Control Point Analysis comparison for 3 different treatment planning and delivery complexity levels using a commercial 3-dimensional diode array

Abstract

To investigate the use of “Control Point Analysis” (Sun Nuclear Corporation, Melbourne, FL) to analyze and compare delivered volumetric-modulated arc therapy (VMAT) plans for 3 different treatment planning complexity levels. A total of 30 patients were chosen and fully anonymized for the purpose of this study. Overall, 10 lung stereotactic body radiotherapy (SBRT), 10 head-and-neck (H&N), and 10 prostate VMAT plans were generated on Pinnacle³ and delivered on a Varian linear accelerator (LINAC). The delivered dose was measured using ArcCHECK (Sun Nuclear Corporation, Melbourne, FL). Each plan was analyzed using “Sun Nuclear Corporation (SNC) Patient 6” and “Control Point Analysis.” Gamma passing percentage was used to assess the differences between the measured and planned dose distributions and to assess the role of various control point binning combinations. Of the different sites considered, the prostate cases reported the highest gamma passing percentages calculated with “SNC Patient 6” (97.5% to 99.2% for the 3%, 3 mm) and “Control Point Analysis” (95.4% to 98.3% for the 3%, 3 mm). The mean percentage of passing control point sectors for the prostate cases increased from 51.8 ± 7.8% for individual control points to 70.6 ± 10.5% for 5 control points binned together to 87.8 ± 11.0% for 10 control points binned together (2%, 2-mm passing criteria). Overall, there was an increasing trend in the percentage of sectors passing gamma analysis with an increase in the number of control points binned together in a sector for both the gamma passing criteria (2%, 2 mm and 3%, 3 mm). Although many plans passed the clinical quality assurance criteria, plans involving the delivery of high Monitor Unit (MU)/control point (SBRT) and plans involving high degree of modulation (H&N) showed less delivery accuracy per control point compared with plans with low MU/control point and low degree of modulation (prostate).

Introduction

Verification of treatment delivery, or quality assurance (QA), is a crucial stage in the radiation therapy treatment process.¹ In the last decade, several tools were developed and applied for delivery QA, including film²; diode arrays, such as MapCHECK (Sun Nuclear Corporation, Melbourne, FL)³; and ion chamber arrays, such as the PTW 2D-array seven29.⁴ Three-dimensional (3D) phantoms and dosimeters, such as 3D diode arrays, solid gels,⁵ and spiral-pattern radiographic films,⁶ have also been developed and studied recently. A new 3D diode array called ArcCHECK (Sun Nuclear Corporation, Melbourne, FL) was implemented for delivery QA. A recent study⁷ evaluated ArcCHECK and showed consistency of response of the individual diodes and minimal field size dependence.

The demand for better QA systems continues to increase. With the development of new treatment techniques and new radiation delivery systems, more intricate QA metrics and tools must be created. The most accepted metric that is currently used clinically in delivery QA is gamma passing percentage, which is based on the definition of gamma introduced by Low et al.⁸ However, recent studies9, 10, 11, 12 showed a lack of correlation between gamma passing percentage and dose differences in regions of interest.

Volumetric-modulated arc therapy (VMAT) is a modern treatment technique13, 14 that requires more sophisticated QA tools. In VMAT, dose delivery is spread over an arc or a subarc. Each arc is divided, during optimization, into a number of control points, each of which has its own multileaf collimator (MLC) pattern and dose weight. During delivery, it is extremely important to verify that the dose delivered per control point matches with that of the plan. Poor agreement between the dose delivered per control point and that of the plan might translate into poor agreement in clinical outcomes. This problem might be overlooked when comparing the composite delivered dose with the planned dose.¹⁵ Varian electronic portal imaging devices (Varian Medical Systems, Palo Alto, CA) have been used for VMAT QA.16, 17 Sun Nuclear (Sun Nuclear Corporation, Melbourne, FL) recently developed a new tool called “Control Point Analysis” that allows the verification of the dose delivered per control point. It is the first available QA tool that has this capability.

In this study, we used the “Control Point Analysis” tool to analyze and compare delivered plans for 3 different treatment planning complexity levels: (1) high MU per control point (MU per control point ≥ 15)—(L1); (2) high degree of modulation (the average open area difference between 2 consecutive control points is more than 3.5 cm² for the whole plan)—(L2); and (3) low MU per control point (MU per control point ≤ 5) plus low degree of modulation (the average open area difference between two consecutive control points is less than 1.5 cm² for the whole plan)—(L3). The consistency of the “Control Point Analysis” tool was examined by comparing its conventional passing percentage of gamma analysis with the “Sun Nuclear Corporation (SNC) Patient 6” results for the 3 treatment planning complexity levels defined earlier. The goal of this work is to study the effect of individual control point QA vs the traditional full arc VMAT QA for the 3 different treatment planning complexity levels.