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Can there be a good external surrogate for
the position of a lung/liver tumour?
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No surrogate is good enough
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Solution 1: use large margins ( approx equal to free breathing)
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Solution 2: image the tumour position daily with respiration-
correlated (4D) IGRT
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4D-CBCT for gated treatment
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Breath hold CBCT
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2D + markers in the tumour