Author: Jae Won Jung, Choonsik Lee, Jason C. Mace, Sergio Morato Rafet 👨🔬
Affiliation: East Carolina University, Carilion Clinic, National Cancer Institute 🌍
Purpose: Dosimetry for radionuclides used in nuclear medicine can be accurately calculated if sufficient data is available. However, in most cases crucial information cannot or will not be known a priori. Furthermore, the dosimetric process is often arduous, requiring intensive resources with respect to personnel and technology which results in limited attention to a large percentage of patients. Convenient models based on MIRD formalism have been developed to overcome the aforementioned challenges, however estimates are not patient specific. We performed voxel-based dosimetry on 55 adult patients with various body sizes using Monte Carlo methods to access the variance of individual S-values for select tissues and to model their dependency on macroscopic patient characteristics.
Methods: Eight organs were segmented from patient CT images using TotalSegmentator. MCNP was used to calculate patient specific S values for I-131. The results were compared with S values calculated from NCINM calculator, which is based on the reference phantoms. The pharmaceutical kinetics and uptake were held constant during the computational analysis and comparison. The variance and the patient’s body size attributes were analyzed to evaluate trends.
Results: I-131 therapy, the first radiopharmaceutical therapy commonly utilized, showed a large variability in S values between patients. We observed an inverse correlation between S value ratio and patient BMI. A shielding effect between source and target tissues may partially explain the dependency. A polynomial fit was calculated post outlier removal as a phantom extension method. Additional radiomic analytics may allow for more accurate approximations in the future.
Conclusion: Personalized dosimetry must account for radiation type, energy and anatomical characteristics of a given patient. If estimates are provided to consent and/or make care decisions, they must accurately reflect an individual’s radiation burden, we believe macroscopic properties in conjunction with phantom results allow for improved dose estimates prior to therapy.