Glossary

510(k) clearance

US FDA clearance of a device shown substantially equivalent to a legally marketed device.

A clearance is not an approval.

Also written as: 510k, 510(k), clearance

Absorbed dose

Energy deposited by radiation per unit mass of tissue.

Measured in gray. Patient-specific estimates are made by qualified professionals with calibrated data.

Also written as: absorbed dose

Administered activity

The amount of radioactivity given, in becquerels (often MBq or GBq).

Appears in labels and protocols as source-reported text. RadioGx never recommends an activity.

Also written as: injected activity

Adverse event report

A report that an unwanted event occurred during use of a product.

Reports do not establish causation or frequency.

Also written as: faers, maude

ALARA

As low as reasonably achievable: the optimisation principle of radiation protection.

Applied through time, distance and shielding and by facility programmes.

Also written as: as low as reasonably achievable

Alpha particle

A helium nucleus emitted in decay, travelling tens of micrometres in tissue.

Deposits dense energy (high LET); used in radium-223 and targeted alpha therapy research.

Also written as: alpha, α

Annihilation

The conversion of a positron and an electron into two photons of 511 keV.

The photons travel in nearly opposite directions, which PET exploits.

Approval

A regulator’s decision authorising marketing of a drug, biologic or high-risk device.

Not the same as a device clearance.

Attenuation correction

Correcting for photons absorbed or scattered in the body before reaching the detector.

In PET/CT and SPECT/CT the CT provides the attenuation map.

Auger electron

A very low-energy electron released after electron capture or internal conversion.

Travels nanometres to micrometres; studied for therapy at cellular scales.

Beta-minus (β−) particle

An electron emitted from the nucleus, travelling up to millimetres in tissue.

Used in therapy with radionuclides such as lutetium-177, yttrium-90 and iodine-131.

Also written as: beta particle, β−

Chelator

A molecule that holds a metal ion, such as a radiometal, securely.

DOTA is a widely used chelator; others suit specific metals such as zirconium.

Also written as: dota, nota, dfo

Collimator

A shielded plate with channels that only lets photons from certain directions reach the detector.

Essential for forming an image; it also discards most photons.

Community developer reference

RadioGx’s label for public repositories: useful references, not curated knowledge or clinical evidence.

Always check licence, maintenance and validation yourself.

Controlled access

Data available only to approved users under a data use agreement.

Never used in RadioGx Public Mode.

Also written as: dbgap, ega

Copyleft

Licence terms requiring derived works to carry the same licence when distributed.

The GPL family is the best-known example.

Cyclotron

A particle accelerator used to produce radionuclides such as fluorine-18.

Many PET radionuclides are cyclotron-produced, some near the point of use because of short half-lives.

De Novo classification

An FDA pathway that classifies a new type of low- to moderate-risk device.

Creates a new classification that later devices can reference.

De-identification

Removing or altering information that could identify a person.

For images this includes headers, burned-in text and, for head scans, facial features.

Also written as: deidentification, anonymisation, defacing

Deauville five-point scale

Visual FDG-PET uptake scoring used in lymphoma response assessment.

Compares lesion uptake with reference regions.

DICOM

The standard format and protocol for medical images and related data.

Files contain pixel data and headers that can include identifying information.

Also written as: dicom standard, rtstruct, dicom seg

Source: DICOM standard (opens in a new tab)

DOI

Digital Object Identifier, a persistent identifier for a published object.

Resolves to the official record.

Dosimetry

The estimation of radiation dose to tissues.

Internal dosimetry for radiopharmaceuticals combines activity over time with energy absorption.

Also written as: internal dosimetry

DOTATATE

A somatostatin analogue peptide with a DOTA chelator.

Labelled with gallium-68, copper-64 or lutetium-177.

Also written as: dotatate

Effective dose

A weighted sum of organ equivalent doses used for radiation protection.

Designed for populations; not suitable for estimating an individual’s risk.

Also written as: effective dose

Effective half-life

The combined rate at which activity falls in the body through decay and elimination.

Shorter than both the physical and biological half-lives. Estimated from measurements by qualified professionals.

Also written as: biological half-life, effective half life

External validation

Testing a model on independent data from different sites or times.

The key step before a model can be trusted beyond its origin.

FAP

Fibroblast activation protein, expressed by cancer-associated fibroblasts.

Target of FAPI imaging agents; therapeutic agents are investigational.

Also written as: fibroblast activation protein, fapi

FDG

Fludeoxyglucose F 18, a glucose analogue PET tracer.

