Lung tumours show larger intra-fractional variations than lymph nodes and lymphoma targets.