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Dosimetric Comparison of Different Beam Energy in Breast Field-In-Field Forward Treatment Planning

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T Tseng

T Tseng*, J Tam , A Powers , V Dumane , S Green , Y Lo , Mount Sinai Medical Center, New York, NY


SU-I-GPD-T-381 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Using high photon energies in breast treatment planning is a practical way to improve dose homogeneity, especially for patients with large breast separation. This work performed a dosimetric comparison of 10MV versus a combination of 6MV+16MV or 16MV only for breast treatment planning.

Methods: Fourteen patients (maximum breast separation: 23.4 – 29.2cm) planned with 6+16MV (n=11)(16MV weighting: 20-72%) or 16MV only (n=3) were re-planned with 10MV. All the plans used field-in-field planning technique and were calculated with Eclipse® AAA 13.6.23. Prescription isodose was converted to a structure in the original plan, which was used as a guide in the new plan to achieve the same coverage. New field in fields were developed to reduce the hot spot as much as possible, without sacrificing coverage. The plan maximum dose, mean skin dose , and homogeneity index, were used to evaluate plan quality. Wilcoxon sign-rank test was used to test the statistical significance of the results.

Results: The average maximum dose of 6+16/16MV plans, 110.8%±1.3%, was slightly lower than that of 10MV plans, 111.2%±1.4%. The average mean skin dose for the original plans were 86.3%±1.7% compared to 86.8%±1.5% for 10MV, and the HI was 8.91±0.88 compared to 9.21±1.11. 16MV only plans were expected to deviate most from 10MV. However, the average maximum dose in 10MV plans was only 1.2%±0.2% higher than the plans with only 16MV. 10MV plans increase the average skin mean dose by 3.3±0.9%. With the same tangent fields, dose to lung and heart were the same.

Conclusion: Dose homogeneity and subcutaneous tissue coverage are the two main quality indices used to evaluate breast plans. This study demonstrates 10MV can achieve comparable quality to 6+16MV or 16MV with a marginal increase in maximum dose. Subcutaneous tissue coverage of 10MV plans is superior to highly weighted 16MV or 16MV only.

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