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

Abstract

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. 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 2 mm. Four DIBH CT-scans were acquired: one for planning (CT_DIBH3) and three additional (CT_DIBH1,2,4) to assess the intra-DIBH target shifts at scanning by registration to CT_DIBH3. During treatment, pre-treatment (CBCT_pre) and post-treatment (CBCT_post) scans were acquired. For each pair of CBCT_pre/post, the target intra-DIBH shift was determined. For lung cancer, tumour (GTV-T_lung) and lymph nodes (GTV-N_lung) were analysed separately. Group mean (GM), systematic and random errors, and GM for the absolute maximum shifts (GM_max) were calculated for the shifts between CT_DIBH1,2,3,4 and between CBCT_pre/post. For GTV-T_lung, GM_max was larger at CBCT than CT in all directions. GM_max in cranio-caudal direction was 3.3 mm (CT)and 6.1 mm (CBCT). The standard deviations of the shifts in the left-right and cranio-caudal directions were larger at CBCT than CT. For GTV-N_lung and CTV_lymphoma, no difference was found in GM_max or SD. Intra-DIBH shifts at planning-CT session are generally smaller than intra-DIBH shifts observed at CBCT_pre/post and therefore underestimate the intra-fractional DIBH uncertainty during treatment. Lung tumours show larger intra-fractional variations than lymph nodes and lymphoma targets.