Quality Assurance of the Radiotherapy Workflow Integrating a Dedicated Wide-bore 3T MRI Simulator A. Xing1, G.P. Liney1, 3, L. Holloway1, 2, 3, S. Arumugam1, R. Rai1, E. Juresic1, and G. Goozee1 1
Cancer Therapy Centre & Ingham Institute for Applied Medical Research, Liverpool Hospital, Sydney, Australia 2 Institutes of Medical Physics, School of Physics, University of Sydney, Sydney, Australia 3 Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia
Abstract—With the advent of high-field MRI scanners specially designed for modern radiotherapy planning and simulation (also known as MRI simulator), more and more oncology centres are introducing MRI into their radiotherapy workflow. A dedicated wide-bore 3T MRI simulator was integrated into our clinical workflow for various treatment sites including head and neck, pelvis, breast, cervix and lung. Patients are scanned first on a 4D CT simulator and then on the MRI simulator in the same setup position as treatment. CT and MRI images are transferred to the treatment planning system for registration. The target volume is then contoured on MRI and transferred to CT images for dose calculation and planning. In order to test the accuracy and consistency of this procedure a quality assurance (QA) program was developed using a suitable MR and CT compatible phantom. Results demonstrate the MRI-integrated workflow implemented in our centre was consistent and reproducible. It is recommended that this QA be performed quarterly or after MRI simulator major repair or maintenance. Keywords— MRI simulator, MRI-integrated radiotherapy workflow, Quality assurance
I. INTRODUCTION
In the last two decades, radiotherapy has evolved rapidly from three-dimensional conformal therapy (3D-CRT) to more advanced treatment modalities such as IMRT, VMAT and Tomotherapy. These advanced treatment techniques can deliver more conformal dose distributions tailored to the shape of the tumor volume while reducing dose to organs at risk (OAR). To achieve the planned clinical goal by utilizing the advanced radiotherapy treatment, the target volume is required to be delineated as accurately as practically achievable [1]. Computed tomography (CT) has been playing a primary role for defining tumor volume as well as the volume of OARs. However, the images suffer from the poor soft tissue contrast and it can be difficult to discriminate adjoining soft tissue structures. The imaging parameters in MRI scanning are very flexible compared to CT scanning, resulting in the ability to vary soft tissue contrast and enhance the difference in proton density, spin-lattice (T1) and spin-spin (T2) relaxation times. Tumors often have similar x-ray attenua-
tion properties as surrounding normal tissue but differ in their MRI properties. MRI-based planning via registration of MRI to CT has been shown to provide more consistent target delineation than CT alone for a variety of sites, reducing inter-physician variation [2]. Driven by the advanced treatment technology and the advantage of MRI imaging over CT imaging, there has been a growing interest and effort in recent years to utilizing MRI acquired on a diagnostic MRI scanner into radiotherapy. More recently, MRI scanners specially designed for radiotherapy planning (also known as MRI simulator) have become commercially available from several vendors. As a consequence, more and more oncology centres have now integrated MRI into their radiotherapy workflow. In line with all other radiotherapy equipment and procedures (CT simulator, linear accelerator and treatment planning system), the MRI simulator and its integration into the RT workflow requires a quality control on a regular basis. Quality assurance (QA) of radiotherapy procedure and equipments is critical for achieving consistent clinical outcome as well as ensuring patient safety. A dedicated MRI simulator was installed and integrated into our routine practice in June, 2013 [3]. There are a number of existing international protocols regarding the quality assurance of MRI in terms of diagnostic performance [4], but there is lack of guidelines concerning the QA of the proposed MRI-RT workflow. The purpose of this study was to establish such a QA program for quality control in our centre.
II.
