Dosimetry of source stepping for intravascular brachytherapy

Abstract

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.