(Monday, 7/15/2019) 9:30 AM - 10:00 AM
Room: Exhibit Hall | Forum 3
Purpose: Intraoral lead shielding is used to protect oral mucosa in treatment of superficial facial lesions. However, dose enhancement due to electron backscatter can cause skin irritation upstream of shielding. Lead is not accurately represented by CT, and backscatter cannot be modeled accurately with conventional treatment planning systems. However, measurements and analytical equations have been fit to model the backscatter. The backscatter was measured in our clinic and compared it to the analytical fits in the literature.
Methods: Backscatter enhancement was measured at 100cm source-to-surface distance for 300MU of 6MeV and 9MeV electrons in solid water phantom with layers of wax and lead. The thickness of wax and solid water (representing bolus and tissue) were varied between 1.5-4mm and 10-15mm, respectively. The aperture defining cut-out and cone sizes were kept constant at 7x7 cm² and 10x10 cm,² respectively. Calibrated film was used for dose measurements. The film was sandwiched between the wax and solid water, representing the buccal-wax interface.
Results: The measured backscatter resulted in dose enhancement of 13-49% at the buccal-wax interface. The measured and analytically derived backscatter enhancement agreed within 5.9% (p=0.001, Wilcoxon-Test). The backscatter at the lead-wax interface for 10mm solid water was 61% and 58% for 6MeV and 9MeV respectively. Dose enhancement is comparable between 6MeV and 9MeV with the same setup (7.9%, p>0.05, Wilcoxon-Test).
Conclusion: Electron backscatter from intraoral lead shielding was measured for a number of clinically relevant geometries. The relative backscatter values are similar for 6MeV and 9MeV electrons due to the trade-off between the amount of backscatter and the electron penetration upstream. The agreement between measured and analytically derived values was reasonably close. Therefore, the analytical method can be used in clinic for estimation of dose enhancement due to backscatter from lead shield.
Keywords
Electron Therapy, Backscatter, Radiochromic Film
Taxonomy
TH- External beam- electrons: dose measurement
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