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Radiation-induced epilation due to couch transit dose for the Leksell gamma knife model C
Carla D Bradford1, Brian Morabito, Douglas R Shearer
1Department of Medical Physics, Rhode Island Hospital, 593 Eddy Street, Room 317, Providence, RI 02903, USA. cbradford@lifespan.org
Purpose:
To determine the cause of epilation at the top of the head for 2 patients with acoustic neuromas after undergoing fractionated radiosurgery with the Leskell Gamma Knife model C. This epilation was unexpected, because the treatment planning program stated the dose at this location was <0.1 Gy.
Methods And Materials:
The radiation dose along a central axis, parallel to the couch, from the helmet's focus to the helmet cap was measured during couch transit.
Results:
Transit doses of 4.4 cGy/shot at 10 cm and 5.6 cGy/shot at distances >15 cm from the helmet's focus were measured. It was estimated that the 2 patients with epilation received approximately 6-7 Gy to the scalp. A shield was constructed and shown to reduce the transit dose by as much as 60%.
Conclusion:
The design of the helmet allows the uncollimated beams to reach areas of the patient, superior to the target, just before and after couch docking with the housing. For treatment involving a large number of shots (i.e., fractionation), off-target doses < or = 8 Gy can result. For these cases, the transit dose should be considered and some form of additional shielding should be used.