Dosimetric penumbra effects in catheter-based intravascular brachytherapy using a centered photon or beta line source

In catheter-based intravascular brachytherapy, either photon or beta emitters are often used in a linear arrangement so that blood vessels of 10-30 mm lengths can be treated. With a line source, the dose gradient in the radial direction and longitudinal direction depend on the type of radionuclides used in the treatment. The purpose of this study was to investigate the dose fall-off at the edges of a linear source in a blood vessel for different types of photon and beta emitters.

Dose distributions were calculated on cylindrical blood vessels of various radii. Radioactive sources of 192Ir, 125I, 103Pd, 188Re, 32P, and 90Y/Sr were studied. All the sources were assumed to be in the form of a line. The dose rate at a point in space produced by a radioactive source was computed by integrating the point dose rate kernel of the corresponding radionuclide over the radioactive line. The point dose rate kernel was computed with Monte Carlo simulation of radiation transport. The edge effects were characterized with three newly defined quantities: longitudinal dose uniformity (LDU), effective coverage length (ECL), and margin length (ML). LDU was defined as the ratio of dose at a distance along the long axis of the vessel to the dose at center. ECL was defined as the length over which the LDU was greater than 0.95. ML was defined as half of the length difference between source length L and ECL, which is essentially the length segment at each edge that is covered by the source physical length but is being underdosed.

All beta emitters provided more uniform dose distributions and covered a larger portion of blood vessels longitudinally than photon emitters. Typical MLs were 2-3 mm for beta emitters and 4-6 mm for gamma emitters. As the radial depth of the point of interest increased, both the LDU and ECL decreased and ML increased. The ML increased from 2 to 3 mm for beta emitters and from 4 to 6 mm for photon emitters when the radial depth of the point of interest increased from 1.5 to 2.5 mm (typical proximal and distal media points for a 3-mm diameter lumen). The ML increased with increasing source length for all radionuclides. For beta emitters the ML increased initially from 1.5 mm to more than 2.5 mm as source length increased from 5 to 10 mm. When the source length was longer than 15 mm, the ML remains nearly constant, about 3 mm. For photon emitters, ML increased continuously from 1.5 mm to more than 6.0 mm, as source length increased from 5 to 50 mm.

A formalism to quantify the dose uniformity along the length of a blood vessel undergoing catheter-based intravascular brachytherapy has been developed. This formalism was used to study the edge effects at the ends of several beta and photon sources. The results indicated that for a centered source the ML at each end due to penumbra effects was about 2 to 3 mm for beta emitters; about 4-6 mm for photon emitters. The ML increases as the radial depth of point of interest in the vessel increases. The ML increases also with increasing source length, especially for photon sources.