MATERIALS AND METHOD
A. MRI simulator Siemens Skyra 3T MRI scanner installed in our centre is a dedicated MRI simulator specially designed for radiotherapy simulation and planning. As shown in Fig. 1(a), it has a wide 70cm bore size to accommodate immobilization device for treatment setup and make patient more comfort during the scanning. External goal-post localization lasers were also included as part of the installation, which is considered to be critical for radiotherapy simulation and plan-
© Springer International Publishing Switzerland 2015 D.A. Jaffray (ed.), World Congress on Medical Physics and Biomedical Engineering, June 7-12, 2015, Toronto, Canada, IFMBE Proceedings 51, DOI: 10.1007/978-3-319-19387-8_132
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ning using MRI simulator. Its unique flat top is matched with the couch of linear accelerator and TomoTherapy unit.
of treatment sites in combination with other coils. A range of coils specially designed for oncology simulation were purchased: two 18 body array coils with long cable and short cable, large and small 4 channel flexible coils, 20 channel spine coils, 20 channel head and neck coil, 20 channel breast coils and standard transmitter/receiver headneck coils. These coils were selected carefully to meet the demand by oncologists to use MRI simulator for a variety of treatment sites including head, neck, prostate, pelvis, lung and breast. B. MRI-integrated RT workflow The MRI simulator was integrated into our current clinical workflow in following way: patient is first scanned on a Philip 4D CT simulator on treatment position, and then scanned at same position on the MRI simulator which is next door to CT scanner as shown in Fig.1. The CT and MRI image are sent to Pinnacle treatment planning system (version 9.8). The MRI is initially auto-coregistered with CT images using mutual information algorithm implemented in Pinnacle and then manually adjusted by radiotherapist (RT) and oncologist. Once the MRI images are registered with CT images, the oncologists delineate the target and organs at risk on MRI images and transfer the contours to CT images. These CT images along with CT structure are sent to Tomotherapy planning station for TomoTherapy planning or planning IMRT, VMAT or 3D-CRT treatment in Pinnacle.
(a)
C. Phantom-based QA program
(b) Fig.1 (a) Siemens Skyra 3T MRI Simulator with a flat couch top and a stand post for simulation lasers. (b) Philips 4D CT simulator used for radiotherapy planning and simulation. QA phantom was setup on both MRI and CT simulators.
With the aid of the localization lasers and flat top, the patient can be kept at same prostitution during MRI simulation and radiotherapy treatment. Sixteen RF coils were integrated into flat top to improve the image quality for variety
To check the consistency and reproducibility of the whole workflow integrating MRI simulator in our centre, a cylindrical phantom was used. As shown in Fig.1, the phantom is a cylinder with diameter of 22 cm and a flat bottom surface. The phantom sits a flat plate with three adjustable support poles and two spirit levels for levelling the phantom. On the surface of the phantom, there are three external markers which are MRI and CT compatible. The phantom was designed in such a way that it is extremely easy to setup on either MRI simulator or CT simulator. Inside the phantom was filled with water. There are four contrast regions at four corners and two cross regions at center. These regions and MRI-compatible markers can be clearly seen on MRI images as shown in Fig.2. The QA procedure is exactly same as the patient workflow. As shown in Fig.1, the phantom base was levelled and setup on the CT simulator flat bed with aid of CT simulation lasers and phantom external markers. The phantom was scanned using CT head-neck protocol. Then The phantom is setup on MRI by aligning its external marks with localisation lasers and scanned using bore coils and a standard
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SE sequence (TE=30ms, TR=3410ms). Both CT and MRI images are sent to Pinnacle for co-registration. Two images set were registered using automaton functionality and mutual information registration algorithm provided by Pinnacle system. The translation and rotation parameters are recorded. This procedure was repeated for a period of 8 days.
whole workflow. The purpose of this check to verify: (1) the external simulation lasers. External simulation laser is Table 1 Quality assurance results for a MRI integrated RT workflow for 8 fractions. Translation(cm)
Fraction Number
Rotation(degree)
X
Y
Z
X
Y
Z
1
0.37
-10.92
-0.32
0.24
0.14
0.04
2
0.24
-11.13
-0.32
0.00
0.00
0.00
3
0.27
-11.14
-0.07
0.00
-0.16
-0.06
4
-0.49
-11.18
-0.69
0.00
0.82
-0.06
5
0.28
-11.12
-0.08
0.00
-0.16
0.00
6
0.07
-11.60
0.53
0.02
1.03
0.00
7
0.28
-10.91
-0.33
0.00
-0.06
0.00
8
-0.29
-10.93
0.41
0.06
0.20
0.05
Fig.2 (Left) QA phantom was scanned with head-first supine with MRI
Average
0.09
-11.12
-0.11
0.04
0.23
0.00
marker on the right side of the phantom. (Right) Phantom was physical rotated 180 degree with the MRI marker on the left side of phantom.
