Theoretical analysis of microdosimetric spectra and cluster formation for Pd-103 and I-125 photon emitters
(2004) World Congress on Medical Physics and Biomedical Engineering — Location: Sydney(Australia) (24.August.2003)
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- Abstract
- In this work we have compared I-125 or Pd-103 from a microdosimetric point of view. The photon spectra at different positions around the seeds have first been calculated using EGSnrc Monte Carlo (MC) code. These photon spectra are used as input for the event-by-event MC code TRION to calculate the microdosimetric lineal energy (y) distribution for each isotope. The microdosimetric dose average lineal energy, y(D), calculated in a sphere of 1 mum is 3.5 keV mum(-1) for I-125 and 4.0 keV mum(-1) for Pd-103, agreeing well with values reported in the literature. y(D) in a 1 mum sphere diminishes slightly with the distance from the seed for Pd-103. This is due to the spectral hardening caused by the presence of a gamma-ray of 357.5 keV in the initial spectrum of Pd-103. In parallel with the calculation of the microdosimetric spectra, we have analysed the distribution of the size of the energy deposition clusters generated by these low energy photons in structures of 2 and 10 nm of radius. Due to Compton interactions, the fraction of very low energy electrons (<5 keV) generated by I-125 photons is 51%, whereas it is only 27% for Pd-103. As these electrons deposit their energy very locally, the pattern of energy depositions contains more clusters of a few nm of radius for I-125 than for Pd-103; the mean cluster orders are respectively 3.3 and 3.0 for 10 nm clusters. This is in opposition with the prediction based on the microdosimetric spectrum and the parameter y(D) and could be of importance for the damage to the cells.
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Reniers, B., Vynckier, S., & Verhaegen, F. (2004). Theoretical analysis of microdosimetric spectra and cluster formation for Pd-103 and I-125 photon emitters. Physics in Medicine and Biology, 49(16), 3781-3795. https://doi.org/10.1088/0031-9155/49/16/022 (Original work published 2004)