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A Study of the Dosimetric Impact of Gold Fiducial Markers in Image Guided Radiation Therapy

L Wang

L Wang*, E Mok, L Xing, L Wang, Stanford University, Stanford, CA

SU-E-T-529 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

During image-guided radiation therapy, gold seeds as fiducial markers are often implanted to the target to track the tumor motion. The presence of the gold seeds could perturb the dose distribution and may cause either cold or hot spots in the target. This study investigates the dosimetric effects of gold seeds by film measurements and Monte Carlo (MC) simulations.

A gold seed of 5mm long and 1mm in diameter was sandwiched between a piece of EBT2 film and a bolus material, with film under the seed, and was positioned at 10 cm depth in a 30x30x20 cm3 solid-water phantom. Three plans were delivered with 6MV radiation beams: (1) a single field in anterior-posterior direction; (2) a single field in posterior-anterior direction; (3) 9 fields in equally-spaced gantry angles. Film dosimetry was performed and the measured dose profiles were compared to MC calculations. CT image-based MC calculations were performed for several lung tumor IMRT cases with gold seeds implanted. Dose comparison was performed between overriding the seed with tissue and with gold material.

For single field irradiations, film measurement shows significant dose enhancement (~25%) in proximal side and dose attenuation (~15%) in the distal side of the seed. However, the attenuation effect disappears for the 9 field irradiation. The dose enhancement for the 9 field plan is up to 7~8% in regions within 1mm to the seed. MC calculations also support the measurements. For the IMRT lung cancer cases, the dose enhancement is around 4~8% adjacent to the seed.

For multiple-field IMRT/Arc treatment, the tissue adjacent to the implanted gold marker will most likely get hot instead of cold. This is because the dose enhancement in the proximal side outruns the attenuation in the distal side of seeds and dose enhancement presents in all the other directions.

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