01 Therapy

Tissue-phantom ratio TPR

Isocentric dose at depth d relative to dose at reference depth.

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Simulation

Tissue-phantom ratio TPR — 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).

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Formula

\mathrm{TPR}(d,d_{\mathrm{ref}})=\frac{D(d)}{D(d_{\mathrm{ref}})}

Variables

D(d) · Dose at depthcGyD(d_ref) · Dose at reference depthcGyd · Depthcmd_ref · Reference depthcm

Results

TPR
Tissue-phantom ratio
0.76
note
Equals TPR20/10 if depths are 20 and 10
0.76

Explanation

\mathrm{TPR}(d,d_{\mathrm{ref}})=\frac{D(d)}{D(d_{\mathrm{ref}})}

What it means

Tissue-phantom ratio is the isocentric dose at depth d divided by the dose at a reference depth (often 5 or 10 cm), both with the same SAD and field size at isocentre. TPR20/10 is the IAEA TRS-398 beam-quality index for MV photons. 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.

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How to use it

Enter the two ionisation or dose readings. For quality index, measure at 20 and 10 cm isocentric in water. Typical TPR20/10: ~0.67 (6 MV), ~0.74 (10 MV), ~0.76–0.78 (15–18 MV). Change one input and watch the curve and the simulation follow.

Symbols

D(d)Dose at depth76 cGy
D(d_ref)Dose at reference depth100 cGy
dDepth20 cm
d_refReference depth10 cm

Worked example

A typical case from the default values: D(d) = 76 cGy (Dose at depth); D(d_ref) = 100 cGy (Dose at reference depth); d = 20 cm (Depth); d_ref = 10 cm (Reference depth). Substituting into the relation gives TPR = 0.76; note = 0.76. These are teaching numbers — align them with your machine.

Typical values give

TPR = 0.76
note = 0.76

Where it comes from

The displayed formula is the working relation. Isocentric dose at depth d relative to dose at reference depth. Usual reference: Khan, Ch. 10. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan, Ch. 10

Assumptions & limits

Same field size and SAD at both depths. Not a substitute for %dd(10)x used in TG-51; the two quality indices are correlated but not identical.

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. Same field size and SAD at both depths. Not a substitute for %dd(10)x used in TG-51; the two quality indices are correlated but not identical.

Keep this

TPR20/10 is a beam-quality index for MV photons (IAEA TRS-398).

In this specialty

Radiotherapy