NucLexNuclear Medicine Lexicon and Theranostics Ontology
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Biodistribution and pharmacokinetics

Where an agent goes, how much is there, and for how long are three questions whose answers depend on how and when someone looked.

MonographBriefNX-M15Proposed length 2,500 to 5,000 words

Writing brief. This monograph has been assigned but the manuscript is not yet written or approved. What follows is the commissioning brief: its question, outline, evidence instructions, and review requirements.

Central question

Where does an agent go over time?

Reader promise

The reader will understand the difference between activity in an organ and concentration in tissue, between a model compartment and an organ, between a fitted clearance rate and an assumed one, and will be able to say what any reported number is missing before reuse.

Required development

  1. Define the quantities. Distinguish administered activity, retained activity, activity concentration, and absorbed dose; state the units of each and why they are not interchangeable. Explain decay correction and how to tell whether a reported value has been corrected.
  2. Explain compartments. Introduce the Compartment model as a mathematical abstraction (a pool with inflow and outflow) and show, with a labelled explanatory diagram, how organs, blood, and tumor are approximated. State the assumptions (instant mixing, first-order transfer) and what they exclude.
  3. Compare a model parameter with a measured result. Show one synthetic quantity, for example a clearance half-time, twice: fitted to synthetic imaging time points with stated uncertainty, and as a literature parameter with population, method, and conditions. Show why the two cannot be substituted silently.
  4. Explain measurement context. Time after administration, imaging method, region definition, calibration, and patient state all condition a measurement. Connect to Context and to Model-ready knowledge exchange for what must travel with a parameter.
  5. Treat time and uncertainty seriously. Show how few time points constrain a curve poorly, how an assumed late-phase behavior changes an integral, and how uncertainty should be reported. Connect to Dosimetry.

Evidence and review

Evidence to obtain: the MIRD (Medical Internal Radiation Dose) committee pamphlets or primer for the formal definitions of cumulated activity, time-integrated activity coefficient, and absorbed dose; EANM dosimetry guidance for measurement protocols and uncertainty methods; a standard pharmacokinetics textbook for compartment model definitions and assumptions; the ICRU or ICRP reports defining the quantities and units cited. Every number must be synthetic and labelled, or quoted from a source with units, population, conditions, and reported uncertainty.

Review concern: a synthetic fitted parameter must not be presented so that a reader could lift it as a reusable value, and no simulated curve may be displayed as patient data.

Biodistribution, Pharmacokinetics, Compartment model, Dosimetry, Parameter, Uncertainty, Context, Model-ready knowledge exchange, Theranostic digital twins, Radiopharmaceutical identity.

Review requirements

Source check of every definition and unit against MIRD, ICRU, or ICRP documents. Domain review by a medical physicist with internal dosimetry experience and a pharmacokineticist. Editorial check that every displayed number carries units, source or synthetic label, conditions, and uncertainty.

Review gate for this monograph

No numerical parameter without units, source, context, and uncertainty.

The assignment exists. The manuscript has not been written or approved.