On the depth of penetration of photons and electrons for intravascular brachytherapy.

PURPOSE

To investigate the depth dose characteristics of various radionuclides under consideration for intravascular brachytherapy (IVB).

MATERIALS AND METHODS

In the past few years, various preclinical studies have shown that 10-30 Gy of ionizing radiation delivered by a brachytherapy treatment may inhibit restenosis following angioplasty. A number of new delivery systems using various radionuclides have been developed and are being investigated for IVB. Typical target size for IVB is in the range of millimeters, in contrast to conventional brachytherapy for cancer in which the target may be 1-5 cm in size. The question addressed in this paper is: whether lower energy photon emitters and even beta emitters, which are not commonly used for intracavitary brachytherapy of cancer, may provide a depth of penetration adequate for IVB. To explore this issue, radial dose functions for photons and electrons in the range of 1-10 mm in water were calculated using Monte Carlo simulation. Reference depth for normalization of the radial dose funtion was chosen to be 2 mm.

RESULTS

Radial dose functions have been calculated for monoenergetic photons with energies of 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.40, and 1.00 MeV and monoenergetic electrons with energies of 0.5, 1.0, 1.5 and 2.0 MeV. Also, the same calculations have been performed for 192Ir, 125I, and 103Pd gamma or x-ray sources as well as 90Sr-90Y, 32P, and 188Re beta-emitting sources. Results are also provided for selected cases in a simulated calcified lesion in water.

CONCLUSIONS

It is concluded that photons above an energy of 20 keV and electrons above an energy of 1.0 MeV are acceptable from the point of view of adequate depth of penetration for IVB in tissue.