SU‐E‐T‐737: The Impact of the Size of Custom Lead Shields On the Dosimetry of Electron Beam Therapy

Purpose: Often the management of deeply penetrated skin cancer (>1cm) involves a standard open election field that is blocked with lead shields at patient’s surface to shield adjacent organs at risk. This may Result in change of the known dose distribution inherent to the respective standard electron cone size. The purpose of this study is to investigate the impact of such shield on the depth dose and profile and to provide more accurate dose distribution. Methods: Gafchromic EBT2 film was used to measure the doses and dose profiles in solid water for 6 MeV and 9 MeV electron beams. The film was calibrated up to 300 cGy. Each film was scanned 24 hours after irradiation. The doses at central axes were measure at 3 different depths (dmax, 90% and 80% isodose lines (IDL)) for two standard circular sizes (5 cm and 8 cm diameters). Each field was either open or partially blocked at the surface of the solid water by 1, 2 and/or 3 cm wide and 5mm thick lead strips from two opposite edges of the light field. Results: For the majority of the cases the dose to the central axes decreases with increase of the blocked area of the field. This difference is more pronounced for smaller field size and higher energy. Compared to the depth of 80% and 90% IDL, the dose at d max is less influenced by increasing of the blocked area. Conclusion: This work accentuates on the important aspect of the use of custom shields in clinical electron radiation therapy. Particularly when smaller field sizes are used, the dose difference at the prescription depth could be as high as 60%.