Author: Chingyun Cheng, Ben Durkee, Carri K. Glide-Hurst, Minglei Kang, Haibo Lin, Bhudatt R. Paliwal, Charles B. Simone, Zhizhen Wei, Tengda Zhang, Xingyi Zhao 👨🔬
Affiliation: University of Wisconsin, Department of Mechanical Engineering, University of Wisconsin-Madison, Departments of Human Oncology and Medical Physics, University of Wisconsin-Madison, New York Proton Center, Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Department of Human Oncology, University of Wisconsin-Madison 🌍
Purpose: Transmission Beam (TB), Single-Energy Bragg Peak (SEBP), and Single-Energy Spread-Out Bragg Peak (SESOBP) are primary proton conformal FLASH techniques. However, each comes with significant limitations, including poor tissue conformity, reduced dose rates, or dependence on costly patient-specific ridge filters. These challenges make them impractical for clinical implementation in human tumors. We hereby propose SEBP-IMPT, a novel hybrid planning strategy combining the high dose rate and conformity of SEBP with the uniformity and flexibility of conventional dose rate Intensity Modulated Proton Therapy (CONV-IMPT).
Methods: An in-house hybrid treatment planning module was developed to integrate SEBP and CONV-IMPT for simultaneous optimization of FLASH treatment plans. The cost function for dose optimization was integrated based on minimum MU requirements of machine beam currents, to ensure FLASH coverage at tumor boundaries and critical OARs. The tumor core was irradiated by CONV-IMPT to maintain the uniformity since the FLASH dose rate is unnecessary. A CONV-IMPT treatment plan was also generated using the same prescription and beam arrangement as reference. Dose and dose rate evaluations were conducted, and clinical dose metrics were analyzed and compared among different approaches for liver SBRT cases.
Results: The SEBP-IMPT hybrid delivery approach simultaneously optimized both dose and dose rate for critical OARs and achieved dosimetry comparable to conventional IMPT. Compared to SEBP alone, SEBP-IMPT reduced the number of beams required, lowered the demand for patient-specific ridge filters and improved the homogeneity at tumor core. For the critical OAR tissue adjacent to the high dose region, the FLASH coverage volume ratio is reported as 100% at a dose threshold of 5 Gy.
Conclusion: The SEBP-IMPT hybrid planning approach demonstrates the potential for low-current proton machines to treat human tumors at FLASH dose rates, achieving plan quality comparable to IMPT with the added potential benefit of enhanced FLASH-sparing effects on OARs.