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Photons (2)

• The presence of tissue heterogeneities, such as air cavities, bony structures can greatly impact the calculated dose distribution. The change in dose is due to the perturbation of the transport of primary and scattered photons and that of the secondary electrons set in motion from photon interactions. • Perturbation of photon transport is more noticeable for lower-energy beams. • Near the interface between a bony structure and waterlike tissue, large hot and cold dose spots can be present.