Session Item

Saturday
August 28
16:45 - 17:45
Online Stream 1
Poster Highlights 7: Quantitative functional and biological imaging
Petra van Houdt, The Netherlands
0520
Poster highlights
Physics
16:53 - 17:01
Repeatability of free breathing diffusion weighted MRI for MR guided liver SBRT
Vivian van Pelt, The Netherlands
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/mmby 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