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, D99% 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.
Highlights
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 presence of these high density markers may perturb the radiation field and introduce undesired dose distributions
Newhauser et al [2] used the Monte Carlo method to calculate the dosimetric effects caused by cylindrical shaped gold fiducial markers (3 mm long and 0.9 mm in diameter) in a single direction spread out Bragg peak (SOBP) proton field
Summary
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 presence of these high density markers may perturb the radiation 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]. Newhauser et al [2] used the Monte Carlo method to calculate the dosimetric effects caused by cylindrical shaped gold fiducial markers (3 mm long and 0.9 mm in diameter) in a single direction spread out Bragg peak (SOBP) proton field. Lim et al [6] confirmed those Monte Carlo results by using diode detectors to measure the dose distributions in a water phantom
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