Reflects glucose uptake in many tissues; uptake is not specific to cancer.

Also written as: fluorodeoxyglucose, 18f-fdg

Feature selection

Choosing a subset of features for a model.

Must happen inside cross-validation to avoid optimistic bias.

First-order feature

A statistic of voxel intensities that ignores spatial arrangement.

Examples: mean, skewness, entropy.

Freedom to operate

An analysis of whether an activity may infringe others’ patents.

Requires a qualified patent attorney; RadioGx does not provide it.

Also written as: fto

Gamma camera

A detector that images gamma photons through a collimator.

The basis of planar and SPECT imaging.

Gamma ray

A high-energy photon emitted from the nucleus.

Escapes the body and is detected by gamma cameras.

Also written as: gamma

Genomic phenotype

Molecular characteristics such as mutations, copy number, expression or methylation.

Measured by laboratory assays on tissue or blood, each with sampling limits and error.

Related: Imaging phenotype

GLCM

Grey-level co-occurrence matrix: counts of intensity pairs at given offsets.

Source of texture features such as contrast and homogeneity.

Also written as: gray level co-occurrence matrix

Gray (Gy)

The unit of absorbed dose: one joule per kilogram.

Physical quantity used in therapy dosimetry research.

Also written as: gy

Half-life (physical)

The time for half of the radioactive atoms in a sample to decay.

A fixed physical property of each radionuclide. Verify values at the IAEA LiveChart of Nuclides.

Also written as: physical half-life

Source: IAEA LiveChart (opens in a new tab)

Harmonisation

Reducing differences between scanners, sites or batches.

Done through acquisition standards or statistical methods such as ComBat.

Also written as: harmonization, combat

IBSI

The Image Biomarker Standardisation Initiative, which standardised radiomic feature definitions.

Publishes reference values so software can be checked.

Source: IBSI (opens in a new tab)

Imaging biomarker

A characteristic measured on images as an indicator of a biological process, disease or response.

Needs technical validation (repeatable measurement) and clinical validation (relationship to the quantity of interest) for each intended use.

Also written as: imaging biomarkers

Imaging phenotype

The appearance or measurable characteristics of tissue on an image.

Can be semantic (described by a reader with a standard vocabulary) or quantitative (computed by software).

Related: Genomic phenotype

Interpretation confidence

RadioGx’s heuristic 0–100 score for how it read your query.

A query-routing score, not a diagnostic, prognostic, dosimetric, regulatory or clinical certainty score.

Also written as: confidence score

Intraclass correlation coefficient

A statistic of agreement between repeated measurements or readers.

Often reported for radiomic feature stability.

Also written as: icc

Krenning score

Visual grading of somatostatin receptor uptake relative to reference organs.

Originally developed for somatostatin receptor scintigraphy.

Label (prescribing information)

The regulator-approved document describing a product’s approved use.

Written for qualified professionals; content varies by jurisdiction.

Also written as: label, uspi, prescribing information, smpc

Ligand (vector)

The part of a radiopharmaceutical that binds the target.

Can be a small molecule, peptide, antibody or antibody fragment.

Also written as: vector

Linear energy transfer (LET)

Energy deposited per unit length of a particle’s track.

Alpha particles have high LET; photons and beta particles have low LET.

Also written as: linear energy transfer

Metabolic tumour volume (MTV)

The volume of tissue above a chosen uptake threshold on PET.

Depends on the segmentation method and threshold.

Also written as: metabolic tumor volume

MIRD schema

A formalism expressing absorbed dose as time-integrated activity multiplied by S values.

Developed by the Medical Internal Radiation Dose committee.

Also written as: mird, mird formalism

Source: MIRD Committee (opens in a new tab)

NCI Genomic Data Commons

The NCI repository for cancer genomic and clinical data.

Has open and controlled access tiers.

Also written as: genomic data commons

NCI Imaging Data Commons

A cloud-based NCI resource for public cancer imaging data.

Offers searchable metadata and built-in viewers.

Also written as: imaging data commons

Source: IDC (opens in a new tab)

NCT number

The ClinicalTrials.gov registry identifier: NCT followed by eight digits.

Lets you find the official record.

Also written as: nct id, nct

NIfTI

A research image format common in neuroimaging and deep learning.

Carries less metadata than DICOM.

NIS (sodium/iodide symporter)

A membrane transporter that moves iodide into thyroid cells.

Basis of radioiodine imaging and therapy.

Also written as: sodium iodide symporter, slc5a5

Open access

A version of a publication that anyone can read for free, legally.

Gold (publisher), green (repository) and other routes exist.

