Ionization Chamber Model Research Articles

Validation of a Monte Carlo model of a NACP-02 plane-parallel ionization chamber model using electron backscatter experiments

The accuracy of Monte Carlo (MC) simulation results relies on validating the MC models used in the calculations. In this work, a MC model for the NACP-02 plane-parallel ionization chamber was built and validated against megavoltage electron backscatter experiments using materials of water, graphite, aluminium and copper. Electron energies ranged between 6–18 MeV and the chamber's air cavity was at the depth of maximum dose, zmax. A chamber model based on manufacturer's specifications resulted in systematic discrepancies of several percents between measured and simulated backscatter factors. Tuning of the MC chamber model against backscatter factors to improve agreement increased the chamber's front window mass thickness by 35% over the reported value of 104 mg cm−2 in the IAEA's TRS-398 absorbed dose protocol. The large increase in chamber window mass thickness was verified by measurements on a disassembled NACP-02 chamber. The new backscatter factor results based on the tuned MC NACP-02 chamber model matched the experimental results within 1–2 standard deviations. We conclude therefore that for MC simulations near zmax, tuning of the NACP-02 chamber model against experimental backscatter measurements is an acceptable method for validating the chamber model.

Multiple‐estimate Monte Carlo calculation of the dose rate constant for a cesium‐131 interstitial brachytherapy seed

The purpose of this study was to calculate a more accurate dose rate constant for the 131Cs (model CS-1, IsoRay Medical, Inc., Richland, WA) interstitial brachytherapy seed. Previous measurements of the dose rate constant for this seed have been reported by others with incongruity. Recent direct measurements by thermoluminescence dosimetry and by gamma-ray spectroscopy were about 15% greater than earlier thermoluminescence dosimetry measurements. Therefore, we set about to calculate independent values by a Monte Carlo approach that combined three estimates as a consistency check, and to quantify the computational uncertainty. The calculated dose rate constant for the 131Cs seed was 1.040 cGy h(-1) U(-1) for an ionization chamber model and 1.032 cGy h(-1) U(-1) for a circular ring model. A formal value of 2.2% uncertainty was calculated for both values. The range of our multiestimate values were from 1.032 to 1.061 cGy h(-1) U(-1). We also modeled three 125I seeds with known dose rate constants to test the accuracy of this study's approach.

Experience on the neutron activation of natural/enriched Re, Sm, and Ho nuclides in a reactor for the production of radiotherapeutic radionuclides.

In recent years, much effort has been concentrated on the use of beta-emitting radionuclides for the treatment of various cancers. The reports suggested the application of 186Re and 153Sm as radiotherapeutic radionuclides for the treatment of palliative widespread skeletal metastases, whereas 166Ho was suggested as an agent for radiation synovectomy. Hence, a study on the production of 186Re, 153Sm, and 166Ho radionuclides was carried out by neutron activation of the appropriate target materials using a Pakistan Atomic Research Reactor (PARR-1) at a neutrons flux of 1 x 10(4) n/cm2 s. These radionuclides were then converted to appropriate radiopharmaceuticals for their use on animals and patients. The targets of natural Re (metal), natural Sm2O3, enriched Sm2O3 (99.06%), Sm(NO3)3 (solid), Sm(NO3)3 (liquid), and Ho2O3 were irradiated in the PARR-1. After irradiation, the purity of these radionuclides were checked by a multichannel analyzer, Canberra series 85 (MCA) coupled with HPGe detector and then measured in radioisotope calibrator Capintec ionization chamber model CRC-5RH. The effect of the irradiation time and amount of target material was investigated on the production yields of the radionuclides. The results showed an increase in the specific activity of Re with an increase in the irradiation time from 1 to 72 h, whereas a decrease in the specific activity was observed with increase in the amount of Re from 10 to 100 mg. Similar results were obtained for 153Sm and 166Ho radionuclides. The results further indicated that the specific activity of powder target was much less than the liquid targets for 153Sm. Their conversion to the appropriate radiotherapeutic radiopharmaceuticals were also carried out by investigating the experimental conditions and acceptable quality of 186Re-HEDP and 153Sm-EDTMP complexes were prepared. These complexes were then used on animals and patients which showed good performance.