Chest Phantom Development for Chest X-ray Radiation Protection Surveys, Internal Beta Dosimetry of an Iodine-131 Labelled Elastin-Like Polypeptide, and I-131 Beta Detection Using a Scintillating Nanoparticle Detector

Project 1: Chest Phantom Development for Chest X-ray Radiation Protection Surveys

Purpose: Develop an acrylic phantom to accurately represent an average adult’s chest for use in radiographic chest unit radiation protection surveys.

Materials and Methods: 6 sheets of 3.81 cm thick acrylic were cut and assembled to form a 30.5 x 30.5 x 20.3 cm hollow box phantom. The acrylic served as tissue equivalent material and the hollow center simulated lungs in a human patient. Six sheets of 1 mm thick aluminum were cut to line the inner walls of the acrylic phantom to potentially boost scatter radiation. Three phantoms underwent posterior-anterior (PA) and lateral chest protocol radiographic scans: the acrylic phantom (with and without the aluminum lining), a 3 gallon water bottle filled with water, and an adult male anthropomorphic phantom. The phantoms were set up as though they were adult patients and scanned with automatic exposure control. Scatter radiation was measured with ion chamber survey meters at 4 points within the room for each phantom and protocol. The scatter data from the acrylic phantom and water bottle were compared to the anthropomorphic phantom to determine which one more accurately represented an adult patient.

Results: For the PA protocol, the average percent difference in measurements between the acrylic phantom and anthropomorphic phantom was 33.3±28.8% with the aluminum lining and 33.0±21.2% without the lining. The percent difference between the water bottle and anthropomorphic phantom was 66.5±42.0%. For the lateral protocol, the average percent difference in measurements between the acrylic phantom and anthropomorphic phantom was 157.6±5.6% with the aluminum lining and 143.0±17.6% without the lining. The percent difference between the water bottle and anthropomorphic phantom was 78.3±22.8%.

Conclusions: The acrylic phantom provided a more accurate comparison to the anthropomorphic phantom than the water bottle for the PA protocol. For the lateral protocol, neither the acrylic phantom nor water bottle provided an adequate comparison to the anthropomorphic phantom.

Project 2: Internal Beta Dosimetry of an Iodine-131 Labelled Elastin-Like Polypeptide

Purpose: Develop a model and simulation to better understand the dosimetry of an I-131 labeled elastin-like polypeptide (ELP) brachytherapy technique.

Materials and Methods: To develop the model, an average scenario based on mouse trials was explored. A 125 mg tumor was approximated as a sphere, with the I-131 ELP injected into its center. The ELP solidifies into a spherical depot – approximately 1/3 the volume of the tumor – and becomes a permanent brachytherapy source. The injected activity of I-131 was 1.25 mCi. I-131 primarily emits β radiation with an average energy of 182 keV, therefore it was determined that all such emissions were confined within the bounds of the tumor. Gamma emissions associated with I-131 were ignored as they were determined to have enough energy to escape the bounds of the tumor without any interaction. This model was implemented into a simulation using the Monte Carlo program FLUKA. From this simulation, the absorbed dose to the tumor and ELP depot, along with the dose profile, was calculated.

Results: The tumor received an absorbed dose of 72.3 Gy while the ELP received 1.14×10^3 Gy. From the dose profile, it was determined that 99% of the absorbed dose to the tumor was highly localized to a 0.3 mm region surrounding the ELP depot.

Conclusions: The model and simulation provided a better understanding of the dosimetry underlying the novel ELP brachytherapy technique. Results obtained demonstrated that the ELP method delivers doses that are comparable to current conventional brachytherapy techniques.

Project 3: I-131 Beta Detection Using a Scintillating Nanoparticle Detector

Purpose: Determine if a scintillating nanocrystal fiber optic detector (nano-FOD) could detect β emissions from I-131.

Materials and Methods: The nano-FOD’s β response was tested using a source vial containing 101 mCi of I-131 in 2 mL of stabilizing solution. A glass vial containing the I-131 was placed inside a lead pig for shielding. A 1 mm diameter hole was drilled through the tops of the vial and pig to allow insertion of the nano-FOD. Measurements were taken every day over a 17 day period by repeatedly submerging the nano-FOD in the I-131 solution and recording the voltage signal it produced. The activity at the time of measurement was calculated based on the time and date of data acquisition. The net signal and signal-to-noise ratio (SNR) were then calculated and plotted as functions of I-131 concentration.

Results: The nano-FOD produced a measurable response when exposed to the β emissions of I-131. The net signal and SNR both demonstrated a linear correlation with the concentration of I-131.

Conclusions: The nano-FOD was demonstrated to be capable of β detection with a linear correlation to activity. If the signals measured can be calibrated to radiation exposure, then the nano-FOD has promising applications as a novel β detector.