  Radiometric surveys in radiotherapy bunkers have been carried out in Brazil
for many years, both by the same radiotherapy facility for verification of
shielding as by the regulatory agency for licensing and control purposes. In
recent years, the Intensity Modulated Radiation Therapy (IMRT) technique has
been gradually incorporated into many facilities. Therefore, it has been
necessary to consider the increased leakage component that has an important
impact on the secondary walls. For that, a radiometric survey method has been
used that considers an increased "time of beam - on" for the secondary walls.
In this work we discuss two methods of doing this: the first considers that
this "time of beam - on" affects the sum of the two components, leakage and
scattered. In another method it is considered that only the leakage component
is affected by this extended "time of beam - on ". We compare the methods and
show that for secondary walls with $U=1$ the first method overestimates dose
rates by important percentages and for secondary walls with $U<1$ it can both
overestimate or underestimate the dose rates, depending on the parameters of
the project. An optimized procedure is proposed, according to the use factor
($U$) of the secondary wall to be measured.
