Cerenkov emission induced by external beam radiation stimulates molecular fluorescence

Abstract

Purpose: Cerenkov emission is induced when a charged particle moves faster than the speed of light in a given medium. Both x-ray photons and electrons produce optical Cerenkov photons in everyday radiation therapy of tissue; yet, this phenomenon has never been fully documented. This study quantifies the emissions and also demonstrates that the Cerenkov emission can excite a fluorophore, protoporphyrin IX (PpIX), embedded in biological phantoms.

Methods: In this study, Cerenkov emission induced by radiation from a clinical linear accelerator is investigated. Biological mimicking phantoms were irradiated with x-ray photons, with energies of 6 or 18 MV, or electrons at energies 6, 9, 12, 15, or 18 MeV. The Cerenkov emission and the induced molecular fluorescence were detected by a camera or a spectrometer equipped with a fiber optic cable.

Results: It is shown that both x-ray photons and electrons, at MeV energies, produce optical Cerenkov photons in tissue mimicking media. Furthermore, we demonstrate that the Cerenkov emission can excite a fluorophore, protoporphyrin IX (PpIX), embedded in biological phantoms.

Conclusions: The results here indicate that molecular fluorescence monitoring during external beam radiotherapy is possible.

Figure 1

Schematics of the experimental setups. (a) A water tank and camera where arrows indicate the irradiation direction of the linear accelerator. (b) The cylindrical scattering phantom with the fiber bundle connected to a spectrometer. (c) The linear accelerator with a phantom placed in the target region. (d) A photograph of the cylindrical scattering phantom.

Figure 2

Experimental setup used for spectrometer calibration.

Figure 3

Cerenkov emission pattern induced by electrons at 18 MeV. (a) The Cerenkov emission pattern as seen from horizontal alignment of the camera and the linear accelerator gantry. The bar represents 1 cm. (b) A schematic picture showing the emission angle relative the particle direction, essentially forming an emission cone.

Figure 4

Cerenkov emission induced by electrons and photons with varying energies and field sizes. In (a)–(e) Cerenkov emission due to an electron beam with a circular field size (diameter 3 cm) at 6 MeV, 9 MeV, 12 MeV, 15 MeV, and 18 MeV respectively. In (f)–(j) Cerenkov emission due to an electron beam with a square field size (4 × 4 cm²) at 6 MeV, 9 MeV, 12 MeV, 15 MeV and 18 MeV respectively. In (k)–(l) Cerenkov emission induced by x-ray photons with a square field size (2 × 2 cm²) at 6 MV and 18 MV, respectively. In (m) the average of the intensity within a square region of 1 cm², centered at the maximum intensity pixel of each image shown as a function of electron energy, (•) electrons with circular field size, (▪) electrons with quadratic field size and (▴) photons with square field size. The photon beam energies, at 6 and 18 MV, are replaced by the average secondary electron energies (originating from Compton or photoelectric interaction), given by 1 and 3 MeV. The bars in (a)–(l) represent 1 cm. All images are acquired in the setup seen in Fig. 1a with the beam incident at an angle of 41°.

Figure 5

Fluorescence spectra induced by an electron beam. The intensity (I) as a function of wavelength (λ) collected from the scattering phantom with increasing levels of protoporphyrin IX, induced by an electron beam at (a) 6 MeV and (b) 18 MeV. The legend specifies the PpIX concentrations in nM. In (c) the incremental intensity (ΔI) at 635 nm is seen as a function of protoporhyrin IX concentration, where (•) is electron energy 6 MeV (R² =  0.96) and (▪▪) 18 MeV (R² =  0.99). In d) the intensity at 635 nm is plotted as a function of field size of the electron beam at 18 MeV where (•) circular field with diameter 1.5 cm, (▴) circular field with diameter 3 cm, (▪) square field size of size 4 × 4 cm²and (♦) square field size of size 6 × 6 cm².

Figure 6

Fluorescence spectra induced by a photon beam. The intensity (I) as a function of wavelength (λ) collected from the scattering phantom with increasing levels of protoporphyrin IX, induced by a photon beam (a) 6 MV and (b) 18 MV. The PpIX concentrations are specified in the legend of Fig. 5 (a)–(b). In (c) the incremental intensity (ΔI) at 635 nm is seen as a function of protoporhyrin IX concentration, where (•) is photon energy 6 MV (R² =  0.94) and (▪) 18 MV (R² =  0.95). In (d) the spectra from chicken muscle tissue is seen where the photon energies are 6 MV and 18 MV.