3133 - Comparison of RapidArc Dynamic vs VMAT for Liver Stereotactic Body Radiation Therapy

Purpose/Objective(s): SBRT intends to deliver ablative doses to primary liver cancers and oligometastases while minimizing dose to organs-at-risk (OARs). This includes reducing the mean liver dose to ensure that >700 cc of healthy liver receives <15 Gy to allow for an adequate functional reserve to remain. RapidArc Dynamic (RAD) is a new treatment modality that combines volumetric arc therapy (VMAT) with a dynamic collimator and static angle modulated ports (STAMPs). Early studies showed equivalent or better target coverage and improved OAR sparing compared to standard VMAT for some disease sites. This study provides the first report of RAD for liver SBRT and compares key dosimetric constraints against that of clinical VMAT plans.

Materials/Methods: All plans were created using Varian Eclipse for a Varian TrueBeam with either 6X-FFF or 10X-FFF beam energy and a high-definition MLC (HDMLC). 10 clinical liver SBRT plans (4 HCC, 5 metastases, 1 metastatic cholangiocarcinoma) previously planned with VMAT (v16.1) were replanned using RAD and VMAT (v18.1). Dose calculations were performed using AcurosXB (16.1.2 or 18.1.1) with 1 mm calculation grid. 10 plans per patient were compared: the clinical v16.1 VMAT, a reoptimized v18.1 VMAT plan and 8 unique RAD plans. Table 1 details the combinations of arc(s), STAMPs, avoidance sectors and weightings used for the VMAT and RAD plans. Dose fractionations included 45Gy/3 (6), 42Gy/3 (1), 36Gy/3 (2) and 31.25Gy/5 (1). VMAT plans used 2 coplanar arcs. RAD plans used the same beam energy, arc range and clinical constraints of the VMAT plans. Evaluated constraints included the volume of normal liver (liver-GTV) receiving <15 Gy, mean dose to liver, left and right kidneys, maximum dose to duodenum, bowel, esophagus and heart, the spinal cord R50 and total monitor units. A paired t-test was performed to identify significant differences between the VMAT and RAD for each OAR constraint.

Results: A RAD technique with 2 partial arcs, +2 static dominant weighting and no avoidance sectors was found to be optimal and superior to VMAT. This RAD technique yielded a statistically significant (p<0.05) reduction in mean liver-GTV (7.9Gy-7.2Gy, p=0.001), the volumes receiving <15Gy (232 cc - 207 cc, p=0.001) and <10Gy (25.6% - 22.8%, p=0.017) and RAD reduced the maximum dose (0.3 cc) of heart, bowel and spinal cord PRV. While not statistically significant (p=0.088), RAD reduced the spinal cord R50. RAD MUs increased an average of 69% (19.1%-125.5%) compared to VMAT.

Conclusion: Compared to VMAT, RAD demonstrated a statistically significant improvement in key dose metrics for liver, heart, spinal cord PRV and reduced R50 for this challenging population. Further study is warranted, in a larger patient population, to confirm which OARs will most greatly benefit from the new RAD technology.