Advantage of 3D volumetric dosemeter in delivery quality assurance of dynamic arc therapy: comparison of pencil beam and Monte Carlo calculations

Abstract

Objective: To evaluate the accuracy of pencil beam calculation (PBC) and Monte Carlo calculation (MCC) for dynamic arc therapy (DAT) in a cylindrically shaped homogenous phantom, by comparing the two plans with an ion chamber, a film and a three-dimensional (3D) volumetric dosemeter. Methods: For this study, an in-house phantom was constructed, and the PBC and MCC plans for DAT were performed using iPlan® RT (BrainLAB®, Heimstetten, Germany). The A16 micro ion chamber (Standard Imaging, Middleton, WI), Gafchromic® EBT2 film (International Specialty Products, Wayne, NJ) and ArcCHECK™ (Sun Nuclear, Melbourne, FL) were used for measurements. For comparison with each plan, two-dimensional (2D) and 3D gamma analyses were performed using 3%/3 mm and 2%/2 mm criteria. Results: The difference between the PBC and MCC plans using 2D and 3D gamma analyses was found to be 7.85%and 28.8%, respectively. The ion chamber and 2D dose distribution measurements did not exhibit this difference revealed by the comparison between the PBC and MCC plans. However, the 3D assessment showed a significant difference between the PBC and MCC (62.7% for PBC vs 93.4% for MCC, p 5 0.034).Conclusion: Evaluation using a 3D volumetric dosemeter can be clinically useful for delivery quality assurance (QA), and the MCC should be used to achieve the most reliable dose calculation for DAT. Advances in knowledge:(1) The DAT plan calculated using the PBC has a limitation in the calculation methods, and a 3D volumetric dosemeter was found to be an adequate tool for delivery QA of DAT. (2) The MCC was superior to PBC in terms of the accuracy in dose calculation for DAT even in the homogenous condition.Over the past few years, there has been a considerable growth in interest concerning arc therapy. This includes a range of techniques, such as dynamic arc therapy (DAT), intensity-modulated arc therapy (IMAT), helical tomotherapy and volumetric modulated arc therapy (VMAT). 1-3These developments have been accompanied by the improvement of dose calculation algorithms and a significant increase in treatment efficiency, thus resulting in shorter treatment times and a smaller number of monitor units (MUs) than with intensity-modulated radiotherapy (IMRT) and other conventional methods.Existing arc therapy planning methods, such as pencil beam calculation (PBC), are based on dividing the full rotation angle by a uniform control point and then optimizing the results. 4,5 Only the dose calculation based on a control point below a certain angle has been previously reported to be an accurate dose delivery to the target area.4,6-8 However, the errors from approximating the arc with individual beams could occur in the area where adjacent beams do not overlap. 9 This area could be wide when the phantom size is large or when the field or target size is small. Compared with PBC, Monte Carlo calculation (MCC) has made accurate dose calculations possible because the accuracy of the dose calculation using MCC depends on the number of simulation histories rather than the numb...