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Dose Perturbation by Gold Fiducials in the Prostate During Image-Guided Proton Therapy: A Monte Carlo Study

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C Smith

C Smith1*, S Wan, PhD1,2,3, U Wichoski, PhD1, (1) Laurentian University, (2) Northern Ontario School of Medicine, (3) Health Sciences North, Sudbury, Ontario


SU-I-GPD-T-127 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantify and evaluate dose perturbation by gold fiducial markers in the prostate during image-guided proton radiotherapy.

Methods: Using the Monte Carlo simulation software SRIM (Stopping and Range of Ions in Matter) a phantom was constructed to replicate a male pelvic anatomy with the following layers (starting from the beam entrance): 20 mm human skin (including subcutaneous fat), 17 mm skeletal muscle, 86 mm compact bone, 30 mm skeletal muscle, and 36 mm prostate. A proton beam with energy of 193 MeV was chosen to correspond with the Bragg-peak at the distal edge of the prostate. To simulate a gold seed and a gold coil fiducial marker, one 1 mm thick layer and two 0.1 mm thick layers of gold were placed within the prostate, respectively. Energy loss per unit distance (eV/Angstrom) was calculated throughout the whole phantom (depth from 0 to 189 mm) for both scenarios, and then compared with that of the phantom without gold fiducials.

Results: The simulation without a fiducial marker was used as a control to compare the effects of two different types of markers to. The gold seed, despite of its small thickness of only 1 mm, resulted in an extremely large spike that was over 3 times of the magnitude of the Bragg-peak, due to high density and high atomic number of gold. On the other hand, dose perturbation caused by the gold coil was much lower, resulting in a spike with similar height as the Bragg-peak, thanks to its hollow geometry.

Conclusion: Gold seeds have been routinely used as fiducial markers for image guidance in photon therapy. However, in proton therapy they can cause unacceptable dose perturbation, as demonstrated in this work. Gold coil, on the other hand, due to its hollow geometry, proves itself to be a much better alternative.

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