**A Polymer-Gel Eye-Phantom for 3D Fluorescent Imaging of Millimetre Radiation Beams**

#### **Leonard H. Luthjens, Tiantian Yao and John M. Warman \***

Delft University of Technology, Faculty of Applied Sciences, Department of Radiation Science and Technology, Section Radiation and Isotopes for Health, Mekelweg 15, 2629 JB Delft, The Netherlands; l.h.luthjens@tudelft.nl(L.H.L.);t.yao@tudelft.nl(T.Y.)

 **\*** Correspondence: j.m.warman@tudelft.nl

Received: 30 September 2018; Accepted: 23 October 2018; Published: 26 October 2018

**Abstract:** We have filled a 24 mm diameter glass sphere with a transparent polymer-gel that is radio-fluorogenic, i.e., it becomes (permanently) fluorescent when irradiated, with an intensity proportional to the local dose deposited. The gel consists of >99.9% tertiary-butyl acrylate (TBA), pre-polymerized to ~15% conversion, and ~100 ppm maleimido-pyrene (MPy). Its dimensions and physical properties are close to those of the vitreous body of the human eye. We have irradiated the gel with a 3 mm diameter, 200 kVp X-ray beam with a dose rate of ~1 Gy/min. A three-dimensional (3D) (video) view of the beam within the gel has been constructed from tomographic images obtained by scanning the sample through a thin sheet of UV light. To minimize optical artefacts, the cell was immersed in a square tank containing a refractive-index-matching medium. The 20–80% penumbra of the beam was determined to be ~0.4 mm. This research was a preparatory investigation of the possibility of using this method to monitor the millimetre diameter proton pencil beams used in ocular radiotherapy.

**Keywords:** radiotherapy eye-phantom; radio-fluorogenic gel; X-ray beam imaging; 3D radiation imaging; polymer gel dosimetry
