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Photon Multileaf Collimators for Electron Radiotherapy

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P Sea

P Sea1,2*, G Gill1,2 , P Taylor1 , E Klein1,2 , (1) Northwell Health (2) Hofstra University


SU-I-GPD-T-71 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Eliminating use of applicators and blocks for electron therapy has been investigated, as they can be replaced by inherent Multi-Leaf Collimators (MLCs). We collected dosimetric data for two electron configurations (conventional, MLC), and compared results.

Methods: We used a Varian 2100 with a series III foil system to make measurements using an applicator at 100 cm SSD, and at 75 cm SSD using conventional photon MLC. We performed flatness and symmetry measurements to test whether our beams were within TG 142 specifications, using an ionization chamber (IC) array for five electron energies. We performed TG 51 protocol to measure output using cylindrical and parallel-plate ionization chambers. PDI’s were measured with and converted to PDD using stopping power ratios. Radiochromic film was used to compare beam characteristics using water and water equivalent plastic.

Results: The MLC configuration showed symmetry to be ~1% for 6, 9, 12, 16 and 20 MeV. Flatness was evaluated at 80% of FWHM, and ranged from 10.4% for 6 MeV and 3.0% for 20 MeV demonstrating a significant dependence on energy. Recipes for margins can be extracted from this profile data. Based on TG-51 we found that a greater output for MLC wasn’t solely due to inverse square. For 6 MeV there was a 14.98% difference for expected vs. measured output. However, when considering the effective SSD for MLC, the expected output approaches the measured. For 6 MeV the percent difference decreased from 13.94% to 6.82%. The PDDs show no distinguishable difference when comparing surface dose, buildup region, and d-max. The film results were comparable to PDDs measured with IC.

Conclusion: These results support the expectation that MLCs can replace the use of applicators and blocks. We’ll perform the same tests on a Varian TrueBeam linac.

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