Dosimetry of source stepping for intravascular brachytherapy

Both beta and gamma 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.

The three most commonly used beta and gamma 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.

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.

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.