A new rectal model for dosimetry applications

A revised geometric representative model of the lower part of the colon, in cluding the rectum, the urinary bladder and prostate, is proposed for use i n the calculation of absorbed dose from injected radiopharmaceuticals. The lower segment of the sigmoid colon as described in the 1987 Oak Ridge Natio nal Laboratory mathematical phantoms does not accurately represent the comb ined 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 diame ters of the descending colon are modified to better approximate their true anatomic dimensions. To avoid organ overlap and for more accurate represent ation of the urinary bladder and the prostate gland tin the male), these or gans 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 pha ntom, the prostatic urethra and seminal duct are also included explicitly i n 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 calculate d with the SIMDOS dosimetry package. Results for Tc-99m and other radionucl ides are compared with previously reported values. Results: The new model w as used to calculate S values that may be crucial to calculations of the ef fective dose equivalent. For I-131, the S (prostate <-- urinary bladder con tents) 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. Correspondi ng 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 (tes tes <-- 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 MIRDOSE3, respectively. The value of S (rectum wall <-- prostate) for 99 mTc 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 abd omen region, thus giving improved estimates of absorbed dose. Due to shifti ng the prostate gland, a 30%-45% reduction in the testes dose and the inser tion of the rectum leads to 48%-55% increase in the LLI wall dose when the prostate is the source organ.