Dose perturbation caused by stents: experiments with a model 90Sr/90Y source

PURPOSE

Biological effects of intravascular brachytherapy are very sensitive to discrepancies between the prescription and the applied dose. If brachytherapy is aimed at in-stent restenosis, shielding and shadowing effects of metallic stents may change the dose distribution relative to that produced by the bare source. The development of new generations of stents inspired us to a new experimental study in this field. The effect was studied for 14 stents which we have recently encountered in clinical practice.

METHODS

The model source was a continuous 20-mm column of (90)Sr/(90)Y solution sealed in a 1-mm-I.D. Plexiglas capillary. The dose distribution in the Plexiglas phantom was mapped using GafChromic MD-55-2 film. The stent masses varied from 2.5 to 25 mg; the strut thicknesses, from 0.075 to 0.15 mm; and the atomic numbers of stent materials, from 24 (Cr) to 79 (Au).

RESULTS

Dose perturbations depend on a variety of stent features. Local reduction of the mean dose rates near the reference distance (r(0) = 2 mm) varied from 11% to 47%. No simple correlation was found between these data and stent characteristics, but it seems that the atomic number of the stent material is less important than the strut thickness and mesh density.

CONCLUSION

The results provide a warning that clinical indications for in-stent radiation therapy must always be confronted with another aspect of the patient's history: the kind of implanted stent. Intravascular brachytherapy using pure beta sources may be recommended only for patients "wearing" light, thin-strut stents. The presence of thick-strut stents is a contraindication for this modality, due to excessive dose perturbation.

Dose distribution around a needle-like anode X-ray tube: dye-film vs. planar thermoluminescent detectors

The dosimetry around the X-ray tube with a needle-like anode (NAXT), developed at the Institute of Nuclear Studies, for interstitial brachytherapy has been performed using (1) dye films (Gafchromic XR-T), (2) large-area thermoluminescent (TL) detectors--prepared either by gluing TL powder onto thin Al foil (so-called planar detectors with spatial resolution of 0.1 mm) and (3) miniature (2 mm diameter and 0.5 mm thick) TL detectors. The measurements were performed in following geometries. (1) Needle inside a PMMA cylinder--the planar TL detector mounted on the surface of the cylinder. (2) Needle inside a thick block of PMMA and TL detector mounted vertically 7 mm from needle axis. TL detectors were read with the planar (2D) thermoluminescence reader, developed at IFJ, with a sensitive CCD (charge couple device) camera. Gafchromic films were evaluated with a system based on Agfa Arcus 1200 scanner and calibrated with X rays (35 kV) filtered with 0.03 mm Mo and with Co-60 photons. The intensity distribution of TL light on the planar detector was calibrated in terms of absorbed dose to water, using (137)Cs gamma-rays. TL planar detectors seem to be a promising tool for 2D dosimetry of miniature X-ray sources. Obtained results for TLDs and Gafchromic films seem to be comparable but differences have been found. Both methods are useful for measurements of dose distribution around the NAXT X-rays source.