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A Monte Carlo Method for Determining the Response Relationship between Two Commonly Used Detectors to Indirectly Measure Alpha Particle Radiation Activity
Journal article   Open access   Peer reviewed

A Monte Carlo Method for Determining the Response Relationship between Two Commonly Used Detectors to Indirectly Measure Alpha Particle Radiation Activity

Christopher J. Tichacek, Mikalai M. Budzevich, Thaddeus J. Wadas, David L. Morse and Eduardo G. Moros
Molecules (Basel, Switzerland), Vol.24(18), p.3397
09/01/2019
DOI: 10.3390/molecules24183397
PMCID: PMC6767018
PMID: 31546752
url
https://doi.org/10.3390/molecules24183397View
Published (Version of record) Open Access

Abstract

Using targeted ligands to deliver alpha-emitting radionuclides directly to tumor cells has become a promising therapeutic strategy. To calculate the radiation dose to patients, activities of parent and daughter radionuclides must be measured. Scintillation detectors can be used to quantify these activities; however, activities found in pre-clinical and clinical studies can exceed their optimal performance range. Therefore, a method of correcting scintillation detector measurements at higher activities was developed using Monte Carlo modeling. Because there are currently no National Institute of Standards and Technology traceable Actinium-225 (Ac-225) standards available, a well-type ionization chamber was used to measure 70.3 +/- 7.0, 144.3 +/- 14.4, 222.0 +/- 22.2, 299.7 +/- 30.0, 370.0 +/- 37.0, and 447.7 +/- 44.7 kBq samples of Ac-225 obtained from Oak Ridge National Lab. Samples were then placed in a well-type NaI(Tl) scintillation detector and spectra were obtained. Alpha particle activity for each species was calculated using gamma abundance per alpha decay. MCNP6 Monte Carlo software was used to simulate the 4 pi-geometry of the NaI(Tl) detector. Using the ionization chamber reading as activity input to the Monte Carlo model, spectra were obtained and compared to NaI(Tl) spectra. Successive simulations of different activities were run until a spectrum minimizing the mean percent difference between the two was identified. This was repeated for each sample activity. Ionization chamber calibration measurements showed increase in error from 3% to 10% as activities decreased, resulting from decreasing detection efficiency. Measurements of Ac-225 using both detector types agreed within 7% of Oak Ridge stated activities. Simulated Monte Carlo spectra of Ac-225 were successfully generated. Activities obtained from these spectra differed with ionization chamber readings up to 156% at 147.7 kBq. Simulated spectra were then adjusted to correct NaI(Tl) measurements to be within 1%. These were compared to ionization chamber readings and a response relationship was determined between the two instruments. Measurements of Ac-225 and daughter activity were conducted using a NaI(Tl) scintillation detector calibrated for energy and efficiency and an ionization chamber calibrated for efficiency using a surrogate calibration reference. Corrections provided by Monte Carlo modeling improve the accuracy of activity quantification for alpha-particle emitting radiopharmaceuticals in pre-clinical and clinical studies.
 Biochemistry & Molecular Biology Chemistry Chemistry, Multidisciplinary Life Sciences & Biomedicine Physical Sciences Science & Technology

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