BERKELEY, CA (UroToday.com) - Image-guided radiation therapy using implanted gold fiducial markers has been shown to cause large dose perturbations in patients who receive proton radiotherapy.
The objective of this study was to analyze the magnitude of the dose perturbations caused by carbon-coated ZrO2 and stainless steel fiducials and compare them to perturbations caused by conventional gold fiducials. Dose perturbations were measured downstream of the fiducials using radiochromic film. These measurements showed that perpendicularly-oriented carbon and stainless steel fiducials located near the beam’s center of modulation perturbed the dose by less than 10%, but that the same fiducials in a parallel-orientation near the beam’s end of range 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 range of the beam and if they are oriented perpendicular to the beam axis.
By using these fiducials for prostate alignment we can ensure adequate target localization while reducing the impact of the dose perturbations caused by the introduction of these fiducial markers. However, this study does not take into account proton range uncertainties and daily range variations due to patient anatomy changes. Proton range uncertainties in treatment planning due to CT number to stopping power ratio conversion are not taken into account in our study. Additionally, the added ability to modulate proton range introduces additional complexities to proton treatments that are not factors in traditional x-ray therapies. Therefore, it is important to note that while the localization of the prostate may help to improve target coverage, other factors need to be taken into account to ensure other factors such as patient weight loss or internal anatomical changes does not adversely affect the proton dose distribution. These effects were not examined in our study and each institution should review their own clinical evaluation of these factors and their influence on target margins and patient setup before implementing any protocol for radiation therapy treatment.
The dose perturbation evaluation of the fiducials used in this study was applied only to the case of prostate cancer since this is what we currently use them for in our clinic. Potentially, these fiducial types could be beneficial for other treatment sites as well. Proton beam characteristics that are specific to these other sites such as proton energy and field size should be examined before the use of these markers for those sites.
Joey Cheung* as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
*Medical Physics Graduate Student, University of Texas M.D. Anderson Cancer Center