Author: Arpit M. Chhabra, J Isabelle Choi, Minglei Kang, Haibo Lin, Hang Qi, Charles B. Simone, Shouyi Wei, Irini Yacoub, Francis Yu, Xingyi Zhao, Ajay Zheng 👨🔬
Affiliation: New York Proton Center, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Department of Human Oncology, University of Wisconsin-Madison 🌍
Purpose:
Pencil beam scanning (PBS) proton therapy effectively treats ocular cancer by minimizing exit doses to normal tissues. However, challenges remain in sparing nearby organs-at-risk(OARs). Ultra-high dose rate (UHDR) proton beams demonstrate the "FLASH" effect, potentially improving therapeutic outcomes. This study evaluates integrating beam-specific apertures into PBS Bragg Peak UHDR treatment plans(BP-UHDR-aperture) to enhance dose conformity for ocular tumors while reducing exposure to surrounding tissues.
Methods:
Five ocular cancer patients treated with conventional intensity-modulated proton therapy (CONV-IMPT) or stereotactic body radiation therapy (SBRT) were retrospectively selected, with a total prescription dose of 50Gy delivered in 5 fractions. Research plans used an in-house treatment planning system with proton BP-UHDR beams, with and without beam-specific apertures, calculated using Monte Carlo simulation. UHDR conditions assumed a beam current of 215nA, minimum MU/spot, and 2ns minimum spot dwelling time. Apertures were created with a 3mm margin from the target at the beams eye view, and a 20mm air gap was maintained between the aperture and the patient. Plan quality was compared across CONV-IMPT, BP-UHDR, and BP-UHDR-aperture plans, evaluating CTV coverage, OAR dose metrics, and dose rates.
Results:
BP-UHDR-aperture plans achieved comparable dose conformity to ocular tumors as BP-UHDR and CONV-IMPT. However, higher hot doses in CTV regions increased Dmax for overlapping OARs, including the ipsilateral eye, optic nerves, retina, and lacrimal glands. Apertures reduced ipsilateral cornea doses, with reductions of 32.8% in Dmean and 22.4% in Dmax, compared to CONV-IMPT plans. Dose rates for normal tissues and OARs exceeded 100Gy/s on average. Volumes receiving dose rates above 40 Gy/s included 83.87%(retina), 81.33%(optic nerves), 74.93%(lacrimal gland), and 76.13% (conjunctiva).
Conclusion: BP-UHDR with apertures offers potential for ocular cancer treatment by sharpening the beam penumbra and reducing corneal doses while maintaining target coverage. However, increased hot doses in overlapping OARs require further investigation before clinical application.