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A Dosimetric Comparison Between Volumetric Modulated Arc and 3D Treatment Plans for Synchronous Bilateral Breast Patients Treated with Simultaneous Integrated Boost

D Levin

D Levin*, E Shekel, D Epstein, Y Tova, S Zalmanov-Faermann, R Pfeffer, Assuta Medical Centers, Tel Aviv, N/A

TH-A-116-10 Thursday 8:00AM - 9:55AM Room: 116

Purpose: To evaluate dosimetric differences between standard tangential and volumetric modulated arc (VMAT) plans for synchronous bilateral breast patients treated with simultaneous integrated boost (SIB).

Methods: For 10 bilateral breast patients we created separate left and right tangential plans and a single VMAT plan encompassing both breasts and tumor beds as target volumes. Patients were prescribed to 50 and 60 Gy in 2 and 2.4 Gy daily fractions to breasts and tumor beds respectively.
For 3D plans each breast was planned independently. A sum of the independent plans was generated to ensure no dose overlaps, and for comparison with the VMAT plan.
VMAT plans were normalized to 95% dose covering 95% breast volume. Optimization objectives for lungs, heart, and spine were based on QUANTEC data.
Conformity index (CI) was calculated for all plans. Data were analyzed using the Mann-Whitney Rank-Sum test.

Results: Tumor bed and breast coverage was similar for 3D and VMAT plans. However, CI for VMAT was significantly better (p<0.001) for both breast and tumor bed. All organ at risk (OAR) metrics (Mean Lung Dose, V20, V5, maximal heart and spine doses) were significantly better (p<0.001) in the 3D plans.

Conclusion:Conformity in VMAT plans was superior to 3D plans, indicating that in 3D plans larger volumes of non-target tissue received high doses. In addition, VMAT treatment setup is much simpler than for 3D plans, where treatment is delivered sequentially to each breast, necessitating separate setups for each side. Thus, VMAT treatment is faster, and much easier for patients.
However, 3D plans are overwhelmingly superior to VMAT in terms of doses to OARs.
Thus, possibly because the breast is a peripheral organ, it appears to be a unique site, where OARs are better spared by a 'simple' tangential plan rather than a more sophisticated VMAT technique.

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