Exhibit Hall | Forum 6
Purpose: Radiotherapy for the lung cancer treatment is often planned on the average-intensity image set (AVE) derived from 4DCT, in which organs at risk, namely, the left ventricle (LV) structure is blurred due to breathing/cardiac motion and the lack of contrast inhibits true delineation of LV myocardium (LVM) and exclusion of blood pool. In a first study of its kind, we assessed the relative effects of cardiac/respiratory motion in estimating radiotherapy dose to the LV and LVM.
Methods: 8 lung cancer patients undergoing radiotherapy were recruited for this study. Each patient underwent a 4DCT (10 breathing phases), an exhale breath-hold T1-VIBE, and an exhale breath-hold cine MR (25 cardiac phases). LV/LVW were contoured on 4DCT/cine, respectively. The dose from the planning AVE image set was propagated via the following workflows: (A) every respiratory phase: AVE_CT→0%-90% 4DCT respiratory phases. (B) every cardiac phase (at exhale): AVE_CT→Exh_CT (50% phase)→Exh_T1-VIBE→Exh_average_cine_MRI→cine_MRI cardiac phases. (C) cine arbitrary reference-phase (cumulative LVM dose over cardiac cycle): Cine_MRI cardiac phases→Cine_MRI arbitrary reference-phase (via deformable image registration). All dose values were normalized by the mean dose from AVE_CT.
Results: The normalized max/mean dose (±standard deviations) for the LV respiratory phases and LVM cardiac phases ranged between 4.9(±2.2)-5.8(±2.3)/0.9(±0.1)-1.2(±0.1) and 5.5(±2.2)-8.3(±4)/ 1.1(±0.1)-1.4(±0.3), respectively. At exhale, the normalized max/mean dose from the accumulated dose on cine MR at LVM (cardiac motion only) was insignificantly higher than the dose on non-cardiac-gated 50%-CT at the LV: 5.9(±2.6)/1.3(±0.2) vs 5.2(±2.3)/1.1(±0.1), p>0.17.
Conclusion: The obtained results suggest that the dose distribution on the LV/LVM can differ during various respiratory/cardiac phases. Due to a better LVM delineation (exclusion of blood pool) and dose-accumulation based on cardiac-motion only, Cine MRI application seems to be beneficial in dose-assessments compared to non-cardiac-gated CT only. This finding requires careful consideration when evaluating the risk of cardiotoxicity secondary to radiotherapy.