A New Modality Of Automatic Planning For Breast Cancer Radiation Therapy

Adjuvant radiotherapy after conserving surgery of breast cancer represents a large number of patient planning. The Auto-Planning makes the entire process faster, less labor intensive and more reproducible, but requires the ‘ad hoc’ license purchase. The objective of this study was to evaluate a workflow for the automation and standardization of breast treatment plan, using a commercial TPS without buying expensive licenses. The procedure requires only two user interactions. The Automated tangential volumetric modulated arc therapy (AutotVMAT) technique for breast irradiation was compared to standard tangential field in field method (FinF). Twenty-eight women with left-sided breast cancer were enrolled in this retrospective study. OARs and PTV were delineated on CT data sets acquired in treatment position (supine, arms up). The prescription dose was 50 Gy in 25 fractions. Treatment plans were created for a Linac equipped of 40 leaf pairs of 4 mm width. The treatment energy was 6 MV. Two treatment plans were generated for each patient: 1) standard tangential field-in-field (FinF), 2) Automated tangential VMAT (AutoVMAT) with two dual arcs of 60°. The AutoVMAT, plans were created with TPS based on X-ray Voxel Montecarlo algorithm. The Tamplate plans were standardized with constrained radiobiological cost functions in two steps: multi criterial optimization to improve OAR sparing and Pareto optimization to increase PTV coverage. Minimum segment width of 1.0 cm and high fluence smoothing was used in the optimization. Maximum number of control points per plan was set to 256. A standard deviation of 0.5% was used in Monte Carlo dose calculation with a dose grid of 3.0 mm. The 3D-CRT FinF plans were generated in TPS with Superposition dose calculation algorithm with the same dose grid. Quantitative and dosimetric evaluation of the plans was performed using the DVH. The number of Monitor Units (MUs) was also recorded. In ipsilateral lung AutoVMAT plans decreased the high dose areas (V20VMAT=1,57%±0,6 vs V20FinF=5,3%±1,2), however increased the low dose ones (V5VMAT=12,2%±1,8 vs V5FinF=8,7±2,1). Significant cardiac dose sparing (V25 and the mean dose) was achieved with Auto-VMAT technique compared to FinF, and non-significant reduction of V2 of the heart was achieved with FinF. There were no differences in the mean dose of contralateral breast, but AutoVMAT plan increased the Maximum dose (DmaxVMAT=3,8±0,9Gy vs DmaxFinF=2,1±0,5Gy). The dose coverage (V47.5 Gy) was greatest with AutoVMAT plan (97,8%vs 92,2%) The average number of MUs in one fraction were 659±93MU and 255±5 MU for AutoVMAT and FinF respectively. The AutoVMAT technique is an effective method for achieving a homogeneous dose coverage reducing doses to heart and lung. Furthermore it is a fast planning process that doesn’t require the manual interaction and can be perform by dosimetrist leaving the validation to the physician.