Dose distribution for endovascular brachytherapy using Ir-192 sources: comparison of Monte Carlo calculations with radiochromic film measurements

Optimising treatment distance and treatment area for HDR surface mould brachytherapy

The purpose of this study was to quantify the effect of treatment area and treatment distance on dose distributions for geometrically optimised surface mould plans in order to provide guidance in choosing treatment parameters and constructing moulds for individual patients. Geometrically optimised plans were generated with a typical brachytherapy planning system and measurements were taken with radiochromic film over depths of 5-32 mm with an (192)Ir high dose rate source. Films were calibrated with a cylindrical geometry technique utilising the (192)Ir source and readout was performed with a flatbed scanner. The rate of dose fall-off about the prescription plane, as well as the magnitude and extent of local dose maxima superficial to the prescription plane, increased with decreasing treatment areas when inter-catheter spacing and treatment distance were kept constant. The dose fall-off was highly dependent on treatment distance, with a 16 % reduction in dose 4 mm superficial to the treatment depth occurring when the distance was increased from 10 to 20 mm while maintaining a 10 mm inter-catheter spacing. The table generated using the measured planar geometry data, can be used as an initial guide for mould construction and planning. The properties of high dose regions near to the catheter plane are highly dependent on the treatment area, which must be considered when normal tissue dose tolerances are a concern. Treatment distance is a key variable influencing the overall dose distribution and should be adjusted as a function of the desired tumour to skin dose ratio, controlled by mould thickness.

Physics-aspects of dose accuracy in high dose rate (HDR) brachytherapy: source dosimetry, treatment planning, equipment performance and in vivo verification techniques

This study provides a review of recent publications on the physics-aspects of dosimetric accuracy in high dose rate (HDR) brachytherapy. The discussion of accuracy is primarily concerned with uncertainties, but methods to improve dose conformation to the prescribed intended dose distribution are also noted. The main aim of the paper is to review current practical techniques and methods employed for HDR brachytherapy dosimetry. This includes work on the determination of dose rate fields around brachytherapy sources, the capability of treatment planning systems, the performance of treatment units and methods to verify dose delivery. This work highlights the determinants of accuracy in HDR dosimetry and treatment delivery and presents a selection of papers, focusing on articles from the last five years, to reflect active areas of research and development. Apart from Monte Carlo modelling of source dosimetry, there is no clear consensus on the optimum techniques to be used to assure dosimetric accuracy through all the processes involved in HDR brachytherapy treatment. With the exception of the ESTRO mailed dosimetry service, there is little dosimetric audit activity reported in the literature, when compared with external beam radiotherapy verification.

In vitro comparison of the antiproliferative effects of rhenium-186 and rhenium-188 on human aortic endothelial cells

PURPOSE

Rhenium-186 ((186)Re) and rhenium-188 ((188)Re) are promising radionuclides for the inhibition of restenosis after percutaneous transluminal angioplasty or other vascular interventions. Until now the maximal dose tolerance of endothelial cells has not been clearly known.

MATERIALS AND METHODS

To characterize the effects of local irradiation treatment, human aortic endothelial cells (ECs) were incubated with different doses of (186)Re and (188)Re. Two days after plating, ECs received treatment for a period of 5 days. The total radiation doses applied were 1, 4, 8, 16, and 32 Gy. On days 1, 3, 5, 7, and 12 after initial rhenium incubation, cell growth, clonogenic activity, cell-cycle distribution, and cytoskeletal architecture were evaluated.

RESULTS

From the first day on, a dose-dependent growth inhibition was observed. Cumulative doses of ≥32 Gy caused a weak colony formation and significant alterations in the cytoskeletal architecture. An increased fraction of cells in G2/M phase was seen for cumulative radiation doses of ≥16 Gy. Interestingly, there were no significant differences between (186)Re and (188)Re.

CONCLUSION

Even for low dose rates of β particles a dose-dependent proliferation inhibition of ECs is seen. Doses beyond 32 Gy alter the cytoskeletal architecture with possibly endothelial dysfunction and late thrombosis.

Evaluation of scatter contribution and distance error by iterative methods for strength determination of HDR 192Ir brachytherapy source

High-dose rate (HDR) 192Ir brachytherapy sources are commonly used for management of malignancies by brachytherapy applications. Measurement of source strength at the hospital is an important dosimetry requirement. The use of 0.6-cm(3) cylindrical ionization chamber is one of the methods of measuring the source strength at the hospitals because this chamber is readily available for beam calibration and dosimetry. While using the cylindrical chamber for this purpose, it is also required to determine the positioning error of the ionization chamber, with respect to the source, commonly called a distance error (c). The contribution of scatter radiation (M(s)) from floor, walls, ceiling, and other materials available in the treatment room also need to be determined accurately so that appropriate correction can be applied while calculating the source strength from the meter reading. Iterative methods of Newton-Raphson and least-squares were used in this work to determine scatter contribution in the experimentally observed meter reading (pC/s) of a cylindrical ionization chamber. Monte Carlo simulation was also used to cross verify the results of the least-squares method. The experimentally observed, least-squares calculated and Monte Carlo estimated values of meter readings from HDR 192Ir brachytherapy source were in good agreement. Considering procedural simplicity, the method of least-squares is recommended for use at the hospitals to estimate values of f (constant of proportionality), c, and M(s) required to determine the strength of HDR 192Ir brachytherapy sources.

Dosimetry of the microSelectron-HDR Ir-192 source using PRESAGE and optical CT

Optical CT, using a solid polyurethane (PRESAGE) radiochromic dosimeter, has been used to evaluate dose distributions produced by the microSelectron-HDR Ir-192 source. The anisotropy functions obtained through optical CT are in good agreement with Monte Carlo and previously published results especially at polar angle above 20 degrees. The results indicated an evident potential for using solid polymer dosimetry as an accurate method for 3-D dosimetry, although refinements to the existing methods are necessary before the technique can be used clinically.