A Novel Positive-Contrast Magnetic Resonance Imaging Line Marker for High-Dose-Rate (HDR) MRI-Assisted Radiosurgery (MARS).

Simple Summary: High-dose-rate (HDR) MRI-Assisted Radiosurgery (MARS) is an advanced form of brachytherapy or interventional radiotherapy involving the use of a remote after-loading system to transport an encapsulated radioisotope source to within or near a tumor with MRI precision. An HDR MARS can be used for treating both primary and recurrent prostate cancer with fewer treatment-related toxicities and comparable survival rates. Image-guided HDR brachytherapy has been linked with better local tumor control and overall survival. Magnetic resonance imaging (MRI) is more effective than computed tomography (CT) and ultrasonography in distinguishing the boundaries of a tumor versus the surrounding normal tissues. However, localizing the applicator and source is more challenging with MRI because conventional fiducial markers are not positively visualized on MRI. The aim of our study was to investigate the MRI signal intensities and characteristics of a newly developed positive-contrast MRI line marker (C4:S) for HDR MARS. We found that C4:S MRI line markers were positively visualized on both T1- and T2-weighted MRI sequences. Our findings indicate the value of C4:S line markers for HDR MARS. Magnetic resonance imaging (MRI) can facilitate accurate organ delineation and optimal dose distributions in high-dose-rate (HDR) MRI-Assisted Radiosurgery (MARS). Its use for this purpose has been limited by the lack of positive-contrast MRI markers that can clearly delineate the lumen of the HDR applicator and precisely show the path of the HDR source on T1- and T2-weighted MRI sequences. We investigated a novel MRI positive-contrast HDR brachytherapy or interventional radiotherapy line marker, C4:S, consisting of C4 (visible on T1-weighted images) complexed with saline. Longitudinal relaxation time (T1) and transverse relaxation time (T2) for C4:S were measured on a 1.5 T MRI scanner. High-density polyethylene (HDPE) tubing filled with C4:S as an HDR brachytherapy line marker was tested for visibility on T1- and T2-weighted MRI sequences in a tissue-equivalent female ultrasound training pelvis phantom. Relaxivity measurements indicated that C4:S solution had good T1-weighted contrast (relative to oil [fat] signal intensity) and good T2-weighted contrast (relative to water signal intensity) at both room temperature (relaxivity ratio > 1; r2/r1 = 1.43) and body temperature (relaxivity ratio > 1; r2/r1 = 1.38). These measurements were verified by the positive visualization of the C4:S (C4/saline 50:50) HDPE tube HDR brachytherapy line marker on both T1- and T2-weighted MRI sequences. Orientation did not affect the relaxivity of the C4:S contrast solution. C4:S encapsulated in HDPE tubing can be visualized as a positive line marker on both T1- and T2-weighted MRI sequences. MRI-guided HDR planning may be possible with these novel line markers for HDR MARS for several types of cancer. [ABSTRACT FROM AUTHOR]