Program Information
Precise Monte Carlo Simulations of NaI(Tl) Spectra for Measurement of Actinium-225 Targeted Alpha Particle Bio-Distribution
Christopher J. Tichacek1,2*, Mikalai M. Budzevich3 , Gary V. Martinez3 , David L. Morse1,4 , Eduardo G. Moros1,2,4 , University of South Florida, (1) Department of Physics, Medical Physics, Tampa, FL; H. Lee Moffitt Cancer Center & Research Institute, (2) Radiation Oncology, (3) Small Animal Imaging Core, and (4) Department of Cancer Imaging and Metabolism, Tampa, FL
Presentations
WE-RAM3-GePD-IT-5 (Wednesday, August 2, 2017) 10:30 AM - 11:00 AM Room: Imaging ePoster Theater
Purpose: The use of targeted ligands to deliver radioisotopes directly to tumor cells has become a promising therapeutic strategy. Because of short path length and high LET, targeted alpha-particles are ideal for treating metastatic disease while minimizing damage to surrounding normal tissues. To calculate the risk of radiation dose to patients, the activity of the radioisotopes must be measured in tissues. ²²⁵Ac and its radioactive daughters, ²²¹Fr and ²¹³Bi, have a total combined gamma spectrum with over 75 lines, hence, the precise measurement of their radioactivity requires a high resolution gamma ray detection system. Using a low resolution NaI(Tl) detector ordinarily available in clinics may underestimate radiation dose up to 40%. To overcome these limitations, the accurate spectrum of ²²⁵Ac gamma rays in the NaI(Tl) detector was simulated using Monte Carlo. Later, these results were applied to determine correction coefficients for measured in-vivo activities per organ.
Methods: MCNP6 was used to simulate the 4π-geometry of a NaI(Tl) scintillation detector. The isotropic multi-energetic source was simulated as a standard 5 mL test tube filled with water and uniformly distributed ²²⁵Ac. The energetic bin-width was set to 1-keV to model an ideal high definition detector. The simulated spectrum was then convolved with a quantum efficiency Gaussian energy broadening function specific to the NaI(Tl) gamma detector (BioDex). The resulting spectrum was compared to an experimentally obtained one of known activity and the correction coefficient was calculated.
Results: The simulated NaI(Tl) spectrum of ²²⁵Ac and resulting convolved spectrum were successfully generated. The correction coefficient for measured activity was estimated as 2.6.
Conclusion: In this study, the Monte Carlo spectrum of ²²⁵Ac was obtained for a standard clinical NaI(Tl) gamma detector. Our model is capable of calculating correction coefficients that can be readily applied to available commercial scintillation detectors.
Funding Support, Disclosures, and Conflict of Interest: Funding: Melanoma Research Alliance Team Science Award, NIH/NCI SBIR Phase I, NIH/NCI P50, Moffitt Skin SPORE Career enhancement Program, Moffitt Imaging and Technology COE funds. Disclosures and Conflict of Interest: This work has been performed in part, in collaboration with Modulation Therapeutics Inc.
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