The role of VMAT interplay effects for liver stereotactic body radiation therapy

摘要

Stereotactic body radiation therapy (SBRT) is beneficial in the treatment of solid liver cancers as it safely delivers an ablative tumor dose. Volumetric modulated arc therapy (VMAT) is an ideal modality for hypofractionated treatment regimes such as SBRT as it can efficiently deliver a conformal dose. However, treatment accuracy becomes compromised for liver tumors due to their inherent respiratory motion. Clinicians have raised concerns when treating moving targets with VMAT because the tumor motion relative to the dynamically moving MLCs degrades the plan quality; a phenomenon termed interplay. The purpose of this study is to examine if the amount of beam modulation and breathing cycle length elicits interplay effects that influence plan quality for liver SBRT treated with VMAT. Respiratory motion was modeled for a series of VMAT plans with varying degrees of modulation using treatment planning software. IMRT modulation factors (MFs) (MUs/fraction dose in cGy) ranged from 1.5-3.5 and respiratory cycle lengths of 3, 5, 10 and 20 seconds were simulated for all plans. Minor interplay effects were observed and deemed clinically negligible: the 95% dose coverage of the target was not compromised. A slight degradation in the plan quality (size of 95% isodose) was detected and worsened with longer respiratory cycle lengths and lower MFs. This is attributed to the variation in gantry speed that was coupled to ihe MF. For high MFs (slower gantry speed), more time was available for the tumor motion to average out during each control point and as a result reducing interplay. We concluded in this limited phantom study that treatment plan quality of liver SBRT is degraded the least by plans with high MFs. Plan fidelity was maintained via few partial arcs (to obtain a slow gantry speed) and adequate PTV margins that accounted for respiration.