Feasibility Study for 3-Dimensional Treatment Planning with Flattening Filter Free (FFF) Beam and Jawless Dual-Level MLC Collimation for Breast Irradiation

Abstract

Varian Halcyon system with 6MV flattening filter free (FFF) beam and jawless dual-level MLC collimation is mainly intended for faster IMRT/VMAT delivery. We examined if three- dimensional (3D) treatment planning is feasible for breast irradiation with Varian Halcyon system. Ten left sided breast cancer patients treated with conventional 6MV and 3D field in field (FIF) technique (group1; breast volume < 1000 cc) and 5 patients treated with mixed energy of 6/18 MV and 3D FIF technique (group2; breast volume ≥ 1000 cc) were selected to re-plan with Varian Halcyon system using 3D FIF technique. Treatment plans for both groups with Varian Halcyon system using 3D FIF were generated using a treatment planning system based on reference plan parameters previously used for the treatment. FIF planning process with Halcyon system for each case was similar to conventional 3D FIF technique, and the plan criteria was to achieve the same (or similar) D95% for breast planning target volume (PTV) as the reference plan. Plan quality was assessed using 1) V105% (PTV covered by 105% of prescription dose; dose homogeneity), 2) mean heart dose, 3) maximum heart dose, 4) V20Gy (ipsilateral lung volume covered by 20Gy) and compared to the reference plan for each case. In addition, three tangential beam IMRT breast plans treated with 6/18 MV mixed energies (group3) were re-planned with Halcyon system using IMRT technique. The mean breast volume for group 1 and group 2 were 548 cc (ranges 312-912 cc) and 1172 cc (ranges 1000- 1360 cc), respectively. D95% for PTV was almost identical between reference plan and Halcyon plan for both groups. Other mean dosimetric values with group1 were as following; 1) V105: 5.8 % vs. 8.3 % (p = 0.006), 2) Mean heart dose: 0.88 Gy vs. 1.13 Gy (p = 0.01), 3) Max heart dose: 13.1 Gy vs. 13.5 Gy (p = 0.8), 4) Ipsilateral lung V20 Gy: 8.7 % vs. 10.4 % (p = 0.26) for reference plan and Halcyon plan, respectively. Similarly for group 2; 1) V105: 5.9 % vs.22.2 % (p = 0.03), 2) Mean heart dose: 0.73 Gy vs. 1.18 Gy (p < 0.001), 3) Max heart dose: 7.7 Gy vs. 11.6 Gy (p = 0.2), 4) Ipsilateral lung V20 Gy: 7.0 % vs. 7.4 % (p = 0.85) for reference plan and Halcyon plan, respectively. For the IMRT plan comparison (group 3; breast volume ranges 1255-1687 cc), all 3 reference plans showed better dosimetric results compared to Halcyon plans; 1) V105: 5.2 % vs.18.1 %, 2) Mean heart dose: 0.85 Gy vs. 3.0 Gy, 3) Max heart dose: 10.6 Gy vs. 40.3 Gy, 4) Ipsilateral lung V20 Gy: 5.3 % vs. 14.9 % for reference plan and Halcyon plan, respectively. The 3D treatment planning with Varian Halcyon system is feasible for breast irradiation with comparable dose homogeneity and critical organs dosimetry for breast volume < 1000 cc. However, for large breast volumes the dose homogeneity is significantly worse with both 3D plan and IMRT probably because of having only low energy with non-flattened beam.