01 Therapy
Tissue-air ratio TAR
Dose in phantom at depth d divided by dose in air at the same point.
Listen
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Simulation
Tissue-air ratio TAR — Change the numbers; the scene follows.
Where it works
Linear accelerator
Isocenter
At isocenter, on the central axis through the patient (or a phantom in the same place).
Open this machineFormula
Variables
Results
TAR
Tissue-air ratio
0.78
Explanation
What it means
TAR is the original isocentric quantity: it folds inverse-square, attenuation and scatter into one number measured at a point that stays at a fixed distance from the source. TMR = TAR(d)/TAR(dmax). TAR of a 0×0 field is the primary exponential e^{−μ(d−dmax)} and is the backbone of Clarkson sector integration. This is a working relation in Radiotherapy.
Where it is used
Clinically it sits on the Linear accelerator — Isocenter. At isocenter, on the central axis through the patient (or a phantom in the same place). Radiotherapy equations sit at the console and in the bunker: output, depth dose, equivalent square, and the monitor units that treat the patient. Hand-calc them beside the TPS, never instead of a commissioned plan.
Linear accelerator · Open this machineHow to use it
Enter tissue dose and in-air dose at the same SAD. A 10×10 6 MV TAR at 10 cm is typically ~0.78. Subtract TAR(0) to get SAR for scatter-only calculations. Change one input and watch the curve and the simulation follow.
Symbols
- D_tTissue dose78 cGy
- D_airIn-air dose100 cGy
Worked example
A typical case from the default values: D_t = 78 cGy (Tissue dose); D_air = 100 cGy (In-air dose). Substituting into the relation gives TAR = 0.78. These are teaching numbers — align them with your machine.
Typical values give
- TAR = 0.78
Where it comes from
The displayed formula is the working relation. Dose in phantom at depth d divided by dose in air at the same point. Usual reference: Johns & Cunningham / Khan. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Johns & Cunningham / Khan
Assumptions & limits
In-air dose must be measured with a build-up cap that establishes CPE and is not itself a mini-phantom of the field. Superseded in modern MU calc by TPR/TMR but still used in older TAR-based algorithms and teaching.
Pitfalls
Never mix PDD from one SSD with TMR from another without converting. Field size at the surface is not the size at isocentre. A hand MU is a check, not a treatment. In-air dose must be measured with a build-up cap that establishes CPE and is not itself a mini-phantom of the field. Superseded in modern MU calc by TPR/TMR but still used in older TAR-based algorithms and teaching.
Keep this
Name the SSD, energy, and field size with every PDD or TMR you quote. In-air dose must be measured with a build-up cap that establishes CPE and is not itself a mini-phantom of the field. Superseded in modern MU calc by TPR/TMR but still used in older TAR-based algorithms and teaching.
In this specialty