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Department of Nuclear Medicine, University of Massachusetts Medical Center, Worcester, Massachusetts
Department of Radiation Physics, Lund University, Lund, Sweden
Department of Nuclear and Radiological Engineering, University of Florida, Gainesville, Florida
Medical Science Division, Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee
Instituto de Pesquisas Energeticas e Nucleares, Sao Paulo, Brazil
Faculty of Science, Aleppo University, Aleppo, Syria
Correspondence: For correspondence or reprints contact: George Mardirossian, PhD, University of Massachusetts Medical Center, Department of Nuclear Medicine, 55 Lake Ave. N., Worcester, MA 01655.
ABSTRACT
A revised geometric representative model ofthe lower part of the colon, including the rectum, the urinary bladder and prostate, is proposed for use in the calculation of absorbed dose from injected radiopharmaceuticals. The lower segment ofthe sigmoid colon as described in the 1987 Oak Ridge National Laboratory mathematical phantoms does not accurately represent the combined urinary bladder/rectal/prostate geometry. In the revised model in this study, the lower part of the abdomen includes an explicitly defined rectum. The shape of sigmoid colon is more anatomically structured, and the diameters of the descending colon are modified to better approximate their true anatomic dimensions. To avoid organ overlap and for more accurate representation of the urinary bladder and the prostate gland (in the male), these organs are shifted from their originally defined positions. The insertion of the rectum and the shifting of the urinary bladder will not overlap with or displace the female phantom's ovaries or the uterus. In the adult male phantom, the prostatic urethra and seminal duct are also included explicitly in the model. The relevant structures are defined for the newborn and 1-, 5-, 10- and 15-y-old (adult female) and adult male phantoms. Methods: Values of the specific absorbed fractions and radionuclide S values were calculated with the SIMDOS dosimetry package. Results for 99mTc and other radionuclides are compared with previously reported values. Results: The new model was used to calculate S values that may be crucial to calculations of the effective dose equivalent. For 131I, the S (prostate 
urinary bladder contents) and S (lower large intestine [LLI] wall urinary bladder contents) are 6.7 x 10-6 and 3.41 x 10-6 mGy/MBq x s, respectively. Corresponding values given by the MIRDOSE3 computer program are 6.23 x 10-6 and 1.53 x 10-6 mGy/MBq x s, respectively. The value of S (rectum wall urinary bladder contents) is 4.84 x 10-5 mGy/MBq x s. For 99mTc, we report S (testes prostate) and S (LLI wall prostate) of 9.41 x 10-7 and 1.53 x 10-7 mGy/MBq x s versus 1.33 x 10-6 and 7.57 x 10-6 mGy/MBq x s given by MIR DOSE3, respectively. The value of S (rectum wall prostate) for 99mTc is given as 4.05 x 10-6 mGy/MBq x s in the present model. Conclusion: The new revised rectal model describes an anatomically realistic lower abdomen region, thus giving improved estimates of absorbed dose. Due to shifting the prostate gland, a 30%–45% reduction in the testes dose and the insertion of the rectum leads to 48%–55% increase in the LLI wall dose when the prostate is the source organ.
Key Words: Reference Man • human phantom • modeling • Monte Carlo simulation • S values
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