This work provides full dosimetric data for a new high-strength 192Ir source currently launched by Varian Oncology Systems for use in their high dose rate remote afterloading systems. The active core length of the new source is reduced to 5 mm compared to a value of 10 mm for the existing VariSource source design, with all other geometric source and encapsulation details being similar. Dose-rate constant, radial dose functions, geometry factors, and anisotropy functions, utilized in the AAPM Task Group 43 dose calculation formalism, were calculated using Monte Carlo simulation. Results are compared with corresponding data published for the existing VariSource and microSelectron high dose rate sources. The dose-rate constant for the new Varian source was found to be equal to 1.101 +/- 0.006cGyh(-1) U(-1), compared to values of 1.043 +/- 0.005 and 1.116 +/- 0.006 cGyh(-1) U(-1) calculated for the existing VariSource and microSelectron sources, respectively. The radial dose functions between the three sources are similar with the exception of their values at radial distances very close to the source (r approximately 2 mm) where differences of approximately 3% are observed. The new Varian source demonstrates a smaller anisotropy relative to the existing VariSource source design for polar angles close to the source longitudinal axis, due to its smaller active core length.

Purpose:To evaluate the usefulness of a time-efficient MRI method for the quantitative determination of tissue iron in the liver and heart of ␤-thalassemic patients using spinspin relaxation rate, R2, measurements. Materials and Methods:Images were obtained at 1.5 T from aqueous Gd-DTPA solutions (0.106 -8 mM) and from the liver and heart of 46 ␤-thalassemic patients and 10 controls. The imaging sequence used was a respiratorytriggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spinecho (SE) pulse sequence (TR ϭ 2000 msec, TE min ϭ 5 msec, echo spacing (ES) ϭ 5 msec, matrix ϭ 192 ϫ 256, slice thickness ϭ 10 mm). Liver iron concentration (LIC) measurements were obtained for 22 patients through biopsy specimens excised from the relevant liver segment. Biopsy specimens were also evaluated regarding iron grade and fibrosis. Serum ferritin (SF) measurements were obtained in all patients.Results: A statistically significant difference was found between patients and healthy controls in mean liver (P Ͻ 0.004) and myocardium (P Ͻ 0.004) R2 values. The R2 values correlated well with Gd DTPA concentration (r ϭ 0.996, P Ͻ 0.0001) and LIC (r ϭ 0.874, P Ͻ 0.0001). A less significant relationship (r ϭ 0.791, P Ͻ 0.0001) was found between LIC measurements and SF levels. R2 measurements appear to be significantly affected (P ϭ 0.04) by different degrees of hepatic fibrosis. The patients' liver R2 values did not correlate with myocardial R2 values (r ϭ 0.038, P Ͻ 0.21). Conclusion:Tissue iron deposition in ␤-thalassemic patients may be adequately quantified using R2 measurements obtained with a 16-echo MRI sequence with short ES (5 msec), even in patients with a relatively increased iron burden.

In this work the extent of the linear dose response and the dynamic dose range of N-vinylpyrrolidone-argon based (VIPAR) polymer gels were investigated. VIPAR gels were irradiated using a 6 MV linear accelerator up to 60 Gy and a Nucletron microSelectron 192Ir HDR brachytherapy source to much higher doses to cover a dose range of two orders of magnitude. They were then MR scanned at 1.5 T to obtain T2-maps. VIPAR gel measurements obtained from the two irradiation regimes were calibrated against ion chamber measurements and dose calculations derived using the AAPM TG-43 protocol respectively. A satisfying agreement between the calibration results derived using the 6 MV x-rays and the 192Ir source was found for doses up to 60 Gy, implying that the response of the VIPAR gels is independent of photon energy and dose rate. A linear R2 dose response up to approximately 40 Gy and a dynamic dose range up to at least approximately 250 Gy were observed. VIPAR gel dose measurements derived using the monoexponentially fitted brachytherapy calibration data were found to be quite accurate.