Single Arc VMAT of H&N Cancer

Abstract

A few radiation treatment planning systems are currently able to plan volumetric modulated arc therapy (VMAT). The VMAT algorithm in Pinnacle3 is called SmartArc. The capability of SmartArc to generate complex treatment plans for the head and neck (H&N) region was tested. This retrospective planning study includes patients with oropharyngeal or hypopharyngeal cancer previously treated with IMRT by utilization of the Pinnacle3™ treatment planning system and Elekta Synergy accelerators. Doses between 50Gy and 68Gy were prescribed to individual Planning Target Volumes (PTV). The patients were re-planned with VMAT using the SmartArc algorithm in Pinnacle3™ 8.9c (research version). A constraint for the re-planning was to use only one single arc to create deliverable plans. The objectives were to achieve identical or better target coverage and sparing of the organs at risk (OAR) using VMAT compared to IMRT. The comparison was made by evaluating of 1) dose-volume histograms (DVHs) of PTVs and OARs, 2) monitor units, 3) treatment time, and 4) treatment accuracy as delivered on Elekta Synergy accelerators. Preliminary results based on 11 patients showed that single arc VMAT plans satisfied all the re-planning objectives. The average maximum dose to the spinal cord was 42.1±0.7Gy and 43.4±0.5Gy for the VMAT and IMRT plans, respectively. V26 of the parotic glands was 45±4% with VMAT and 48±5% with IMRT. SmartArc generated VMAT treatment plans with an average conformity index (V95%/Vtarget) of 1.56±0.04 improving the conformity index of 1.71±0.07 by IMRT. This significantly reduced the size of high dose volumes (volumes which received more than 15Gy) in the surrounding normal tissues. VMAT reduced the number of MUs by 10% from mean 503±20MU to 449+/-22MU using IMRT and VMAT, respectively. Delivering the VMAT plans on an Elekta Synergy accelerator the mean treatment time was 250s which was a 40 percent reduction compared to IMRT. Dose measurements of the VMAT treatments with a Delta4 phantom (Scandidos) showed that 99.1±0.1% of the detector points - when corrected for offset in output - satisfied a gamma criterion of 3mm and 3% of 2Gy compared to 99.7±0.1% for IMRT. SmartArc generated single arc VMAT plans with improved target coverage and sparing of OARs compared to IMRT. The conformity was increased by VMAT compared to IMRT reducing high dose volumes in normal tissues. The VMAT plans used fewer MUs compared to the IMRT plans and reduced the treatment time by approximately 40% compared to IMRT. Both IMRT and VMAT radiation treatment plans delivered on an Elekta Synergy accelerator produced clinically acceptable geometric dose precision.