SD
0.31
0.23
0.41
0.08
0.45
0.04
III.
RESULTS AND DISCUSSION
Table 1 shows the registration parameters for 8 consecutive fractions using the QA phantom. The averaged translation parameters in both three directions are less than 1.2mm, whereas the rotation is less than 0.3 degree. Especially along z direction, on average, there is no rotation and less than 1mm translation along x direction. The standard deviation of translation and rotational parameters are less than 0.45, indicating that the MRI-integrated workflow is consistent and highly reproducible. It is recommended that the QA procedure be performed quarterly or in the event of MRI scanner maintenance and repair. There are well-established protocols for CT-only based radiotherapy simulation and planning. It was recommended that simulation and planning procedure starting from CT images acquisition, image transfer, geometric accuracy as well as the image orientation be monitored using a phantom [5]. The proposed methodology and developed QA procedure serves for this purpose but for radiotherapy planning and simulation workflow integrated with a MRI simulator. The performance of MRI simulator itself is monitored by a daily QA program and annual QA program established according to international protocols for MRI scanner [4]. The designed QA program is an end-to-end test for the
essential for radiotherapy simulation using MRI simulator. The laser is aligned with the fiducial marks on immobilization device or patient, which can be verified by the external marks and cross on the central slice as shown in Fig.2. The accuracy requirement for laser movement is ±1mm, which is same as simulation lasers as on CT and linear accelerator; (2) the reproducibility of setup and imaging system. As the tests followed the exactly RT workflow used in our clinic, it not only checked the phantom setup on both MRI and CT simulators but also the stability of two image systems; (3) image transfer and registration. The MRI images have to be transferred to treatment planning system from the local database of MRI console. It is critical that the image data are transferred between two systems with correct geometric orientation as shown in Fig.2. The constancy of autoregistration with the clinically used algorithm was checked as well. It is aware that the proposed QA program is not intended for patient specific QA. For each treatment site, a MRI scanning protocol for RT simulation was established at MRI-simulator commissioning stage providing a sitespecific MRI reference guide. For each patient, the sitespecific protocol parameters were optimized for this specific patient by an experienced MRI technologist and a RT specializing in planning, image registration and MRI image acquisition to achieve the optimal image quality required for radiotherapy simulation and planning. The similar practical
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strategy for patient-specific QA was indepeendently developed and used by Paulson et.al in their centre [6].
2. 3.
IV. CONCLUSIONS 4.
A QA program was established in our cenntre to perform quality control of MRI-integrated workflow using u a CT and MRI compatible phantom. This program was w simple and suitable for routine practice. The initial ressults suggested that this QA procedure may be performed quuarterly or after MRI scanner repair. The further work using the t same phantom and setup for monitoring of MRI scanneer performance in term of image quality is being developed to improve the efficiency of our daily QA program.
5.
6.
Devic S. (2012) MRI simulation for radiotherapy planning. Med. Phys. 39:6701-6711 mugan S, Juresic E, Cassapi Xing A, Liney G, Holloway L, Arum L and Goozee G. (2014) Commisssioning and acceptance of a dedicated 3T MRI simulator for f radiotherapy treatment planning. ESTRO33, Vienna, Austriia. American Association of medical phhysics. (2010) AAPM report 100: Acceptance test and quality asssurance procedure for magnetic imaging facilities.AAPM, MD D. American Association of medical physics. p (1994) Comprehensive QA for radiotherapy oncologyy: report of AAPM radiation committee task group 40. Med. Physs. 31:581-618. Paulson E.S, Erickson B et.al. (20115) Comprehensive MRI simulation methodology using a dedicated MRI scanner in radiation oncology for external beam raadiation treatment planning. Med. Phys. 42:28-39.
CONFLICT OF INTEREST The authors declare that they have no confflict of interest.
REFERENCES 1.
w link in the Njeh C.F. (2008) Tumor delineation: The weakest search for accuracy in Radiotherapy. J Med. Phys. P 33:136–140
Author: Aitang Xing Institute: Liverpool Cancer therapy centree Street: Elizabeth St. City: Sydney Country: Australia Email:
[email protected]
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