Planning Concepts & Evaluation

How closely the prescription isodose matches the size, shape and position of the target — scored with the RTOG ratio (R100%), the Paddick index and others.

The maximum dose at any point 2 cm or more away from the PTV, expressed as a percentage of the prescription dose.

The volume receiving half the prescription dose divided by the volume receiving the full prescription dose — Paddick's measure of how steeply dose falls off outside the target.

A single number describing how uniform the dose is inside the target. Several formulas are in use, so a value means little until you know which one produced it.

Regions receiving more than the prescription dose. Where a hotspot sits matters as much as how hot it is.

Dose spreading into the tissue around the target at levels below the prescription. In lung SBRT it is judged with R50% and D2cm.

The volume enclosed by the 50% prescription isodose divided by the PTV volume — a measure of how quickly dose falls off outside the target.

How much of the target receives the prescription dose — reported either as the share of the PTV receiving it or as the dose received by a set share of the PTV.

The minimum dose received by the hottest small volume of a structure — a more robust stand-in for a single-point maximum.

A graph that summarizes the dose a structure receives as volume against dose — the source of almost every number used to evaluate a plan.

The highest dose in a structure. A single-point maximum depends on the calculation grid, so protocols often define the maximum on a small volume instead.

The average dose across a structure's whole volume — a common limit for organs such as the lungs, heart, parotids, kidneys and liver.

The volume of a structure receiving at least dose x — written with its units, such as V20Gy[%] for the percentage of lung receiving 20 Gy or more.

Repair, redistribution, repopulation and reoxygenation — the four processes between fractions that help explain how fractionation changes the effect of radiation. A fifth R, radiosensitivity, is often added.

Biologically effective dose — a linear-quadratic model quantity that puts schedules with different doses per fraction on one scale, for a stated α/β.

The total dose in 2 Gy fractions that the linear-quadratic model predicts would have the same effect as a given schedule, for a stated α/β.

Linear energy transfer describes how densely a particle deposits energy along its track; relative biological effectiveness compares the dose of one radiation with the dose of a reference radiation that has the same effect. Together they explain why protons are planned in Gy(RBE).

How much more dose hypoxic cells need than well-oxygenated cells for the same effect — about two and a half to three times for photons. It is why tumor hypoxia matters in radiotherapy.

Tumor control probability and normal tissue complication probability — models that turn a dose distribution into an estimated chance of controlling the tumor or causing a complication.

The internal target volume — a target volume that covers the tumor wherever it moves during treatment, most often over the breathing cycle. NRG lung protocols call the motion-encompassing GTV an IGTV.

The planning organ at risk volume — an organ at risk plus a margin for motion and set-up uncertainty, so a dose limit still holds if the organ is not exactly where it was contoured.