The Dosimetric Effect of MRI Positive-Contrast Markers vs. Negative-Contrast Fiducial Markers on Proton Radiation

Abstract

The usage of magnetic resonance imaging (MRI) has been limited in prostate cancer radiation because a lack of effective positive-signal MRI fiducial markers (FM). The FM used in our prostate clinic (gold anchor [GAFM] for photons; carbon [CFM] for protons) are not reliably localized with MRI. The uncertainty of FM localization using MRI affects spatial accuracy of the target volume planning and limits optimum patient treatment positioning. Current FM generate 10-38% proton dose perturbation. We hypothesize that the novel multimodality positive-signal FM (C4 Nova [C4NFM], are visible on kilovoltage, computed tomography, and MR images, with a reduced imaging artifact versus GAFM or CFM) can potentially improve the precision and accuracy of radiotherapy and present more acceptable proton dose perturbation. Herein, dose perturbation of different FM was compared.The negative- (GAFM [0.38 × 8.4 mm], CFM [0.59-0.88 × 3 mm]) and positive-signal (C4NFM-5 [1.0 × 5.5 mm], C4NFM-10 [1.0 × 9.8 mm]) MRI FM were tested. Markers were arranged inside plastic water material at the center of a passively scattered spread-out Bragg peak (160 MeV, range 16 cm, SOBP 10 cm). The dose was measured using radiochromic film placed at 0, 2, 5, 10, 20, 30, and 40 mm downstream from the markers. The FM were oriented perpendicularly (PER) or parallel (PAR) relative to the beam axis. The films were scanned using Epson 11000XL flatbed scanning in 300 depi resolution and were further analyzed using imageJ software.The proton dose perturbation is associated with marker type, orientation, and the depth of measurement (Table). Proton dose perturbation increased with the depth away from the FM before it gradually decreased to zero. Less dose perturbations were observed at the PER vs. PAR orientation. GAFM induced higher dose perturbation versus CFM and C4NFM in both the maximum value and the perturbation depth. Compared to CFM, C4NFM produced a lower maximum dose perturbation (1.4% vs. 3.0% at PER; 5.7% vs. 7.9% at PAR), and shallower perturbation depth (5 vs. 20 mm at PER). Notably, both the longer C4NFM-10 and shorter C4NFM-5 produced the same dose perturbation in both maximum value (5.7% at PER) and the depth (20 mm).C4 Nova MRI positive-contrast fiducial markers have the least proton dose perturbation effect compared with gold and carbon MRI negative-contrast markers. These data may serve as evidence for future clinical trials to study the benefit of MRI from treatment planning to delivery using positive MRI signal C4 Nova FM in preserving tumor control while decrease radiation related toxicities in patients with prostate cancer.