On the other hand, Monte Carlo calculations for organs outside the x-ray field (such as the
prostate) showed significant statistical uncertainty (44%). This poor absorbed dose tally result is
due to the fact that the Monte Carlo did not have enough histories for these results to converge to
low statistical uncertainty in an acceptable time.
Alternatively, as shown in Table 5-2,when running the Monte Carlo for extended running
times (~40 hours on our parallel cluster), the Monte Carlo calculated absorbed dose delivered to
the prostate began to show convergence to the absorbed dose calculated using EDK-SN. This
illustrates that the EDK-SN methodology, executed in 2 hours (SN execution time = 1.5 hr, EDK-
SN execution time = 0.5 hr), is capable of providing an accurate solution and a significant speed
up for a full physics absorbed dose calculation in human phantoms for high energy external
beams.
Table 5-2. Prostate absorbed dose rate (MeV/g.s) calculated for the test problem using EDK-SN
and MCNP5 using pulse height tally (*F8) with (photon, electron mode) at different
MCNP5 Monte Carlo running times
Dose 2o--MC (EDK-SN- Running
(M~ev/gm.Sec) Uncertainty MC)/EDK-SN Time
ED)K-SN 2.29E-08 ----- ----- 2hr
MC1t 2.21E-08 44.00% 3.62% 3.9hr
MC2 2.34E-08 39.20% 2.14% 4.7hr
MC3 2.18E-08 34.40% 5.05% 6;.1hr
MC4 2.23E-08 22.20% 2.69%/ 14.5hr
MC5 2.46E-08 16.86;% 6.91% 28.15hr
MC6 2.406E-08 14.14% 4.58% 38.85hr
5.2.2 Ray-Effects
When considering a forward peaked volumetric (10 cm x 1 cm x 17 cm) X-ray source
placed on top of the left lung area of the phantom, we noted significant differences between the
absorbed dose calculated by EDK-SN and the absorbed dose calculated by full physics Monte