Purpose: A low dose, beam's-eye-view, proton radiographic projection of a patient, with an accurate water equivalent thickness (WET), could help validate patient anatomical changes and guide range-adaptive proton therapy. Towards this end, it might be worth sacrificing transverse spatial resolution to provide WET estimates that are fast, and implementable at existing clinical proton energies of up to 230 MeV.
Methods: A residual-energy focus lens was designed using 4 magnetic quadrupoles, with focus exit energies of 60, 100 and 150 MeV. The radiographic properties of this system are evaluated in Geant4. The total length of the lens is 3.5 m, with 0.75 m of space from patient center to the first quadrupole magnet. The response of the system was simulated on a square water stepwedge with thicknesses of 15, 16, 17, 18 and 19 cm. 10â?· protons were used to acquire an image on a 20 cm Ã— 20 cm detector grid with 500 Âµm pixel spacing.
Results: Lenses focusing residual energies of 100 MeV or less were insufficient to create a coherent image at low dose. The 150-MeV system modelled here provides a 20 cm Ã— 20 cm field of view, with relatively high maximum field gradients of 13.9 T/m. The radiograph deposits 20 ÂµGy of dose in the water phantom, which is much less than a kV planar image acquired on traditional linacs. Spatial resolution ranges from 1.5 to 4 mm, due to chromatic (off focus energy) blur.
Conclusion: The radiographic system provides limited-resolution images, towards providing estimates of the WET from instantaneous proton radiographs, and minimal dose deposition. The limited amount of space available for a lens in a treatment environment limits the flexibility of the system, however, stronger quadrupole magnets enable a reduced footprint. Higher energy proton radiography would improve image quality with less deposited dose.