Room: Exhibit Hall | Forum 3
Purpose: The ring-mounted Linac, Halcyon, only offers 6MV-FFF beam (no electrons) causing treatment of breast cancer patients to be challenging due to the softer beam quality and non-uniform dose profile. This study investigated whether 6MV-FFF is sufficient to generate clinically acceptable plans for hypofractionated left-breast patients treated with deep inspiratory breath hold (DIBH).
Methods: The Halcyon beam model was mimicked using Varian Truebeam 6MV-FFF. Ten left-breast DIBH patients who underwent tangents-style field-in-field (FiF) treatment using mixed-energy flattened beams (6-18 MV,Truebeam) to 42.56 Gy in 16 fractions were re-planned with tangents-style FiF and volumetric modulated arc therapy (VMAT) with 6MV-FFF. PTV coverage, OAR doses, out-of-field doses, and beam-on time were evaluated for all plans.
Results: 6MV-FFF FiF plans presented poorer dose homogeneity (p=0.001) and higher maximum doses (p<0.001) compared to clinical plans. VMAT plans exhibited comparable dose homogeneity (p=0.04) and conformity to clinical plans. VMAT produced higher mean heart doses (2.2Â±0.8Gy) compared to clinical plans (0.9Â±0.1Gy, p=0.001), but still met RTOG-1005 requirements. VMAT gave higher mean dose to the left anterior descending (LAD) artery (4.9Â±1.9Gy) compared to clinical plans (3.0Â±0.7Gy). VMAT produced significantly higher ipsilateral lung V16Gy (p=0.007) and V8Gy (p<0.001) compared to clinical plans, but still met RTOG-1005 constraints. FiF plans were not significantly different from clinical plans in terms of heart, LAD, and ipsilateral lung doses. VMAT significantly decreased skin D5cc (p<0.001), but significantly increased beam-on time and out-of-field doses, such as contralateral breast.
Conclusion: VMAT produced breast plans of similar coverage to clinical plans but with increased OAR doses. 6X-FFF FiF plans spared OARs but decreased target coverage and provided lower dose homogeneity. Results indicate that VMAT may be used to treat selected left-breast DIBH patients on Halcyon. Use of simultaneous integrated boost VMAT to replace sequential electron boost for breast treatments will be further investigated.