Cumulative Dose Reconstruction Using CBCT-Based Synthetic CT for Interfractional Tissue Expander Metallic Port Variability
J Liu1*, C Chen1, P Park1, A Shim1, P Tsai1, C Shi1, H Lin1, JI Choi1,2, (1) New York Proton Center, New York, New York, (2) Memorial Sloan Kettering Cancer Center, New York, NY
Presentations
(Saturday, 3/26/2022) 10:30 AM - 12:30 PM [Central Time (GMT-5)]
Room: Celestin D-E
Purpose: Variation in metallic port (MP) position in tissue expanders (TEs) for breast cancer patients who have undergone mastectomy with two-stage reconstruction may impact the validity of delivered doses. The feasibility of utilizing daily CBCT-based synthetic CT scans (synCTs) to account for change in MP positioning during a pencil beam scanning proton therapy treatment course was evaluated.
Methods: Two treatment plans (50.4 Gy(RBE)/28 fractions) were developed on the simulation CT (pCT) and first verification CT (vCT) with MP in discrete positions and were used alternatively based on the daily MP position. TE delineations were performed on all CT images, including the MP, based on manufacturer specifications with appropriate density assignments. Field-specific targets (FSTs) around the MP were used to avoid protons passing through high-Z materials. Total 28 synCTs generated from daily CBCTs with actual MP positions were used to reconstruct the cumulative dose.
Results: Hot spots >120% were found behind the 2 planned MP positions in the fractional doses on synCTs but decreased to 115% cumulatively. The cumulative D95% of the chest wall CTV decreased by 1.1% and 2.5% on pCT and vCT, respectively, from 95% in the nominal plans. The cumulative V20Gy(RBE) of left lung were 15.3% and 16.8% on pCT and vCT, respectively, lower than the V20Gy(RBE) of 20% in the nominal plans. The cumulative Dmean of heart decreased to 72.8 cGy(RBE) on pCT but increased to 161.3 cGy(RBE) on vCT. The dosimetric impact due to interfractional MP dislocations was spared with the use of 2 treatment plans.
Conclusion: Cumulative dose can be reconstructed using CBCT-based synCTs, which represent patient real-time position and anatomy during daily treatments. The robustness of breast proton plans using FSTs around the MP can be improved with multiple fields and plans, which provides forgiveness of dose heterogeneity incurred from dislocation of high-Z materials.
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