A new rectal model for dosimetry applications.

UNLABELLED: A revised geometric representative model of the 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 of the 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 MIRDOSE3, 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.

Radiation dosimetry in nuclear medicine.

Radionuclides are used in nuclear medicine in a variety of diagnostic and therapeutic procedures. A knowledge of the radiation dose received by different organs in the body is essential to an evaluation of the risks and benefits of any procedure. In this paper, current methods for internal dosimetry are reviewed, as they are applied in nuclear medicine. Particularly, the Medical Internal Radiation Dose (MIRD) system for dosimetry is explained, and many of its published resources discussed. Available models representing individuals of different age and gender, including those representing the pregnant woman are described; current trends in establishing models for individual patients are also evaluated. The proper design of kinetic studies for establishing radiation doses for radiopharmaceuticals is discussed. An overview of how to use information obtained in a dosimetry study, including that of the effective dose equivalent (ICRP 30) and effective dose (ICRP 60), is given. Current trends and issues in internal dosimetry, including the calculation of patient-specific doses and in the use of small scale and microdosimetry techniques, are also reviewed.

Biokinetics of indium-111-bleomycin complex in head and neck cancer--implementations for radiochemotherapy.

Bleomycin (BLM) has been used successfully for treatment of head and neck cancer. Combining radionuclide therapy with chemotherapy is a fascinating possibility. We have studied the biokinetics of BLM labeled with indium-111 (In-111). A complex formed at low pH had an activity of 100 MBq/mg BLM. This substance was intravenously injected into 10 head and neck cancer patients in escalating doses of 75, 175, and 375 MBq. Scintigraphic data from these patients were compared with tissue samples obtained at surgery. The activity distribution and penetration into tumor tissue was not affected by increasing the injected activity. In-111-BLMC uptake was directly proportional to Ki-67/MIB-1 activity and number of mitoses in tumor tissue. Based on the biokinetics, dosimetric calculations for In-111 and In-114m have been performed. S values for realistic geometry (a phantom designed from Patient CT) have been calculated. In-114m could deliver a 4-fold absorbed radiation dose into the tumor compared with In-111. We think that In-111-BLMC could be used for radiochemotherapy in head and neck cancer or adjuvant Auger-electron therapy using In-114m combined with BLM. Further studies on cellular dosimetry should be undertaken.

Indium-111 bleomycin complex for radiochemotherapy of head and neck cancer--dosimetric and biokinetic aspects.

Bleomycin (BLM) is used for the treatment of head and neck cancer. In order to improve the effectiveness of this chemotherapeutic drug, BLM was combined with indium-111. A complex of these agents (111In-BLMC), formed at low pH, was injected intravenously into ten head and neck cancer patients in escalating activities of 75, 175 and 375 MBq. The internally delivered dose to the tumours varied from 0.20 to 2.73 mGy at 75 MBq, from 0.33 to 2.51 mGy at 175 MBq, and from 0.87 to 31.3 mGy at the 375 MBq activity level. Uptake of radioactivity was 0.45+/-0.24x10(-3)% ID/g in primary tumours and 0. 52+/-0.20x10(-3)% ID/g in metastases (at 48 h). Tumour volumes varied from 0.51 to 49.0 cm3. The radioactivity half-lives in the tumours were 30+/-7 h. The activity distribution and penetration into tumour tissue were not affected by increasing the injected activity. There was a positive correlation between BLMC uptake and Ki-67/Mib activity as well as number of mitoses in tumour tissue. These data indicate that 111In-BLMC has potential as a radiochemotherapeutic agent in head and neck cancer and that adjuvant Auger-electron therapy is possible using 114mIn-labelled BLMC.

A Monte-Carlo program converting activity distributions to absorbed dose distributions in a radionuclide treatment planning system.

In systemic radiation therapy, the absorbed dose distribution must be calculated from the individual activity distribution. A computer code has been developed for the conversion of an arbitrary activity distribution to a 3-D absorbed dose distribution. The activity distribution can be described either analytically or as a voxel based distribution, which comes from a SPECT acquisition. Decay points are sampled according to the activity map, and particles (photons and electrons) from the decay are followed through the tissue until they either escape the patient or drop below a cut off energy. To verify the calculated results, the mathematically defined MIRD phantom and unity density spheres have been included in the code. Also other published dosimetry data were used for verification. Absorbed fractions and S-values were calculated. A comparison with simulated data from the code with MIRD data shows good agreement. The S values are within 10-20% of published MIRD S values for most organs. Absorbed fractions for photons and electrons in spheres (masses between 1 g and 200 kg) are within 10-15% of those published. Radial absorbed dose distributions in a necrotic tumor show good agreement with published data. The application of the code in a radionuclide therapy dose planning system, based on quantitative SPECT, is discussed.