Also written as: oa

Open-source licence

Terms under which source code may be used, changed and shared.

Permissive and copyleft families differ in obligations.

Also written as: licence, license

Organ at risk

A normal organ whose radiation exposure limits or shapes therapy.

For some radioligands, research focuses on kidneys, salivary glands and bone marrow.

Also written as: organs at risk

Overfitting

A model that learns noise in its training data and fails on new data.

Common when features greatly outnumber patients.

Paired imaging-genomic data

Imaging and genomic data from the same cases.

RadioGx only reports pairing when documentation says so.

Also written as: paired data, matched imaging genomics

Partial volume effect

Underestimation of activity in objects small relative to scanner resolution.

Small lesions can appear less intense than they are.

Patent family

Patent documents in several jurisdictions that share a priority.

Each member has its own legal status.

Patient release criteria

Regulatory conditions for releasing people who have received radioactive material.

Set by national regulators; instructions come from the treating facility.

Also written as: release criteria

Paywall

A barrier requiring payment or subscription to read content.

RadioGx never bypasses paywalls.

Peptide receptor radionuclide therapy (PRRT)

Radionuclide therapy using radiolabelled peptides, typically somatostatin analogues.

Used in somatostatin receptor-expressing neuroendocrine tumours.

Also written as: prrt

PERCIST

PET Response Criteria in Solid Tumors: a framework for measuring metabolic response.

Specifies measurement and thresholds for research and trials.

PET (positron emission tomography)

Imaging that detects pairs of annihilation photons from positron emitters.

Offers quantitative 3D images of tracer distribution.

Also written as: positron emission tomography

PET/CT

A combined scanner acquiring PET and CT in one session.

CT supplies anatomy and attenuation correction.

Also written as: pet/ct, petct

PET/MR

A combined PET and MRI scanner.

Adds soft-tissue contrast without the CT radiation component.

Also written as: pet/mr, pet/mri

PHI

Protected health information: health data that can identify a person.

Never type, upload or paste PHI into RadioGx.

Also written as: protected health information, patient identifiers

Planar scintigraphy

A two-dimensional gamma camera image.

Includes static and whole-body views, such as many bone scans.

Also written as: scintigraphy, planar imaging

PMID

PubMed identifier for a citation record.

Lets you find the PubMed entry.

Positron (β+) emission

Decay that emits a positron, which annihilates with an electron to make two 511 keV photons.

The basis of PET imaging.

Also written as: beta plus, β+

Post-therapy imaging dosimetry

Using quantitative imaging after therapy to estimate where dose was delivered.

Common with lutetium-177 because of its gamma emissions.

Premarket approval (PMA)

The FDA approval pathway for higher-risk devices.

Requires evidence of safety and effectiveness.

Preprint

A manuscript shared before peer review.

Labelled as a preprint in RadioGx; can change before or never reach publication.

Provenance

Where a piece of information came from: source, record, time retrieved and how it was matched.

Every important RadioGx fact carries provenance.

PSMA

Prostate-specific membrane antigen, a cell-surface enzyme encoded by FOLH1.

Also expressed in several normal tissues and in some non-prostate tumour vasculature.

Also written as: prostate-specific membrane antigen, folh1, gcpii

Quantitative imaging biomarker

An imaging biomarker expressed as a number on a defined scale.

Examples include SUV, ADC and volumes. Profiles from standards groups specify acquisition and measurement so values are comparable.

Also written as: qib

Radiogenomics

The study of associations between imaging features and genomic or molecular features.

In imaging research, radiogenomics relates semantic or quantitative imaging features to mutations, expression or other molecular data. In radiation oncology the word also describes genetic variation linked to radiotherapy response.

Caveat: Associations are research findings, not diagnostic tests.

Also written as: imaging genomics, radiogenomic

Related: Radiomics, Imaging phenotype

Source: Radiopaedia (opens in a new tab)

Radioligand therapy (RLT)

Therapy using a radiolabelled ligand that binds a target.

Includes PSMA- and SSTR-directed therapies.

Also written as: radioligand therapy, targeted radionuclide therapy

Radiomics

Extraction of many quantitative features from medical images.

Radiomics converts images into tables of shape, intensity and texture features that can be related to outcomes. Values depend on acquisition and processing choices.

Caveat: Radiomic features are research context, not standalone diagnostic tools.

Also written as: radiomic, quantitative imaging

Related: IBSI, Texture feature

Radionuclide

An unstable atomic species that decays and emits radiation.

Described by element, mass number and decay properties, for example lutetium-177. "Radioisotope" is often used loosely as a synonym.

