Level 0:	no biological optimization.
Level I:	individualization of prescription dose for specified level of toxicity [i.e., dose escalated in plans where possible using fixed normal tissue control probability (NTCP) level for the organ at risk (OAR)].
Level II:	the same as above, but the number of fractions is adjusted as well as the prescription dose based on an isotoxic basis.
Level III:	biological cost functions used in the actual optimization of the dose distribution. Equivalent uniform dose (EUD), TCP, and NTCP parameters are used alongside conventional DVH parameters used in the optimization (e.g., Dmax, D99, V50%, mean).
Level IV:	individual patient-specific data is used in the optimization of the patient's plan (e.g., use of functional imaging to highlight areas of hypoxia and other areas of increased radio-resistance).
Level V:	using individual patient biology to optimize dose prescriptions in conjunction with any of the other levels.