Effect of Multiple Arcs on Planned Dose Distributions Delivered using Volumetric Modulated Arc Techniques - A Treatment Planning Study

Abstract

A preliminary study has been carried out, using a development version of the Philips Pinnacle treatment planning system, into the dosimetric effects of increasing the number of delivered arcs in Volumetric Modulated Arc treatments (VMAT) for a series of lesions within the head and neck. Three head-and-neck cases were chosen, which had previously been treated to multiple dose levels with a simultaneous integrated boost technique using fixed-angle, step-and-shoot IMRT on an Elekta linac at 6MV. These were replanned using the SmartArc VMAT inverse planning option within Pinnacle (v8.9). Plans were created with 1, 2 or 3 arcs, and then re-optimized with the aim of achieving equivalent plans with VMAT. Analysis of the resulting DVHs allowed comparison of plans for the following values; PTV maximum (1cc) and minimum dose, spinal cord and brainstem maximum (1cc) dose, and the patient volume receiving 5Gy. Of the three cases evaluated, the maximum PTV dose was similar for all plans. However moving from 1 to 2 arcs increased the minimum dose to the high-dose boost PTV on average by 1.9Gy (1.6-2.2Gy), and from 1 to 3 arcs by 4.6Gy (2.6-6Gy). When compared to the minimum dose in the boost PTV volume achieved by the IMRT plan, 1 VMAT arc often resulted in a reduction in coverage, on average by 2.4Gy (-0.06-4.35Gy). The maximum dose to critical OARs was reduced by increasing the number of VMAT arcs from 1 to 2 arcs. On average the maximum spinal cord dose was reduced 5.7Gy (5.4-6Gy) and 3.2Gy (2.6-3.8Gy) to the brainstem for the cases considered. The reduction in OAR dose was less pronounced when employing 3 arcs as the required level of dose sparing to the OARs was already achieved with 2 arcs. In the low dose region, the mean increase in total volume irradiated to 5Gy using VMAT with 3 arcs, compared to the clinical IMRT plan, was only 3.9% (1.3-8.9%). Delivery times for VMAT plans ranged from 75secs for a single arc to approximately 4 mins for the delivery of 3 arcs. For complex head and neck cases, VMAT can provide comparable treatment plans to those delivered using conventional IMRT if a minimum of 2 arcs per fraction are used. This can be achieved without a significant increase in the patient volume irradiated to low dose, compared to a conventional step-and-shoot IMRT technique. VMAT offers the potential for good quality treatment plans with delivery times significantly reduced from traditional IMRT techniques.