International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2012, 1, 8-13 doi:2660001:10.2436/ijmpcero.2012.11002 Published Online May 2012 (http://www.SciRP.org/journal/ijmpcero) Dose Perturbations of Gold Fiducial Markers in the Prostate Cancer Intensity Modulated Proton Radiation Therapy (IMPT) Miao Zhang 1 , Sung Kim 1 , Ting Chen 1 , Xiaohu Mo 2 , Bruce G. Haffty 1 , Ning J. Yue 1 1 Department of Radiation Oncology, The Cancer Institute of New Jersey, UMDNJ-Robert Wood Johnson University Hospital, New Brunswick, USA 2 Medical Physics Department, University of Wisconsin-Madison, Madison, USA Email: zhangm3@umdnj.edu Received April 7, 2012; revised May 13, 2012; accepted May 22, 2012 ABSTRACT The objective of this study is to investigate the dose perturbations introduced by the implanted gold fiducial markers in the prostate cancer intensity modulated proton therapy (IMPT) and the impacts of different plan designs on the pertur- bations. Five proton plans: a single lateral field 3D-modulation (3D-mod) plan, 2 fields laterally opposing 3D-mod plan, 6-, 9-, and 18-field distal edge tracking (DET) plans were designed on the CT images of a prostate patient. The dose distributions were first generated for the plans free of fiducial markers with 78 Gy prescribed to 95% of the PTV. To derive the dose perturbations of the gold fiducial markers, three cylindrical shaped gold fiducial markers (3 mm long and 1 mm in diameter) were artificially inserted into the prostate, and the dose distributions were re-computed. Monte Carlo method was used for dose computation. It was found that the gold fiducial markers perturbed the dose distribu- tions, especially along the beam paths. The markers caused a shadowing effect reducing the doses in the areas beyond the markers. Overall, due to the presence of the fiducial markers, D 99% of prostate were reduced by 2.96 Gy, 4.21 Gy, 0.16 Gy, 0.34 Gy, 0.15 Gy for the plans of single field 3D-mod, 2-field parallel opposed 3D-mod, 6-, 9-, and 18-field DET respectively. Our study showed these dose perturbation effects decreased with the increase of number of beam angles. Up to 6 beam angles may be required to reduce the dose perturbations from the gold fiducial markers to a clini- cally acceptable level in IMPT. Keywords: Gold Fiducial Marker; Intensity Modulated Proton Therapy; Prostate Cancer; Monte Carlo 1. Introduction In prostate radiotherapy, high density fiducial markers are often used on a daily basis for accurate localization of prostate gland. While high density fiducial markers are preferred for providing high contrast images, the pres- ence of these high density markers may perturb the ra- diation field and introduce undesired dose distributions. For conventional megavoltage photon radiotherapy, both theoretical and experimental studies have shown the presence of small high density materials, like gold, does not introduce significant dose perturbations due to the strong penetrating power of megavoltage photons and the multiple beam angles arrangement [1]. However, the use of high density fiducial markers in proton therapy may require careful consideration [2-5]. Newhauser et al. [2] used the Monte Carlo method to calculate the dosimetric effects caused by cylindrical shaped gold fiducial mark- ers (3 mm long and 0.9 mm in diameter) in a single di- rection spread out Bragg peak (SOBP) proton field. Com- pared to the dose distribution without markers in water, they found that gold markers casted a “dose shadow” to the area behind the markers. The magnitude of the under- dose depended on the orientation of the gold markers with respect to the radiation field, as well as the relative locations of the gold markers to the distal edge of the SOBP. In the worst case scenario (marker was parallel to the beam direction and close to the distal end of the SOBP) the under-dose to the tissue behind the marker could be as large as 50%. Even when the orientation of the marker was perpendicular to the beam direction, the under-dose to the tissue behind the marker was still around 20%. In a later investigation, Lim et al. [6] con- firmed those Monte Carlo results by using diode detec- tors to measure the dose distributions in a water phan- tom. Recently, more sophisticated proton delivery systems become available. The introduction of the active scan- ning technology in the proton delivery system allows Copyright © 2012 SciRes. IJMPCERO