A Real-Time Monte Carlo System for Internal Dose Evaluation Using an Anthropomorphic Phantom with Different Shapes of Tumors I

In order to evaluate internal dose, MIRD system was proposed by Society of Nuclear Medicine in 1976. MIRDOSE3 computer software based on this concept is currently the most frequently used tool to calculate the organ dose in nuclear medicine community. Unfortunately it uses fixed anthropomorphic phantoms, so the geometric parameters of organs are short of individual characteristics. Although the tumor dose can be calculated by nodule module, the organ doses contributed from tumors are still not included. To conquer this problem and acquire more accurate dose information, a internal dose evaluation system based on real-time Monte Carlo simulation was developed. The INER anthropomorphic phantom was modified from the adult reference man proposed by Cristy and Eckerman. Different shapes of tumor models can be arranged everywhere within the phantom. And an input file for MCNP was then created automatically to simulate the S values of organs and tumors simultaneously. To verify this system, results show that 84% of all organs are within 20% of dose difference with MIRDOSE3. The ratio of thyroid dose is about 1.01 and the effective dose ratio is 0.95. The self S value for a sphere tumor model inserted in abdomen is 5.916E-3 mGy/MBq×s. And additional S values for normal organs are also provided. The agreement of S values with MIRDOSE3 results shows the validity of our system for most target organs with I administration. An example of the inserted tumor model indicates the ability to provide more dosimetric information for patient-specific dose evaluation. Our real-time Monte Carlo internal dose evaluation system could be a good help for more accurate dose estimation.