Also written as: radioisotope, radionuclides

Radionuclide generator

A device holding a longer-lived parent that decays to a short-lived daughter eluted on site.

Examples: molybdenum-99/technetium-99m and germanium-68/gallium-68.

Also written as: generator

Radiopharmaceutical

A medicinal product containing a radionuclide, used for imaging or therapy.

Usually combines a targeting vector, a chelator or prosthetic group, and a radionuclide. Regulated as a drug in many jurisdictions.

Also written as: radiopharmaceuticals, radiodrug

Radiotracer

A radiolabelled compound used in tracer amounts to study a biological process.

Given in a mass so small that it does not disturb the process it measures.

Also written as: tracer

RECIST 1.1

Response Evaluation Criteria in Solid Tumors: anatomic size-based response categories.

Widely used to define trial endpoints.

Source: RECIST (EORTC) (opens in a new tab)

S value

Absorbed dose to a target region per radioactive decay in a source region.

Computed for reference anatomical models.

Segmentation

Outlining a region of interest on an image, by hand, semi-automatically or with AI.

A major source of variability in radiomics.

Also written as: contouring, roi

Shape feature

A radiomic feature describing the geometry of a region.

Examples: volume, sphericity, elongation.

Sievert (Sv)

The unit of equivalent and effective dose.

A radiation protection unit, not a measure of individual risk.

Also written as: sv, msv

Software as a medical device (SaMD)

Software intended for medical purposes without being part of a hardware device.

Regulated in many jurisdictions; research code is not automatically SaMD-cleared.

Also written as: samd

SPECT

Single-photon emission computed tomography: 3D imaging of gamma emitters with a rotating gamma camera.

Uses collimators; widely available.

Also written as: single photon emission computed tomography

SPECT/CT

A combined SPECT and CT scanner.

Improves localisation and enables quantitative SPECT.

Also written as: spect/ct

SSTR2

Somatostatin receptor subtype 2, expressed by many neuroendocrine tumours.

Target of DOTATATE and DOTATOC radiopharmaceuticals.

Also written as: somatostatin receptor, sstr

Standardised uptake value (SUV)

PET activity concentration normalised to injected activity per body size.

SUVmax, SUVpeak and SUVmean are common variants. Sensitive to timing, glucose, reconstruction and partial volume.

Also written as: suvmax, suvmean, suvpeak, standardized uptake value

Targeted alpha therapy

Therapy with alpha-emitting radionuclides attached to targeting molecules.

Largely investigational except for specific products; check status live.

Also written as: tat, alpha therapy

TCGA

The Cancer Genome Atlas, a landmark cancer genomics programme.

Data are in the NCI GDC; some cases have imaging in TCIA.

Also written as: the cancer genome atlas

TCIA

The Cancer Imaging Archive, a public repository of de-identified cancer images.

Each collection has its own usage policy and citation.

Also written as: cancer imaging archive

Source: TCIA (opens in a new tab)

Test-retest repeatability

Agreement between measurements repeated under the same conditions.

Public same-day repeat scans help assess radiomic feature stability.

Texture feature

A radiomic feature describing spatial patterns of intensity.

Computed from matrices such as GLCM; sensitive to discretisation and voxel size.

Theranostic pair

An imaging and a therapeutic agent aimed at the same target.

Sometimes identical chemistry with different isotopes (iodine-123/iodine-131; copper-64/copper-67), sometimes different radiometals on the same ligand.

Related: Theranostics

Theranostics

Imaging a target and treating it with closely related radiopharmaceuticals.

Also called "see what you treat". Imaging uptake is a guide, not a guarantee of response.

Also written as: theranostic, theragnostics

Time-integrated activity

The total number of decays in a region over time.

Estimated from serial measurements and fitted curves.

Also written as: cumulated activity

Time-of-flight PET

PET that uses photon arrival-time differences to localise events along each line of response.

Improves image quality, especially in larger patients.

Also written as: tof, time of flight

Total lesion glycolysis (TLG)

MTV multiplied by the mean SUV of that volume.

Shares MTV’s dependence on segmentation.

Tracer principle

The idea that a labelled compound in tiny amounts follows biology without disturbing it.

The foundation of nuclear medicine imaging.

Trial phase

The stage of drug development a study represents, from phase 1 to phase 4.

Diagnostic and device studies may not use phases.

Also written as: phase

Voxel-level dosimetry

Dose estimation at the level of image voxels using kernels or Monte Carlo methods.

Captures non-uniform dose within organs and lesions.