AbstractID: 6640 Title: Shielding Radiation Oncology Treatment Rooms for IMRT
Accepted procedures for designing radiation shielding for radiotherapy facilities are well
established. However, intensity modulated radiation therapy (IMRT) alters the
conventional treatment parameters to such an extent that special shielding considerations
are warranted for this new technique. This paper addresses the issue of designing a
facility to accommodate IMRT by extending conventional shielding design concepts
outlined in NCRP 49. Ten consecutive IMRT prostrate treatments that were delivered in
a clinic were analyzed to determine typical parameters for the Workload and Use Factors.
The leaf sequences for generation of the intensity patterns were calculated for delivery by
a step-and-shoot method on a 15-MV linac equipped with a multileaf collimator (MLC)
containing 40 leaf pairs. The plans employed 7-11 gantry angles. The conventional
methods for determining barrier thicknesses were extended for IMRT. In particular, the
concept of Workload was unchanged, but was greater than conventional workloads as
determined by the monitor units used for daily IMRT treatments. The result was an
increase in the shielding for Secondary Radiation by one to two half-value layers and an
impact on maze and door shielding for neutrons. The Use Factor for radiation reaching a
barrier for this variant of IMRT was extracted from the leaf sequence and was found to be
on average equal in each octant of gantry angle but lower than the Use Factor of
conventional therapy in proportion to the increase in Workload. The practical
consequence was that Primary Barrier requirements for IMRT were unchanged from
those for conventional treatments.