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Dose Perturbations by Electromagnetic Transponders in the Proton Environment

D Dolney

D Dolney*, J McDonough, N Vapiwala, J Metz, Univ Pennsylvania, Philadelphia, PA

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

Purpose: Surgically implanted electromagnetic transponders have been used in external beam
radiotherapy for target localization and position monitoring in real time. The effect of transponders
on proton therapy dose distributions has not been reported. This study determines the level of the
dose deficit that results from the presence of Calypso Beaconâ„¢ transponders in the path of the
treatment beam.

Methods: A Monte Carlo implementation of the transponder geometry is validated against mea-
surements using Gafchromic EBT2 film in a proton field and subsequently used to generate dose
distributions for transponders at different positions and orientations in the proton spread-out Bragg
peak. The maximum dose deficit is extracted in each case.

Results: Dose shadows of up to 60% occur for transponders positioned very near the end of
range of the Bragg peak. However, if transponders are positioned further than 5 mm from the end
of range, and are not oriented parallel to the beam direction, then the dose deficit can be kept
below 10%.

Conclusion: Beacon electromagnetic transponders could be used for patient setup and motion
management for proton therapy provided some guidelines regarding their placement and orien-
tation with respect to the beam can be met. In cases of difficult transponder positions, options
to minimise the shadow include careful plan design with multiple beam angles to distribute the
shadow over a larger volume, or possibly increasing the dose in the expected shadow region to
offset the deficit.

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