The role of intravascular ultrasound imaging in vascular brachytherapy

Intracoronary brachytherapy has recently emerged as a new therapy to prevent restenosis. Initial experimental work was achieved in animal models and the results were assessed by histomorphometry. Initial clinical trials used angiography to guide dosimetry and to assess efficacy. Intravascular ultrasound (IVUS) permits tomographic examination of the vessel wall, elucidating the true morphology of the lumen and transmural components, which cannot be investigated on the lumenogram obtained by angiography. This paper reviews the use of IVUS in the clinical studies of brachytherapy conducted to date. IVUS allows clinicians to make a thorough assessment of the remodeling of the vessel and appears to have a major role to play in facilitating understanding of the underlying mechanisms of action in this emerging field. The authors propose that state-of-the-art IVUS techniques should be employed to further knowledge of the mechanisms of action of brachytherapy in atherosclerotic human coronary arteries.     

Standards for intravascular brachytherapy dosimetry.

The dosimetry of intravascular brachytherapy sources is quite difficult due to the small size of the sources and the close proximity at which measurements are necessary. The development of National Standards for the dosimetry of these sources is described in this paper as well as predictions of how these standards will be changed and improved upon in the future.     

Dosimetry of source stepping for intravascular brachytherapy

Purpose: Both β and γ sources of fixed length are currently used in the catheter-based intravascular brachytherapy (IVBT). Source stepping is often used to treat a lesion longer than the effective treatment length of the source. A major challenge for the stepping procedure is to attain a perfect dosimetric match (uniform dose) at the source junction. This work presents a quantitative and systematic dosimetric analysis for source stepping during an IVBT procedure. Materials and Methods: The three most commonly used β and γ sources (¹⁹²Ir by BEST, ⁹⁰Sr by NOVOSTE and ³²P by Guidant) were studied using the EGSnrc Monte Carlo code. Dose distributions were calculated for a perfect end-to-end match and for a range of end-to-end gaps and overlaps between consecutive steps. Results: It is found that a perfect end-to-end match during source stepping yields uniform dose distribution in the region of source junction. The doses in the case of a mismatch (in the presence of an end-to-end gap or overlap) were found to be significantly different from those with the perfect end-to-end match. The dose deviation depends on the size of the gap or overlap, radial distance and type of source. The dose deviation decreases with radial distance for a given gap/overlap. For example, for a gap/overlap of 2 mm, dose decreases/increases of 30%, 55% and 60% were found at the radial distance of 2 mm from source for ¹⁹²Ir, ⁹⁰Sr and ³²P, respectively. These dose deviations are reduced by approximately 10% when the radial distance increases from 2 to 3 mm. The dose deviations for gaps or overlaps in the range of 0–5 mm are presented. Conclusions: During an IVBT procedure involving source stepping, a perfect end-to-end match is always desired. Significant underdosing or overdosing can occur in the case of a source mismatch. A considerable caution should be exercised to ensure that sources are properly matched.     

Ytterbium-169: A promising new radionuclide for intravascular brachytherapy

Purpose: To explore the feasibility of ¹⁶⁹Yb (γ, 93 keV) as a new radionuclide for intravascular brachytherapy (IVBT) in terms of dose distribution, penetration power, and radiation safety features as compared with ¹²⁵I and ¹⁹²Ir. Methods: The dose distributions for catheter-based sources, ¹⁶⁹Yb, ¹²⁵I, and ¹⁹²Ir, in homogeneous water and in the presence of calcium and a steel stent have been determined and compared using the Monte Carlo method (MCNP4B2 code). The dose rates of the sources were evaluated from 0.02 to 100 cm. Results: In the short distance range (0.02<r<1.0 cm), the dose distributions in homogeneous water are very similar for the three radionuclides when the dose rates are normalized at 2 mm. Between 1 and 20 cm, the relative dose rates fall off similarly for ¹⁶⁹Yb and ¹⁹²Ir, whereas for ¹²⁵I, it decreases much more rapidly. At a distance further away (r∼100 cm), the dose rate of ¹⁶⁹Yb is about 10 times lower than that of ¹⁹²Ir, indicating the cathlab radiation shielding requirement for ¹⁶⁹Yb is substantially reduced as compared with ¹⁹²Ir. Calcified plaques and stents cause a drastic dose reduction in the arterial wall for ¹²⁵I, but have no effect for ¹⁹²Ir γ-rays. Only slight dose reductions were detected for ¹⁶⁹Yb beyond a layer of 1.0-mm calcium (2–3%), and behind a steel stent strut (5%). Conclusion: ¹⁶⁹Yb is a promising new radionuclide for IVBT. It has a much better penetrating power through calcified plaques and stents compared with the low-energy source ¹²⁵I. It also provides easier radiation protection measures for cardiac cathlab personnel than the high-energy source ¹⁹²Ir, while preserving a favorable dose distribution in tissues surrounding an arterial vessel.     

The Beginnings of the American Society of Zoologists

The American Society of Zoologists takes its origins from severalpreceding organizations, especially the American Society ofNaturalists and the American Morphological Society. Leadersof these parent groups were prominent among the founders ofthe American Society of Zoologists. The development of our presentSociety from its predecessors, and the roles played by our earlyleaders are briefly described in this paper.