Deliverable proton conformal FLASH radiotherapy treatment planning for head and neck re-irradiation patients

Clinical translation of ultra-high dose rate (UHDR) delivery to harness potential FLASH effect requires a treatment planning system (TPS) to optimize and calculate dose and dose rate in patients. Proton conformal FLASH treatment aims to deliver pencil beam scanning (PBS) Bragg Peaks to the tumor region with UHDR. In this work, we conducted a treatment planning study for head and neck (H&N) re-irradiation patients using a research-version of a commercial TPS paired with conformal FLASH hardware integrated into a nozzle of a clinical cyclotron-based system.

Fifteen H&N patients were planned for re-irradiation of 40 GyRBE in 5 fractions to the area of intact tumor. The TPS was configured with validated UHDR beam measurement to generate optimized patient FLASH plans with one or two beams, delivered as single-beam-per-fraction (SBPF). Each beam consists of a deliverable mono-energetic PBS map, a 3D-printable conformal energy modulator design, a selection of aluminum range shifter plates, and a brass aperture. Python scripts with machine-specific delivery timing parameters were used for Monte Carlo dose and dose rate calculations. Clinical VMAT and IMPT plans were also generated for dosimetric comparison.

All plans met the tumor target and OAR planning objectives. Conformal FLASH plans showed very similar dose distributions to the clinical IMPT plans. Compared to VMAT plans, both IMPT and FLASH plans have reduced low dose region, maximum cord dose D0.03 cc (8.37 ± 0.94 vs. 3.19 ± 3.81 and 4.32 ± 3.12 GyRBE, respectively), contra-lateral parotid mean dose (1.88 ± 0.99 vs. 0.00 ± 0.01 and 0.00 ± 0.00 GyRBE, respectively) and contra-lateral submandibular gland mean dose (2.49 ± 1.06 vs. 0.14 ± 0.13 and 0.19 ± 0.19 GyRBE, respectively). With 500 nA quasi-continuous nozzle beam current, the mean dose-averaged dose rate in CTVs of these 15 patients achieved 95.75 ± 22.78 Gy/s.

We report the deliverable proton conformal FLASH treatment plans for H&N re-irradiation patients using the innovative hardware configuration and measured beam data in our institution. The FLASH plans have very similar plan qualities to clinical IMPT proton plans and were deliverable with our proton machine. The machine specific 3D dose rate distribution can be calculated and displayed in the TPS.