Organs at risk

 These are critical normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose. Any movements of the organs at risk (OAR) or uncertainties of set-up may be accounted for with a margin similar to the principles for PTV, to create a planning organ at risk volume (PRV). The size of the margin may vary in different directions. Where a PTV and PRV are close or overlap, a clinical decision about relative risks of tumour relapse or normal tissue damage must be made. Shielding of parts of normal organs is possible with the use of multi-leaf collimation (MLC). Dose–volume histograms (DVHs) are used to calculate normal tissue dose distributions.

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