Therefore, this is an example that highlights that tissue heterogeneities can be difficult to discern
in these problems using the Monte Carlo method.
As explained previously, one of the most important issues limiting the use of the Monte
Carlo techniques for clinical absorbed dose calculations is achieving a statistically reliable result
in a reasonable time. This problem is even greater when a photon-electron mode is required,
when CPE is not applicable, causing computer running times needed to reduce the statistical
error to increase rapidly. Improved variance reduction techniques may reduce the computation
times. However, such techniques are to be used with caution, as improper application of
statistical variance reduction may cause wrong answers. Moreover, variance reduction
techniques may not reduce the time required to reduce the variance to acceptable levels,
especially when accurate absorbed dose estimations are needed over the entire problem.
100 Minutes (Pulse height tally using photon-
4.00E-04 -electrn mode)
3.50E-04 --*-- SBLLBSSS
Ilu -o- SSBBSSS
(0 3.00E-04
as 2.50E-04-
1.50E-04-
ur1.00E-04 -
5.00E-05-
0.00E+00
O 10 20 30 40 50
X(cmR)
Figure 5-2. Monte Carlo derived total absorbed dose rate distributions along the X-axis
computed using *F8 MCNP5 tally for three different phantoms. MCNP5 uncertainty
(20) average (20.0%), 100 Minute run