Abstract
PurposeDaily quantitative MR imaging during radiotherapy of cancer patients has become feasible with MRI systems integrated with linear accelerators (MR-linacs). Quantitative images could be used for treatment response monitoring. With intravoxel incoherent motion (IVIM) MRI, it is possible to acquire perfusion information without the use of contrast agents. In this multicenter study, daily IVIM measurements were performed in prostate cancer patients to identify changes that potentially reflect response to treatment.Materials and MethodsForty-three patients were included, treated with 20 fractions of 3 Gy on a 1.5 T MR-linac. IVIM measurements were performed on each treatment day. The diffusion coefficient (D), perfusion fraction (f), and pseudo-diffusion coefficient (D*) were calculated based on the median signal intensities in the non-cancerous prostate and the tumor. Repeatability coefficients (RCs) were determined based on the first two treatment fractions. Separate linear mixed-effects models were constructed for the three IVIM parameters.ResultsIn total, 726 fractions were analyzed. Pre-treatment average values, measured on the first fraction before irradiation, were 1.46 × 10−3 mm2/s, 0.086, and 28.7 × 10−3 mm2/s in the non-cancerous prostate and 1.19 × 10−3 mm2/s, 0.088, and 28.9 × 10−3 mm2/s in the tumor, for D, f, and D*, respectively. The repeatability coefficients for D, f, and D* in the non-cancerous prostate were 0.09 × 10−3 mm2/s, 0.05, and 15.3 × 10−3 mm2/s. In the tumor, these values were 0.44 × 10−3 mm2/s, 0.16, and 76.4 × 10−3 mm2/s. The mixed effects analysis showed an increase in D of the tumors over the course of treatment, while remaining stable in the non-cancerous prostate. The f and D* increased in both the non-cancerous prostate and tumor.ConclusionsIt is feasible to perform daily IVIM measurements on an MR-linac system. Although the repeatability coefficients were high, changes in IVIM perfusion parameters were measured on a group level, indicating that IVIM has potential for measuring treatment response.
Highlights
Integrated MR-linac systems combine an MRI scanner with a linear accelerator, allowing acquisition of MRI scans of the patient on each treatment fraction of a radiotherapy (RT) course
Forty-three patients from three institutes with intermediate- and high-risk biopsy-proven prostate cancer were included in this study according to the EAU risk classification [21]
intravoxel incoherent motion (IVIM) parameters were calculated from the median signal intensities of the tumor and non-cancerous prostate
Summary
Integrated MR-linac systems combine an MRI scanner with a linear accelerator, allowing acquisition of MRI scans of the patient on each treatment fraction of a radiotherapy (RT) course. Quantitative MRI enables the characterization of tissue properties in a quantitative manner By measuring this on a daily basis, two exciting ideas for personalized radiotherapy come within reach. The first is to adapt the dose distribution of a treatment plan on a daily basis according to the changing patient biology [4], and the second is to base the total dose that a patient receives on the biological response [5]. For this to become clinical practice, the performance of MR-linacs regarding quantitative MRI first needs to be validated [6]. It needs to be established if daily changes in imaging biomarkers are detectable and if these changes are associated with clinical outcome
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