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Electron Beam Dosimetry for Standard Circular Cutouts to Ensure Adequate Target Coverage in the Treatment of Small Superficial Lesions

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Y Lee

Y Lee*, Y Kim , University of Arizona, Tucson, AZ


SU-I-GPD-T-70 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Although isodose distributions and penumbra characteristics at a depth of interest are important dosimetric information, the current practice of clinical setup in electron beam therapy is usually based on percent depth dose (PDD) curves and cutout factors. The purpose of this study is to establish electron beam dosimetry at clinically relevant depths for standard circular cutouts to ensure adequate target coverage in the treatment of small superficial lesions.

Methods: PDD curves of 6 MeV and 9 MeV electron beams for circular cutouts with diameters of 4 cm, 5 cm, 6 cm, 7 cm and 8 cm as well as for a 10 × 10 cm2 open applicator were measured at source-to-surface distance (SSD) 100 cm in water. Profiles were also measured at dmax (depth of maximum dose) and four clinically relevant depths (R95 (distal depth of 95% maximum dose), R90, R85 and R80) in the same setup. Cutout factors were determined in water as well.

Results: The PDD curves for 4 cm cutout at 6 MeV and for 4 cm and 5 cm cutouts at 9 MeV were shifted toward water surface. The width of isodose lines (IDLs) and penumbra obtained from the profiles increased with depth but the dependence of penumbra on cutout diameter was minimal at all depths for both energies. Output factors for 4 cm cutout were more than ± 5% different from unity for both energies.

Conclusion: Due to own beam characteristics at each of clinically relevant depths for standard circular cutouts, caution is advised in determining cutout size for the treatment of small legions. Therefore, it is recommended that PDD curves and profiles (isodose distributions and penumbra characteristics) at clinically relevant depths for small cutouts be measured and carefully reviewed for clinical setup to ensure adequate target coverage.

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