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 (StO2) 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 (pO2). Thus, it is feasible to probe both hemoglobin StO2 and pO2 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 StO2 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 StO2 and pO2 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.
Highlights
Recent work demonstrated the occurrence of Čerenkov emission light during external beam radiotherapy [1], using linear accelerator (LINAC) irradiation from electrons and photons
Simulation results about the detection sensitivity distribution and effective sampling depth for both couple Čerenkov Radiation (CR) and CREL will be shown and discussed
The lifetime of PtG4, an oxygen sensitive phosphor, recovered using CREL was calculated by CREL correctly by fitting the decay and could be potentially adopted to reveal the tissue pO2 value in the sampling region
Summary
Recent work demonstrated the occurrence of Čerenkov emission light during external beam radiotherapy [1], using linear accelerator (LINAC) irradiation from electrons and photons. While a previous study demonstrated that spectroscopy of tissue hemoglobin oxygen saturation (StO2) is feasible using Čerenkov emission as an internal light source, the spatial origins of the signal and the detection sensitivity distribution were not examined. The present study considers this, as well as the potential to induce phosphorescence of exogenous oxygen-sensitive probes [3] in tissue, thereby measuring both tissue StO2 and oxygen partial pressure (pO2) [4]. This combined approach may be useful in tumor radiotherapy, as it may allow comprehensive quantification of tissue hypoxia as well as changes in hypoxia during the course of fractionated radiation therapy. The spatial origins of both absorption-based StO2 and phosphorescent-based pO2 signals have been quantified in our study through experiments and Monte Carlo modeling
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