Repeatability of free breathing diffusion weighted MRI for MR guided liver SBRT
PH-0213
Abstract
Repeatability of free breathing diffusion weighted MRI for MR guided liver SBRT
Authors: Vivian van Pelt1, Robin J. M. Navest1, Marlies E. Nowee1, Uulke A. van der Heide1, Petra J. van Houdt1
1The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands
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Purpose or Objective
Online adaptive MR-guided Stereotactic Body
Radiotherapy (SBRT) of liver metastases is an upcoming approach1. Daily
functional imaging on a MR-linac provides the opportunity to measure tumor
biology changes during treatment. The apparent diffusion coefficient (ADC)
derived from diffusion weighted MRI (DWI) is a potential candidate to monitor
such changes. Key for success of a response biomarker is that the measurements
are accurate and precise over time2.
Therefore, the aim of this study was to assess the repeatability of ADC values from
free-breathing DWI measured on an MR-linac.
Material and Methods
18 patients
were treated with 3x20Gy on a 1.5T MR-linac (Unity, Elekta AB, Stockholm,
Sweden) in a prospective trial. DWI scans were performed in 2 separate sessions
(test-retest) before treatment delivery (free breathing, b-values 0, 30, 150,
and 500s/mm2, FOV=510x319x98mm3, voxel size= 4x4x4mm3). The
ADC was calculated from the high b-value images (150 and 500 s/mm2).
The tumor was delineated on the b= 500s/mm2 by two observers. The mean of ADC values
were obtained for each tumor delineation. The test-retest scans were used to
determine the repeatability coefficient (%RC) from the within-subject
coefficient of variation (wCV)2. The inter-observer variation was
assessed by wCV of the ADC values from the two delineations on the test scans. To
determine whether the repeatability is influenced by the amount of tumor
motion, the motion amplitude from the 4D T2-weigthed MRI was related to the
percentage difference in ADC of the test-retest data with Pearson correlation
coefficient.
Results
Fig. 1 shows an example of the test-retest data
including delineations. The size of the tumors and mean ADC values ranged
between 4.0-80.0 cm3 and 0.4-1.7 mm2/s. The %RC of the
ADC values was 41% (wCV 15%) and 45% (wCV 16%) for observer 1 and observer 2, respectively.
The inter-observer wCV for ADC was 3.4%. The average motion in the test-retest had
a vector length of 1.2cm (0.4-3.3cm). A moderate correlation between motion
amplitude and ADC difference was observed for both observers (R = 0.43 and 0.45)(Fig.2).
Conclusion
The ADC values for liver metastases measured on
a MR-linac were in the same range as ADC values reported on diagnostic systems3,4,5. The %RC values for this
dataset are larger than those reported for free-breathing DWI on diagnostic
systems (26%2 and 30%3). The inter-observer variation was
smaller than the variation between the test and retest measurements, suggesting
that the test-retest variation is not dominated by delineation variation. We
also did not find a strong relation with the amount of tumor motion. However,
the ADC values might be affected by misalignment between individual b-value images.
The repeatability could be improved by correction for motion by triggered
acquisitions or retrospective registration3.
References
1. Padgett et al. Phys. Med. 2020
2. Shukla-dave et al. JMRI 2019
3. Pathak et al. Sci. Rep. 2019
4. Deckers et al. JMRI 2014
5. Lahrsow et al. CIRSE 2017