Čerenkov radiation emission and excited luminescence (CREL) sensitivity during external beam radiation therapy: Monte Carlo and tissue oxygenation phantom studies

Abstract

Radiotherapy generates Čerenkov radiation emission in tissue, and spectral absorption features appearing in the emission spectrum can be used to quantify blood oxygen saturation (S(t)O(2)) from the known absorptions of hemoglobin. Additionally, the Čerenkov light can be used to excite oxygen-sensitive phosphorescence of probe PtG4, whose emission lifetime directly reports on tissue oxygen partial pressure (pO(2)). Thus, it is feasible to probe both hemoglobin S(t)O(2) and pO(2) using external radiation therapy beam to create as an internal light source in tumor tissue. In this study, the sensitivity and spatial origins of these two signals were examined. Emission was detected using a fiber-optic coupled intensifier-gated CCD camera interfaced to a spectrometer. The phosphorescence lifetimes were quantified and compared with S(t)O(2) changes previously measured. Monte Carlo simulations of the linear accelerator beam were used together with tracking of the optical signals, to predict the spatial distribution and zone sensitivity within the phantom. As the fiber-to-beam distance (FBD) varied from 0 to 30 mm, i.e. the distance from the fiber tip to the nearest side of the radiotherapy beam, the effective sampling depth for CR emission changed from 4 to 29 mm for the wavelengths in the range of 600-1000 nm. For the secondary emission (phosphorescence) the effective sampling depth was determined to be in the range of 9 to 19 mm. These results indicate that sampling of S(t)O(2) and pO(2) in tissue should be feasible during radiation therapy, and that the radiation beam and fiber sampling geometry can be set up to acquire signals that originate as deep as a few centimeters in the tissue.

Keywords: (000.2170) Equipment and techniques; (300.0300) Spectroscopy; (300.6390) Spectroscopy, molecular.

Fig. 1

The geometry of the measurement system and temporal acquisition process are shown with (A) the fast time gated spectrometer system, (B) the time line of how the linear accelerator works in pulsed mode and the way to measure decays of CREL. The data were acquired while choosing gate width (D to E) of 3 ms and a gate delay varying (A to D) starting with 10 μs and ending up with 200 μs.

Fig. 2

The geometry of simulations and optical properties of phantom are shown with (A) a 60x60x100 mm cuboid container defined and voxelised into 0.5 mm cubes. The coordinate system was as indicated here, and a fiber with diameter 1.2 mm and numerical aperture 0.22 was posited right on the surface of the container at position (x = 30 mm, y = 0 mm, z = 75 mm). The external electron beam irradiated the phantom from the top surface. In (B) the intersections of a typical simulation to show how the sensitivity distribution appeared in 3D, and in (C) the optical properties are shown of the tissue mimicking phantom made of water, 1% Intralipid, 1% whole blood and PtG4 with a concentration of 5 μM.

Fig. 3

Experimental measurements of CREL are shown for PtG4 in different pO₂ levels. In (A) the Molar extinction coefficient of PtG4 is shown, with (B) the emission spectrum and (C) a gated spectrum measurement of CR shown from tissue mimicking phantoms with different S_tO₂ (S_tO₂ = 92% and S_tO₂ = 5%). (D) A continuous wavelength and gated spectrum measurement of CREL shown from tissue mimicking phantoms with different pO₂ levels (pO₂ = 141.95 Torr and pO₂ = 2.31 Torr). In (E) the lifetime fitting of the fast time gated CREL intensity data is shown.

Fig. 4

Detective sensitivity distribution of Čerenkov radiation emission and sensitivity vs. depth profiles. In (A)-(D) the Detective sensitivity distribution of Čerenkov radiation emission is shown in y-z plane while x = 30 mm for broad electron beam with 18 MeV energy and fiber-beam distances = 0 mm, 10 mm, 20 mm and 30 mm. The coordinate system was the same shown in Fig. 2(A). The external radiotherapy beam propagated in –z direction initially and the fiber was put at (y = 0, z = 75), pointing +y direction. For each fiber-beam distance, wavelength from 300 nm to 1000 nm with 25 nm increment have been investigated and shown here (left to right, top to bottom). In (E)-(H) the sensitivity vs. depth profiles for broad 18 MeV electron beams are shown with fiber-beam distances of 0 mm, 10 mm, 20 mm and 30 mm and wavelength from 300 nm to 1000 nm with 100 nm increment.

Fig. 5

In (A) the effective sampling depth vs. wavelength are shown for fiber-beam distances of 0 mm, 10 mm, 20 mm and 30 mm. In (B) the normalized intensity of Čerenkov radiation emission vs. wavelength is shown (i.e. simulated spectrum).

Fig. 6

Calculated sensitivity distribution and sensitivity vs. depth profile of CREL are shown in (A)-(D) in the y-z plane while x = 30 mm for fiber-beam distances of 0 mm, 10 mm, 20 mm and 30 mm. In (E) the sensitivity vs. depth profiles for the same FBDs are shown.