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Dose Shadowing Due to Gold Fudicial Markers in Proton Therapy

1R Regmi*, 1D Maes, 1J Saini, 1A Egan, 2S Bowen, 1T Wong, 1SCCA Proton Therapy Center, 2University of Washington, School of Medicine, Seattle, WA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Fudicial markers do not typically migrate after implantation for prostate image guided radiation therapy. However, when in the proton beam path, dose shadowing can occur. Objective of this study is to investigate the magnitude of dose shadowing in clinical proton beams for a specific gold fudicial marker.

Methods: Two fiducials markers [gold anchors straight, gold anchor folded] with the size of 0.28 mm diameter and 10 mm in length were evaluated in this study. To quantify the dose shadowing effect, Gafchromic film was placed behind each marker, then sandwiched in a solid water phantom. A proton beam of field size 10 x 10 cm2 with range and modulation of 16 cm and 10 cm respectively were shot through the fiducial markers. Markers were placed at the center of the SOBP. Films were then analyzed to estimate dose shadowing effects.

Results: X and Y lateral profiles, centered at the marker locations were extracted from the film planes. We marked the position of the anchors on the film so that profile could be analyzed in the vicinity of the anchor position. If there would have been any dose shadowing, we would have seen dip in the profile where the position of anchor was. However, we didn’t see any dose perturbation on both x and y profiles measured just behind the markers. Flatness and symmetry in both directions were also within the tolerance of 3%.

Conclusion: There is negligible dose shadowing effect due to gold anchors placed in the proton beam path. So, dosimetrically, these anchors are safe to use since there is insignificant effect on the dose distribution.

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