ESTRO 36 Abstract Book

S130

ESTRO 36 2017

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Purpose or Objective

Currently the first MR-Linac systems (Elekta AB, Sweden)

are being installed at several clinical institutions

worldwide. In order to introduce this new technology

safely into clinic it is imperative that the imaging

component is rigorously tested. In this work we present a

comprehensive set of tests for clinical acceptance and

commissioning of a high field (1.5 Tesla) hybrid MR-Linac

(MRL) system. Guidelines as well as initial results are

presented.

Material and Methods

The complete test protocol consists of a series of general

MRI hardware tests, radiotherapy specific test, and hybrid

tests. General MRI hardware and radiotherapy specific

tests include established tests to allow comparison with

diagnostic and radiotherapy planning 1.5T MRI systems.

The hybrid tests are unique to the design and application

of the MRL and were developed in on a non-clinical MRL

prototype (U1) and performed on the clinical prototype

(U2) after installation in September 2016. Hybrid tests

include: Spurious Noise check, MR-MV alignment, Gantry

influence on B0 homogeneity, and radiation effects.

Finally, sequence specific tests are included to ensure

geometric fidelity of the MRI protocols that will be

performed during clinical use.

Results

Table 1 lists the tests included in the commissioning

protocol, subdivided into six sections. The first four

sections contain quality control (QC) tests, which test the

individual components of the system. The final two

sections include quality assurance (QA) measurements,

which probe overall image quality, and thus test a

combination of several components. The QC

measurements serve as a characterisation of the system,

whereas the QA tests serve as a null measurement before

the system is introduced into clinic.

Fig. 1 shows the results from two independent geometric

fidelity measurements, with a) the vendor provided

geometric fidelity phantom, and b) a third party phantom

[Modus Medical Devices Inc, London, ON, Canada].

Geometric distortions were found to be 0.94mm, 1.82mm,

and 2.35mm for diameter spherical volume (DSV) of

300mm, 350mm, and 400mm, respectively. The maximum

geometric distortion occurred at the edge of the DSV.

Further tests revealed that the influence of the gantry on

magnetic field homogeniety was negligible (<0.1 µT) and

RF flip angle accuracy was within spec and comparable to

our MRI-RT 1.5T Philips Ingenia scanner. Finally, the ACR

resolution, geometry, and low contrast detectability tests

all passed the ACR criteria using diagnostic imaging

protocols (i.e., without additional averaging of the data).

No spurious noise was observed during operation of the

Gantry and Linac, suggesting good decoupling of the two

systems.

Conclusion

A comprehensive acceptance and commissioning protocol

was developed and performed for clinical acceptance of

the first 1.5T MR-Linac within the Atlantic consortium.

Overall system performance is extremely similar to our

diagnostic 1.5T MRI scanner, used for radiotherapy

planning. Hybrid tests showed good decoupling of the two

systems.

OC-0258 Investigation of magnetic field effects on 3D

dosimeters for MR-IGRT applications

H.J. Lee

1,2

, Y. Roed

1,3

, S. Venkataraman

, M. Carroll

1,2

, G.

Ibbott

The University of Texas MD Anderson Cancer Center,

Radiation Physics, Houston, USA

University of Texas at Houston, Graduate School of

Biomedical Sciences, Houston, USA

University of Houston, Physics, Houston, USA

Purpose or Objective

Conventional QA tools lack the ability to report changes in

volumetric dose distributions and discrepancies out of the

plane of measurement. The strong magnetic field in the

integrated pre-clinical 1.5T MRI – 7MV linear accelerator

system (MR-linac, Elekta AB, Stockholm, Sweden)

influences secondary electrons resulting in changes in dose

deposition in three dimensions. The purpose of this study

was to investigate strong magnetic field effects on 3D

dosimeters for magnetic resonance image-guided

radiation therapy (MR-IGRT) applications.

Material and Methods

There are currently three main types of 3D dosimeters:

radiochromic plastic, radiochromic gel, and polymer gel.

For this study, the following three dosimeters were used:

PRESAGE®, FOX (Fricke-type), and BANG

(MGS Research

Inc). For the purposes of batch consistency, an

electromagnet was used for same-day irradiations with