In the search for water-equivalent detectors, researchers have long pursued the idea of active scintillation dosimeters. Plastic scintillators in particular have demonstrated water equivalence in megavoltage beams, thereby avoiding the additional corrections required to account, for instance, for air in ionization chambers or high-density materials in diodes that are present right at the point of measurement. As ever smaller fields became desirable for stereotactic treatments, use of traditional detectors became increasingly complex, since confounding factors such as changing beam quality with decreasing field size now had to be considered. Here again, the size of scintillators offered an intriguing option.
So why were no commercial scintillators available until Standard Imaging released the Exradin W1 in 2014?
Organic scintillators produce visible light when irradiated. The amount of light produced is directly proportional to the dose delivered to the scintillator. This light is transmitted through an optical fiber to a detection system that converts the visible light into electrical current, again directly proportional to the amount of radiation delivered to the scintillator. So far so good.
The difficulty with scintillators, and the primary factor that hindered commercialization despite their long history in the research sphere, is the stem effect caused by the production of Cerenkov light in the optical transfer fiber. Since Cerenkov light also falls within the visible spectrum, it overlaps the light produced by the scintillator itself and adds an undesirable stem effect signal. Eliminating this signal with techniques like two-fiber methods or hollow fibers came with its own difficulties. The second fiber in the two-fiber method is offset from the measurement fiber and may not receive identical dose, particularly in steep dose gradients. Hollow fibers introduced an air cavity close to the measurement location.
In 2011, Guillot et al. published a manuscript1
describing a two-color chromatic method for Cerenkov removal, which greatly simplified the Cerenkov correction and enabled Standard Imaging to develop the Exradin scintillation detectors. This method relies on the stability of the shape of the Cerenkov spectrum as the amount of fiber in the field changes. If two measurements are made with identical dose to the scintillator but different lengths of optical fiber within the field, the only changes in the measured composite spectrum are due to the Cerenkov signal. By splitting the measured signal into two portions, call them Blue and Green, the relative changes in the integrated signal in each of these regions can be attributed solely to Cereknov. A correction factor, called the Cerenkov Light Ratio (CLR) is generated based on the changes in the measured signal in those two regions.
CLR = (BlueMAX – BlueMIN)/(GreenMAX – GreenMIN)
Where BlueMAX and GreenMAX are blue and green channel signals with the maximum amount of fiber in the field, and BlueMIN and GreenMIN are the blue and green channel signals with the minimum amount of fiber in the field.
Using this known relationship, subsequent measurements can be corrected for Cerenkov using the CLR factor.
M = Blue – (Green*CLR)
Where M is the corrected measurement, and Blue and Green are the integrated signals from the corresponding portions of the spectrum.
A proportional amount of the green signal is subtracted from the blue signal, and what is left is the scintillator signal itself. The correction can be performed rapidly: for scanning measurements, the Exradin W2 system takes in the raw optical signal, applies the Cerenkov correction, and converts the corrected signal into a proportional electronic current that can be read by a water tank electrometer.
So while small fields are still daunting to most physicists, the availability of commercial organic scintillator detectors have greatly simplified the process of small field dosimetry.
About the Author
Shannon Holmes, Ph.D., is a Medical Physicist with Standard Imaging. She obtained her graduate degrees in Medical Physics from the University of Wisconsin – Madison, and bachelor’s degree in Physics from the University of Puget Sound in Tacoma, WA, USA.
1 – M. Guillot, L. Gingras, L. Archambault. “Spectral method for correction of the Cerenkov light effect in plastic scintillation detectors: A comparison study of calibration procedures and validation in Cerenkov light-dominated situations.” Med. Phys. 38(4); 2140-2150 (2011).