Correlation of phantom-based and log file patient-specific QA with complexity scores for VMAT.

Christina E Agnew,Conor K Mcgarry,Denise M Irvine

doi:10.1120/jacmp.v15i6.4994

Abstract

The motivation for this study was to reduce physics workload relating to patient‐specific quality assurance (QA). VMAT plan delivery accuracy was determined from analysis of pre‐ and on‐treatment trajectory log files and phantom‐based ionization chamber array measurements. The correlation in this combination of measurements for patient‐specific QA was investigated. The relationship between delivery errors and plan complexity was investigated as a potential method to further reduce patient‐specific QA workload. Thirty VMAT plans from three treatment sites — prostate only, prostate and pelvic node (PPN), and head and neck (H&N) — were retrospectively analyzed in this work. The 2D fluence delivery reconstructed from pretreatment and on‐treatment trajectory log files was compared with the planned fluence using gamma analysis. Pretreatment dose delivery verification was also carried out using gamma analysis of ionization chamber array measurements compared with calculated doses. Pearson correlations were used to explore any relationship between trajectory log file (pretreatment and on‐treatment) and ionization chamber array gamma results (pretreatment). Plan complexity was assessed using the MU/ arc and the modulation complexity score (MCS), with Pearson correlations used to examine any relationships between complexity metrics and plan delivery accuracy. Trajectory log files were also used to further explore the accuracy of MLC and gantry positions. Pretreatment 1%/1 mm gamma passing rates for trajectory log file analysis were 99.1% (98.7%–99.2%), 99.3% (99.1%–99.5%), and 98.4% (97.3%–98.8%) (median (IQR)) for prostate, PPN, and H&N, respectively, and were significantly correlated to on‐treatment trajectory log file gamma results (R=0.989,p<0.001). Pretreatment ionization chamber array (2%/2 mm) gamma results were also significantly correlated with on‐treatment trajectory log file gamma results (R=0.623,p<0.001). Furthermore, all gamma results displayed a significant correlation with MCS (R>0.57,p<0.001), but not with MU/arc. Average MLC position and gantry angle errors were 0.001±0.002mm and 0.025°±0.008° over all treatment sites and were not found to affect delivery accuracy. However, variability in MLC speed was found to be directly related to MLC position accuracy. The accuracy of VMAT plan delivery assessed using pretreatment trajectory log file fluence delivery and ionization chamber array measurements were strongly correlated with on‐treatment trajectory log file fluence delivery. The strong correlation between trajectory log file and phantom‐based gamma results demonstrates potential to reduce our current patient‐specific QA. Additionally, insight into MLC and gantry position accuracy through trajectory log file analysis and the strong correlation between gamma analysis results and the MCS could also provide further methodologies to both optimize the VMAT planning and QA process.PACS number: 87.53.Bn, 87.55.Kh, 87.55.Qr

Highlights

Volumetric-modulated arc therapy (VMAT) offers equivalent or higher conformity to target volumes and faster delivery times compared to step and shoot or dynamic intensity-modulated radiotherapy (IMRT).(1-2) VMAT is a system for intensity-modulated radiotherapy treatment delivery that achieves high dose conformality by optimizing the dose rate, gantry speed, and the leaf positions of the dynamic multileaf collimator
The delivery of VMAT is inherently more complex than fixed gantry IMRT deliveries. This additional complexity has led to recommendations for additional machine-specific commissioning tests.[3,4] 2D patient-specific quality assurance (QA) methods used for IMRT delivery verification have been found adequate for verifying VMAT delivery accuracy.[5,6,7] Despite this, 3D verification procedures have been adopted into routine use to readily assess the effect of the rotating gantry on plan delivery.[8,9,10]
A number of complexity metrics have been proposed for step-and-shoot IMRT[29,31,32] and, of these, the modulation complexity score (MCS), which assesses the variability of leaf positions and aperture areas between segments, was found to be most sensitive to delivery and plan parameters.[33]. The MCS has since been successfully adapted for assessment of planning parameters on VMAT plan complexity;(30) the relationship between VMAT plan complexity and deliverability has not yet been explored in the literature

Summary

Introduction

Volumetric-modulated arc therapy (VMAT) offers equivalent or higher conformity to target volumes and faster delivery times compared to step and shoot or dynamic intensity-modulated radiotherapy (IMRT).(1-2) VMAT is a system for intensity-modulated radiotherapy treatment delivery that achieves high dose conformality by optimizing the dose rate, gantry speed, and the leaf positions of the dynamic multileaf collimator. The delivery of VMAT is inherently more complex than fixed gantry IMRT deliveries This additional complexity has led to recommendations for additional machine-specific commissioning tests.[3,4] 2D patient-specific quality assurance (QA) methods used for IMRT delivery verification have been found adequate for verifying VMAT delivery accuracy.[5,6,7] Despite this, 3D verification procedures have been adopted into routine use to readily assess the effect of the rotating gantry on plan delivery.[8,9,10]. The MCS has since been successfully adapted for assessment of planning parameters on VMAT plan complexity;(30) the relationship between VMAT plan complexity and deliverability has not yet been explored in the literature

Methods

Results

Discussion

Conclusion