Clinical evaluation of an anatomy‐based patient specific quality assurance system

Pascal Hauri,Stephanie Lang,Shaun Graydon,Sarah Verlaan,Linda Ahnen,Stephan Klöck

doi:10.1120/jacmp.v15i2.4647

Abstract

The Delta4DVH Anatomy 3D quality assurance (QA) system (ScandiDos), which converts the measured detector dose into the dose distribution in the patient geometry was evaluated. It allows a direct comparison of the calculated 3D dose with the measured back‐projected dose. In total, 16 static and 16 volumetric‐modulated arc therapy (VMAT) fields were planned using four different energies. Isocenter dose was measured with a pinpoint chamber in homogeneous phantoms to investigate the dose prediction by the Delta4DVH Anatomy algorithm for static fields. Dose distributions of VMAT fields were measured using GAFCHROMIC film. Gravitational gantry errors up to 10° were introduced into all VMAT plans to study the potential of detecting errors. Additionally, 20 clinical treatment plans were verified. For static fields, the Delta4DVH Anatomy predicted the isocenter dose accurately, with a deviation to the measured phantom dose of 1.1%±0.6%. For VMAT fields the predicted Delta4DVH Anatomy dose in the isocenter plane corresponded to the measured dose in the phantom, with an average gamma agreement index (GAI) (3 mm/3%) of 96.9±0.4%. The Delta4DVH Anatomy detected the induced systematic gantry error of 10° with a relative GAI (3 mm/3%) change of 5.8%±1.6%. The conventional Delta4PT QA system detected a GAI change of 4.2%±2.0%. The conventional Delta4PT GAI (3 mm/3%) was 99.8%±0.4% for the clinical treatment plans. The mean body and PTV‐GAI (3 mm/5%) for the Delta4DVH Anatomy were 96.4%±2.0% and 97.7%±1.8%; however, this dropped to 90.8%±3.4% and 87.1%±4.1% for passing criteria of 3 mm/3%. The anatomy‐based patient specific quality assurance system predicts the dose distribution correctly for a homogeneous case. The limiting factor for the error detection is the large variability in the error‐free plans. The dose calculation algorithm is inferior to that used in the TPS (Eclipse).PACS numbers: 87.56.Fc, 87.56.‐v

Highlights

Patient-specific quality assurance (QA) for intensity-modulated techniques is often performed by comparing measured with calculated dose distributions in a QA phantom
The dose calculation algorithm is inferior to that used in the treatment planning system (TPS) (Eclipse)
An advantage compared to conventional QA is that the gamma agreement index (GAI) can be individually evaluated based on the patient body, the planning target volume (PTV), and organs at risk (OAR)

Summary

Introduction

Patient-specific quality assurance (QA) for intensity-modulated techniques is often performed by comparing measured with calculated dose distributions in a QA phantom. A solution might be a patient anatomy-based QA system The principle of such a system is to measure the dose of the patient plan as for conventional QA in a phantom, and remap the measured dose distribution to the CT dataset of the patient. An advantage compared to conventional QA is that the GAI can be individually evaluated based on the patient body, the planning target volume (PTV), and organs at risk (OAR). Organ specific dose-volume histograms (DVH) can be compared with the TPS predicted DVHs (Fig. 1). This allows discussion regarding potentially necessary clinical plan adjustments with the clinicians, in case of poor agreement in the patient’s DVH Anatomy analysis. Patient anatomy-based QA systems could replace independent monitor units (MU) checks in the patient geometry

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Discussion

Conclusion