Dose perturbations and image artifacts caused by carbon-coated ceramic and stainless steel fiducials used in proton therapy for prostate cancer

Image-guided radiation therapy using implanted fiducial markers is a common solution for prostate localization to improve targeting accuracy. However, fiducials that are typically used for conventional photon radiotherapy cause large dose perturbations in patients who receive proton radiotherapy. A proposed solution has been to use fiducials of lower atomic number (Z) materials to minimize this effect in tissue, but the effects of these fiducials on dose distributions have not been quantified. The objective of this study was to analyze the magnitude of the dose perturbations caused by select lower-Z fiducials (a carbon-coated zirconium dioxide fiducial and a plastic-coated stainless steel fiducial) and compare them to perturbations caused by conventional gold fiducials. Sets of phantoms were used to assess select components of the effects on dose. First, the fiducials were assessed for radiographic visibility using both conventional computed tomography (CT) and an on-board kilovoltage imaging device at our proton therapy center. CT streak artifacts from the fiducials were also measured in a separate phantom. Second, dose perturbations were measured downstream of the fiducials using radiochromic film. The magnitude of dose perturbation was characterized as a function of marker material, implantation depth and orientation with respect to the beam axis. The radiographic visibility of the markers was deemed to be acceptable for clinical use. The dose measurements showed that the perpendicularly oriented zirconium dioxide and stainless steel fiducials located near the center of modulation of the proton beam perturbed the dose by less than 10%, but that the same fiducials in a parallel orientation near the end of the range of the beam could perturb the dose by as much as 38%. This suggests that carbon-coated and stainless steel fiducials could be used in proton therapy if they are located far from the end of the range of the beam and if they are oriented perpendicular to the beam axis.