Volumetric Modulated Arc Therapy for Stereotactic Body Radiation Therapy: Initial Clinical Experience

Abstract

Volumetric modulated arc therapy (VMAT) is a relatively new treatment modality characterized by variable angular dose rate dynamic arc delivery. Our institution is one of the first to apply this technique clinically for treatment of stereotactic body radiation therapy (SBRT). The purpose of this work is to present our initial clinical experience with VMAT for SBRT of the lung and spine. Six SBRT patients (3 lung, 3 spine) have been treated with VMAT under IRB approved institutional protocols. Each patient had a conformal or IMRT plan created for comparison purposes. Patients were selected based on expected tolerable treatment time, quality of the initial IMRT or 3DCRT plan, time available between simulation and treatment, and physician preference. Lung SBRT was prescribed as 48-60 Gy in 4-5 Fx, and spine SBRT was prescribed as 15-18 Gy in 1 Fx. Planning time, plan quality, QA procedure and results, and treatment efficiency for VMAT vs. conventional techniques were evaluated. Seven courses of SBRT were delivered to 6 patients (1 lung patient had 2 isocenters) using VMAT. Planning time was increased by approximately 30% and plan quality was equivalent or superior for VMAT in all patients compared to 3DCRT or IMRT. In 2 lung cases, VMAT reduced the lung and/or heart dose low enough to allow SBRT treatment that would not have been possible due to DVH constraints. PTV D95 was increased by 0.8-3.1%, and GTV mean dose was increased by an average of 3.0% for the 4 lesions treated with VMAT vs. 3DCRT. Lung V20, V12.5, and mean dose were lower in all patients with max reductions in V20 and V12.5 of 2.6% and 5.8%, respectively. In 3 spine patients, OAR doses met protocol guidelines for both techniques, but PTV D90 was improved by 1.8-3.4%, and GTV and PTV mean doses increased by 1.1-4.3% when using VMAT vs. IMRT. QA on a cylindrical diode phantom showed excellent results for all techniques (> 95% passing, 3% / 2 mm gamma). QA time was increased with VMAT compared to our standard planar dose QA for IMRT. MU were increased slightly with VMAT vs. 3DCRT in lung SBRT (0.4-5.3%), but treatment times were reduced by 44-59% (mean = 7.9 min). MU were reduced with spine VMAT vs. IMRT by up to 28%, and treatment times were reduced by 37-52% (mean = 9 min). We have treated 6 lung or spine SBRT patients with VMAT. Compared with alternative IMRT or conformal plans, VMAT plans had equivalent or better plan quality, and the treatment times were reduced up to 13 min or 59%. Despite an increase in time required for planning and QA, VMAT is an excellent option for treatment using SBRT. In addition to the positive impact on patient comfort and clinic workload, treatment time reductions also reduce the potential for intrafraction motion and may make it possible for patients to better tolerate SBRT times.