**4. Conclusions**

We have shown that it is possible to produce a three-dimensional, fluorescent image of the energy deposited by a 3 mm beam of high-energy radiation in a gel medium of physical properties and dimensions close to those of the vitreous body of the human eye. A 3D video image of the beam, with submillimetre spatial resolution, can be produced in-house within minutes of irradiation using a portable tomographic fluorescence-scanning apparatus with refractive index matching. We intend to apply the method to the study of energy deposition by proton pencil beams and to the control of radiotherapy protocols and equipment when the Holland Proton Therapy Clinic is eventually commissioned (www.hollandptc.nl/en/).

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/10/11/1195/ s1, Video S1: "Movie full color.MP4" and "Movie gray.MP4".

**Author Contributions:** Conceptualization, L.H.L., T.Y. and J.M.W.; Methodology, L.H.L., T.Y. and J.M.W; Validation, L.H.L., T.Y. and J.M.W.; Formal Analysis, L.H.L., T.Y. and J.M.W.; Writing-Review & Editing, J.M.W.

**Acknowledgments:** The authors wish to acknowledge Gertjan Bon of the glassblowing department of the University of Amsterdam who realized the form of the eye phantom cell.

**Conflicts of Interest:** The authors declare no conflict of interest.
