Establish a comprehensive plan robustness evaluation platform for intensity modulated proton therapy for bilateral head and neck cancer based on the daily cone-beam computed tomography: a dose accumulation study between the PTV based IMPT (PTV-IMPT) and CTV based robust optimized IMPT (ro-IMPT) planning strategies

Background: A comprehensive plan evaluation platform was established based on the daily cone-beam computed tomography (CBCT) to assess the treatment robustness quality between planning target volume-based intensity modulated proton therapy (PTV-IMPT) and clinical target volume (CTV)-based robust optimized IMPT (ro-IMPT) planning strategies in bilateral head and neck cancer (HNC) treatment. Methods: Nine bilateral HNC patients' CT structure sets were used in this study. Daily CBCTs were converted into synthetic-CT (sCT) for dose reconstruction. The accuracy of the proton dose calculation in sCT is cross-validated via the same day's verification-CT sim (vCT) with 3D gamma index comparison. PTV-IMPT and ro-IMPT were generated on the initial planning CT (pCT). CTV high-risk volume (CTV_high) received 70 Gy and CTV low/intermediate-risk (CTV_low) received 60 Gy. For PTV-IMPT, the PTVs were expanded 3 mm from the CTV; for ro-IMPT, robust optimization used a 3 mm setup and 3.5% range uncertainties. Dose accumulations were then calculated on the 35 sets of daily sCT, and the target coverages were compared to the initial plans. Results: The 3D gamma index dose comparison (3 mm/3%) showed an average pass rate of 98.2%±1.5% comparing the same day's pair of sCT and vCT with both plans (total 38 pairs). Through the dose accumulation of 35 treatment fractions, the PTV-IMPT plan group's mean V100 of CTV_high/CTV_low coverage degraded to 80.70%/85.73% compared to 96.72%/96.13% of the ro-IMPT group (P<0.002). One patient did have suboptimal coverage (CTV_low <90%) even with ro-IMPT. Significant weight loss was noted for this patient during the treatment course (>5 lbs). Conclusions: A comprehensive plan robustness evaluation platform based on the CBCT is established in our clinical workflow and enables dose accumulation and plan robustness evaluation on a daily basis. ro-IMPT demonstrated an optimal planning strategy over PTV-IMPT for bilateral HNC treatment. However, special cautions are needed for patients with significant weight or geometry changes.