Author: Sean P. Boyer, Shae Gans, Mingcheng Gao, Draik B. Hecksel, Mark Pankuch 👨🔬
Affiliation: Northwestern Medicine Proton Center, Northwestern Medicine Chicago Proton Center 🌍
Purpose: Protons deposit a large portion of their energy at the end of their range in a region known as the Bragg peak, which may increase the relative biological effective (RBE) dose in this region. A larger volume of high RBE dose can occur with thin targets of shallow range where the dose in the Bragg peak accounts for a larger proportion of the total dose delivered. We investigated the effects of plan optimization methods on RBE dose for a thin intracranial target.
Methods: A thin intracranial clinical target volume with a diameter of 1.5 cm was created on the right side of brain phantom. Treatment plans were created using optimization that included robust optimization with 2.5mm of positional robustness and 3.5% range robustness, a low dose limiting ring structure, and a PTV requiring 95% coverage. For each plan, RBE dose was calculated using a scaled version of the model by McNamara D(RBEMcNamara) and dose average LET (LETd) was calculated using the Raystation treatment planning system.
Results: A single en face treatment field resulted in a 110% D(RBEMcNamara) volume of 20.51 cc and a D1% LETd of 6.7 keV/µm. By using two fields with a 110% hinge angle, single field uniform dose, robust optimization, PTV coverage of 95% of prescription dose and a maximum MU of 3 MU/spot, we were able to reduce the 110% D(RBEMcNamara) to 0.1 cc and the D1% LETd to 4.2 keV/µm. Similar techniques in a previous study reduced the 110% D(RBEMcNamara) from 2.52 cc to 0 cc using multi-field optimization, a hinge angle of 180⁰, and a max MU/spot of 1.
Conclusion: In a fashion similar to a centrally located intracranial target, it may be possible to use optimization techniques to reduce RBE dose in regions of the brain distal to an intracranial target.