Development and evaluation of an end-to-end test for head and neck IMRT with a novel multiple-dosimetric modality phantom.

Viatcheslav V Zakjevskii,Michael G Snyder,Cory S Knill,Joseph T Rakowski

doi:10.1120/jacmp.v17i2.5705

Viatcheslav V Zakjevskii, Michael G Snyder + Show 2 more

Open Access

https://doi.org/10.1120/jacmp.v17i2.5705

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Abstract

A comprehensive end‐to‐end test for head and neck IMRT treatments was developed using a custom phantom designed to utilize multiple dosimetry devices. Initial end‐to‐end test and custom H&N phantom were designed to yield maximum information in anatomical regions significant to H&N plans with respect to: (i) geometric accuracy, (ii) dosimetric accuracy, and (iii) treatment reproducibility. The phantom was designed in collaboration with Integrated Medical Technologies. The phantom was imaged on a CT simulator and the CT was reconstructed with 1 mm slice thickness and imported into Varian's Eclipse treatment planning system. OARs and the PTV were contoured with the aid of Smart Segmentation. A clinical template was used to create an eight‐field IMRT plan and dose was calculated with heterogeneity correction on. Plans were delivered with a TrueBeam equipped with a high definition MLC. Preliminary end‐to‐end results were measured using film, ion chambers, and optically stimulated luminescent dosimeters (OSLDs). Ion chamber dose measurements were compared to the treatment planning system. Films were analyzed with FilmQA Pro using composite gamma index. OSLDs were read with a MicroStar reader using a custom calibration curve. Final phantom design incorporated two axial and one coronal film planes with 18 OSLD locations adjacent to those planes as well as four locations for IMRT ionization chambers below inferior film plane. The end‐to‐end test was consistently reproducible, resulting in average gamma pass rate greater than 99% using 3%/3 mm analysis criteria, and average OSLD and ion chamber measurements within 1% of planned dose. After initial calibration of OSLD and film systems, the end‐to‐end test provides next‐day results, allowing for integration in routine clinical QA. Preliminary trials have demonstrated that our end‐to‐end is a reproducible QA tool that enables the ongoing evaluation of dosimetric and geometric accuracy of clinical head and neck treatments.PACS number(s): 87.55.Qr

Highlights

498 Snyder et al.: End-to-end test for H&N IMRTtypically found in patients, more conformal treatments can be designed with greater dose escalation to the target and improved normal tissue sparing in mind — which could potentially lead to improved patient outcomes.Current commonly used quality assurance protocols for IMRT and VMAT accuracy make little or no effort to account for anatomical reality
Our goal is to develop new, standardized end-to-end testing protocols for conventional linear accelerators, designed to evaluate clinically realistic head and neck plans with multiple dosimetric tools in a novel head and neck phantom which effectively simulates real-life tissue heterogeneities
The creation of a standardized end-to-end test for head and neck IMRT involved the creation of a custom anthropomorphic phantom, an anatomical structure set consisting of realistic target and normal tissue volumes, a clinically feasible treatment plan, and a consistently repeatable measurement and assessment methodology

Summary

Introduction

498 Snyder et al.: End-to-end test for H&N IMRTtypically found in patients, more conformal treatments can be designed with greater dose escalation to the target and improved normal tissue sparing in mind — which could potentially lead to improved patient outcomes.Current commonly used quality assurance protocols for IMRT and VMAT accuracy make little or no effort to account for anatomical reality. Adopted IMRT QA techniques recalculate a patient-specific plan to a geometrically simple homogeneous slab of tissue- equivalent plastic. These do nothing to verify the treatment planning system’s ability to account for heterogeneities, such as bone or air cavities, or realistic patient body contours. The most widely adopted end-to-end tests for linear accelerator IMRT are the IROC Houston (formerly RPC) credentialing tests. IROC Houston phantoms (MD Anderson Cancer Center, Houston, TX) are anthropomorphically shaped; the head and neck phantom does not account for tissue heterogeneities such as bone or air-filled sinuses. Even with 7% dose-difference and 4 mm distance-to-agreement passing criteria, ~ 18% of participating institutions fail IROC Houston credentialing.[3]

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