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Secondary barriers are designed to protect individuals beyond the treatment room from:
Leakage:
dependent on design, typically limited to 0.1 to 0.2% of the primary beam
originates from target - not necessarily via the isocentre. IMRT -> the leakage
radiation can be significantly increased (a factor of 10 is often assumed).
Scatter:
assumed to come from the patient and room walls. Difficult to calculate - use
largest field size for measurements the lower the radiation energy, the more of a
concern for photon beams.
Secondary Sources in External Beam RT