2973 - Introducing a New Spot-Scanning Proton Arc Optimization Algorithm with a Variable Tolerance Window to Improve Plan Quality for Bilateral Breast Cancer
Presenter(s)
X. Cong1, Z. Zhang1, G. Liu2, P. Liu3, X. Cao3, X. Li3, P. Chen4, J. T. Dilworth5, and X. Ding3; 1Corewell Health William Beaumont University Hospital, Royal Oak, MI, 2Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, 430023, China, 3Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI, 4Beaumont University Hospital Corewell Health System, Royal Oak, MI, 5Corewell Health William Beaumont University Hospital, Royal Oak, Royal Oak, MI
Purpose/Objective(s): This study aims to introduce a new SPArc algorithm with a variable tolerance window (SPArc-variable) to further improve plan quality for patients receiving bilateral breast and regional nodal irradiation, by allowing extra energy layers across specific arc trajectories.
Materials/Methods:
Starting with the multifield-IMPT plan with 20 degrees apart, the SPArc-variable algorithm iteratively selects a subset of energy layers with decreasing energy layer filtration factor. Then it resamples the energy layers based on the gantry’s movement and irradiation sequence. Finally, spot weighting optimization is applied. Five cases with bilateral breast and lymph nodes treatment are retrospectively selected. Three treatment planning groups were generated, including IMPT with 2 isos, SPArc planning with a fixed distance of 2.5 degrees between adjacent control points(SPArc-original), and SPArc planning with non-fixed distance(SPArc-variable), with a prescription of 5000cGy(RBE). DVH metrics are included to evaluate performance over OARs and targets, and dynamic delivery is simulated via a published dynamic arc system controller.Results: Both SPArc-original and SPArc-variable plans showed slightly better target coverage than IMPT plans, but the difference was not statistically significant. In addition, SPArc-variable showed significantly better performance than IMPT and SPArc-original in sparing the heart(P<0.01 for D1%) and significantly better performance in sparing the left and right lungs(P<0.01 for V20Gy and V5Gy) than both IMPT and SPArc-original. In terms of delivery efficiency, the SPArc-variable shows a slightly longer delivery time than SPArc-original and is superior to IMPT(P=0.01).
Conclusion: This study introduces a novel SPArc optimization algorithm with a variable window to enhance dosimetric performance for bilateral breast cancer patients. More specifically, it reduces the dynamic delivery time compared to IMPT, while achieving equivalent target coverage and significantly superior heart and lungs sparing compared to IMPT and SPArc-original.
Table 1: Dosimetric analysis of IMPT, SPArc-original, and SPArc-variable| Evaluation Metrics | IMPT | SPArc-original | SPArc-variable | |
| CTV | D95%(cGy) | 4582.80±182.59 | 4596.96±194.46 | 4695.14±258.63 |
| CI | 0.73±0.10 | 0.74±0.13 | 0.74±0.13 | |
| Heart | V5Gy(%) | 10.67±5.91 | 7.59±4.90(-28.87%) | 3.28±3.75(-69.29%) |
| Mean(cGy) | 185.56±119.43 | 153.59±128.17(-17.23%) | 72.34±70.06(-61.02%) | |
| D1%(cGy) | 2196.89±1129.16 | 1965.61±1209.91(-10.53%) | 1193.34±1144.12(-45.68%) | |
| Left Lung | V20Gy(%) | 23.33±5.90 | 15.19±4.49(-34.89%) | 7.96±2.20(-65.89%) |
| V5Gy(%) | 48.13±4.05 | 38.40±4.50(-20.22%) | 22.81±3.86(-52.61%) | |
| Mean(cGy) | 1062.31±183.37 | 798.18±129.86(-24.86%) | 460.77±84.57(-56.63%) | |
| Right Lung | V20Gy(%) | 26.08±3.92 | 15.12±5.49(-42.03%) | 8.17±3.74(-68.68%) |
| V5Gy(%) | 50.91±2.19 | 37.52±7.72(-26.29%) | 24.10±7.45(-52.65%) | |
| Mean(cGy) | 1142.60±88.88 | 798.05±216.42(-30.15%) | 481.45±168.35(-57.86%) |