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PTV: disadvantages
The PTV concept works only when
a)The dose is homogeneous
b)The dose is invariant after translations/rotations
c)Margins are defined correctly w.r.t. the geometrical
uncertainties
The tradeoff between target coverage and OAR
sparing is unbalanced
N.B. IGRT aims at reducing PTV margins but does
not address the shortcomings of PTV-based RT