Dose modification factors for 192Ir high-dose-rate irradiation using Monte Carlo simulation.

A recently introduced brachytherapy system for partial breast irradiation, MammoSite, consists of a balloon applicator filled with contrast solution and a catheter for insertion of an 192Ir high-dose-rate (HDR) source. In using this system, the treatment dose is typically prescribed to be delivered 1 cm from the balloon's surface. Most treatment-planning systems currently in use for brachytherapy procedures use water-based dosimetry with no correction for heterogeneity. Therefore, these systems assume that full scatter exists regardless of the amount of tissue beyond the prescription line. This assumption might not be a reasonable one, especially when the tissue beyond the prescription line is thin. In such a case, the resulting limited scatter could cause an underdose to be delivered along the prescription line. We used Monte Carlo simulations to investigate how the thickness of the tissue between the surface of the balloon and the skin or lung affected the treatment dose delivery. Calculations were based on a spherical water phantom with a diameter of 30 cm and balloons with diameters of 4 cm, 5 cm, and 6 cm. The dose modification factor is defined as the ratio of the dose rate at the typical prescription distance of 1 cm from the balloon's surface with full scatter obtained using the water phantom to the dose rate with a finite tissue thickness (from 0 cm to 10 cm) beyond the prescription line. The dose modification factor was found to be dependent on the balloon diameter and was 1.098 for the 4-cm balloon and 1.132 for the 6-cm balloon with no tissue beyond the prescription distance at the breast-skin interface. The dose modification factor at the breast-lung interface was 1.067 for the 4-cm balloon and 1.096 for the 6-cm balloon. Even 5 cm of tissue beyond the prescription distance could not result in full scatter. Thus, we found that considering the effect of diminished scatter is important to accurate dosimetry. Not accounting for the dose modification factor may result in delivering a lower dose than is prescribed.

Contrast effects on dosimetry of a partial breast irradiation system.

MammoSite is a high-dose rate brachytherapy procedure for partial breast irradiation, which uses a balloon filled with radiopaque iodine-based contrast solution and catheter for insertion of 192Ir high-dose-rate source. The radiopaque material helps visualizing the balloon contour, catheter, and source position within the balloon, which is essential for computerized tomography-based treatment planning and for daily QA using x-ray radiographs. Because of the high content of iodine in contrast media, increased absorption and attenuation of photons may take place within the balloon, which would affect the resultant dose rates outside the balloon. The impact of the concentration of the radiopaque solution on the physical dosimetry of this brachytherapy procedure is investigated in this study using MCNPX (version 2.4) Monte Carlo simulation. Calculations were based on a 30 cm diameter water sphere phantom. The source geometry was that of the Nucletron microSelectron HDR v2 192Ir source. Concentration of the iodine-based radiopaque solution was varied from 5% to 25% by volume, a range recommended by the balloon's manufacturer. Balloon diameters of 4, 5, and 6 cm were simulated. Dose rate per unit air-kerma strength was calculated in 1 mm scoring bin steps. The dose rate reduction at the typical prescription line of 1 cm away from the balloon surface ranged from - 0.8% for the smallest balloon diameter and contrast concentration to a maximum of - 5.7% for the largest balloon diameter and contrast concentration, relative to a water-filled balloon. Limiting the contrast concentration to 10% would insure less than 3% reduction in the prescription dose, regardless of balloon diameter.

Resrad-recycle: a computer model for analyzing radiation exposures resulting from recycling radioactively contaminated scrap metals or reusing radioactively surface-contaminated materials and equipment.

RESRAD-RECYCLE is a computer code designed by Argonne National Laboratory (ANL) to be used in making decisions about the disposition of radioactively contaminated materials and scrap metals. It implements a pathway analysis methodology to evaluate potential radiation exposures resulting from the recycling of contaminated scrap metals and the reuse of surface-contaminated materials and equipment. For modeling purposes, it divides the entire metal recycling process into six steps: (1) scrap delivery, (2) scrap melting, (3) ingot delivery, (4) product fabrication, (5) product distribution, and (6) use of finished product. RESRAD-RECYCLE considers the reuse of surface-contaminated materials in their original forms. It contains representative exposure scenarios for each recycling step and the reuse process; users can also specify scenarios if desired. The model calculates individual and collective population doses for workers involved in the recycling process and for the public using the finished products. The results are then used to derive clearance levels for the contaminated materials on the basis of input dose restrictions. The model accounts for radiological decay and ingrowth, dilution and partitioning during melting, and distribution of refined metal in the various finished products, as well as the varying densities and geometries of the radiation sources during the recycling process. A complete material balance in terms of mass and radioactivity during the recycling process can also be implemented. In an international validation study, the radiation doses calculated by RESRAD-RECYCLE were shown to agree fairly well with actual measurement data.