A Planning Comparison of 3D Conformal Multiple Static Field, Conformal Arc, and Volumetric Modulated Arc Therapy for the Delivery of Stereotactic Body Radiation Therapy for Lung Cancer

Utilization of Stereotactic Body Radiation Therapy (SBRT) for early stage non-small cell lung cancer (NSCLC) has increased. A number of different treatment delivery techniques are clinically utilized. The primary objective of this study was to compare treatment time, monitor units and dosimetric non-inferiority of 3D conformal multiple static field (MSF) to conformal arc (CA) and volumetric modulated arc therapy (VMAT) plans. This retrospective study identified 23 NSCLC patients previously treated with SBRT using a MSF technique. Plans consisted of 9-11 static coplanar and non-coplanar fields. Each patient was replanned using CA and VMAT techniques. The ratio of the prescription isodose volume to PTV volume (R100%), the maximum dose 2cm away from the PTV (D2cm), percentage of lung receiving 20Gy or more (V20Gy), van’t Riet conformity number (CN), total monitor units (MU) and treatment time were evaluated. Non-inferiority comparisons to detect significance were performed using a two-tailed paired t-test. A p-value of 0.017 (0.05/3) was used to adjust for multiple hypothesis testing. The VMAT plans had significantly lower R100% than either the MSF or CA, and the CA plans were significantly better than MSF plans [mean difference: MSF-v-CA = 0.07 (p<0.0001); MSF-v-VMAT = 0.15 (p<0.0001); CA-v-VMAT = 0.08 (p<0.0001)]. Looking at D2cm, the VMAT plans had significantly lower values than either the MSF or CA, and the CA plans were significantly better than MSF plans [mean difference (percentage of total dose): MSF-v-CA = 2.73 (p = 0.005); MSF-v-VMAT = 4.80 (p<0.0001); CA-v-VMAT = 2.08 (p = 0.006)]. Comparing the van’t Riet conformity numbers, the higher VMAT values were significantly better than either CA or MSF plans, while again the CA plans were better than MSF plans [mean difference: MSF-v-CA = -0.5 (p<0.0001); MSF-v-VMAT = -0.11 (p<0.0001); CA-v-VMAT = -0.06 (p<0.0001)]. Total monitor units for the plans revealed that VMAT had the highest MUs, and CA had the lowest, with significant differences between all three techniques [mean MUs: 2687, 2164 and 2072 (p<0.0001)]. Evaluation of treatment times (not including imaging) demonstrated that conformal arcs were the quickest to deliver, significantly faster than VMAT or MSF [mean treatment time: 3.91 min, 4.73 min and 8.56 min respectively (p<0.0001)]. This study supports the use of CA for lung SBRT in eligible patients with significant dosimetric advantages over multiple static fields. VMAT demonstrated additional gains except increased monitor units and treatment time. Considering the additional optimization time during planning, treatment delivery verification, dedicated machine requirements and conflicting reports on the safe use of hypofractionated VMAT in the thorax, conformal arcs provide an adequate alternative to MSF for delivering SBRT.