(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Radiation-induced cataract formation has little to no dose threshold value or fractionation effect. Patients receiving electron therapy to the hand while standing beside the electron applicator will receive some lens dose from scattered electrons, possibly at consequential levels. This study investigates typical lens dose, its variation with setup parameters, and some simple mitigation strategies.
Methods: Measurements were made with a parallel-plate ionization chamber at 3 mm depth in solid water to approximate patient lens dose on a Varian Trilogy linear accelerator. A typical setup was chosen with eye height 160 cm, eyes 40 cm lateral to central axis, 9 MeV beam, and 10 cm applicator. A solid cerrobend block without cutout was used to set an upper limit. The height, distance, beam energy, applicator size, and cutout fill were all varied separately from that setup. A head phantom was used to test mitigation strategies, with an OSLD placed on the phantom eye under 3 mm of bolus.
Results: For the typical setup the lens would receive 0.15% ± 0.01% of prescription dose. Strong dependence was found with applicator size (0.59% ± 0.04% for 20 cm applicator), distance (0.83% ± 0.05% for 20 cm at 144 cm eye height), and block fill (0.08% ± 0.03% for open cutout). Reduction in dose is negligible when closing eyelids; 2.5–4-fold by turning the head 90 degrees to the side or 45 degrees up or adding 1–2 mm lead shielding to glasses; and essentially complete from placing 1 mm Pb shield on the applicator or turning away.
Conclusion: Typical setups will deliver lens doses of 10’s of cGy over a course, which has been previously shown to increase cataract risk. Several reasonably achievable mitigation strategies are effective at reducing lens dose.
Keywords
Electron Therapy, Scatter, Patient Positioning
Taxonomy
TH- External Beam- Electrons: out of field dosimetry/risk analysis
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