Advanced Physics for Brachytherapy 2018

Type B uncertainty … cross sections

➢ Probability distributions are in the form of a cross section data base . ➢ Total partial and differential photon cross sections are required including atomic form factors F(x, Z) and incoherent scatter factors S(x, Z) ➢ These cross sections must be: complete (in terms of E, Z) self-consistent and up to date/accurate. ➢ Self-consistency is also required with mass energy absorption data used for kerma calculation from energy fluence ➢ Up to date cross sections are used in all current versions of benchmarked MC codes: EPDL97 (LLNL) in the ENDL or ENDF/B-VI formats or the DLC-146 format (ORNL) or XCOM (NIST) database. Mass energy-absorption coefficients for water by Seltzer and Hubbell (available on line at NIST). Data readily available online through NIST ( http://www.nist.gov/pml/data/xraycoef/ )

➢ TG-138 (AAPM/ESTRO) cites ~1% cross section type B that contributes to dosimetric uncertainty at r=5 cm about 0.76 % (low E) and 0.12% (high E)