Radiation dose in computed tomography of the pelvis: comparison of helical and axial scanning.

An anthropomorphic Rando phantom was used to compare radiation doses sustained during helical and conventional axial CT of the pelvis. The values obtained with the Rando phantom were validated against cadaveric phantoms, and show good agreement. For the authors' particular CT unit, helical scanning was found to deliver a lower radiation dose than conventional axial scanning. This was most prominent at 1.0-s tube rotation times (average dose ratio 1.24). For realistic scanning parameters and exposure factors, the ratio of radiation dose to pelvic organs can be expected to lie in the range of 40-100 mGy. The whole-body effective dose (ED) depends on selection of scanning parameters and patients anatomy. In a favourable case scenario, the ED for CT scanning of the pelvis in a male can be expected to be between 10 and 20 mSv if the scrotum is not included in the radiation field, while the ED in a female will be approximately 20 mSv. An examination of scatter radiation fall-off curves from a single slice shows that the spread of scatter radiation is only marginally affected by slice thickness. A total of 10-12 cm of human soft tissue acts as a good barrier against internal scattered radiation. The use of such scatter fall-off curves, together with manufacturers' dosimetry specifications, allows a fast estimate of absorbed dose.

Technetium-99m-methylene diphosphonate uptake in the fetal skeleton at 30 weeks gestation.

Retention of 99mTc-MDP in the fetal skeleton and placenta at 30 and 32 wk gestation was observed during bone scan examination of the maternal skeleton for staging of malignant tumors. The implications and significance of these observations are discussed.

Samarium-153-labelled EDTMP for bone metastases from cancer of the prostate.

A Phase I dose-escalation trial of a bone seeking phosphonate labelled with Samarium-153 was conducted on heavily pretreated patients with widespread bony metastases from cancer of the prostate. The bone marrow dose was calculated initially on a 4:1 uptake of cortical to cancellous bone, but subsequent information suggested the distribution was approximately equal so that the marrow received about three times the dose that has been prescribed. As a result, what were at first thought to be doses of 0.5 Gy, 1.0 Gy and 1.5 Gy were probably 1.5 Gy, 3.0 Gy and 4.5 Gy respectively. Three patients were treated at each dose level. The dose was repeated in the first three. Pain relief was delayed for 2 weeks, and was maximal by 4 weeks. All but one patient gained some benefit but this was transitory, lasting only 4-6 weeks in most. A recalculated dose of 3 Gy proved the most effective. The major toxicity was to platelets. Thrombocytopenia was fatal in four cases (two with repeat doses). All patients died but three survived more than 6 months with the help of third line hormonal measures and local radiotherapy to maintain comfort. A second group of five patients, not previously irradiated, were given 153Sm-EDTMP to a marrow dose of 3 Gy and all have survived for more than 4 months and achieved minimal to excellent relief of pain. The technique is recommended as initial therapy in unirradiated patients with good bone marrow function and with significant pain above and below the diaphragm, making half-body irradiation less likely to be a useful palliative procedure.

The relation of radionuclide uptake by bone to the rate of calcium mineralization. I: Experimental studies using 45Ca, 32P and 99Tcm-MDP.

The study of the uptake of radionuclides by bone has been undertaken in a mouse tail graft model using 45Ca, 32P and a routine bone scanning agent 99Tcm-MDP, together with serial calcium determinations. The model provided an experimental system in which the calcium mineral content and the rate of mineralization both changed progressively throughout its development. A significant linear correlation was found between 45Ca and 32P uptake and the rate of calcium mineralization, which held for all stages of the graft's growth. Both radiotracers therefore accurately reflected the calcium mineral deposition. In the case of 99Tcm-MDP, the correlation with mineralization rate only applied for the most active growth period of the graft when the rate was increasing. For all radiotracers, the peak in bone uptake corresponded to the maximum in mineralization rate.

The relation of radionuclide uptake by bone to the rate of calcium mineralization. II: Patient studies using 99Tcm-MDP.

The relation of radionuclide uptake by bone and rate of calcification has been studied in normal vertebrae and in vertebral metastases from cancer of the prostate. Specifically, the determination of 99Tcm-MDP uptake by radionuclide scanning and the estimation of calcium concentration of trabecular bone by dual-energy computed tomography have provided the means of obtaining a relation between these parameters which was similar to that found in an animal model, in which the dependence of radionuclide uptake on the rate of mineralization was established. This relationship has enabled the experimental findings to be extrapolated to those in patient studies and has shown that in sclerotic bone lesions, the increase in 99Tcm-MDP uptake accompanying the progression of the metastases was proportional to the rate of calcification.