01 Therapy

TMR from PDD

Convert percent depth dose to tissue-maximum ratio.

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Simulation

TMR from PDD — 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{TMR}(d) = \frac{\mathrm{PDD}(d)}{100}\left(\frac{\mathrm{SSD}+d}{\mathrm{SSD}+d_{\max}}\right)^2

Variables

PDD · Percent depth dose%SSD · Source-to-surface distancecmd · Depthcmdₘₐₓ · Depth of dose maximumcm

Results

TMR
Tissue-maximum ratio
0.7869

Curve

Explanation

\mathrm{TMR}(d) = \frac{\mathrm{PDD}(d)}{100}\left(\frac{\mathrm{SSD}+d}{\mathrm{SSD}+d_{\max}}\right)^2

What it means

TMR is the isocentric analogue of PDD: dose at depth in phantom divided by dose at dmax, both at the same SAD. The conversion shown applies an inverse-square factor between the SSD setup and the isocentre. 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

Use when you have a PDD table but need TMR for an SAD MU calc. Example: PDD 67%, SSD 100, d=10, dmax=1.5 → TMR ≈ 0.76. Full clinical conversion also uses Sp(surface)/Sp(depth). Change one input and watch the curve and the simulation follow.

Symbols

PDDPercent depth dose67 %
SSDSource-to-surface distance100 cm
dDepth10 cm
dₘₐₓDepth of dose maximum1.5 cm

Worked example

A typical case from the default values: PDD = 67 % (Percent depth dose); SSD = 100 cm (Source-to-surface distance); d = 10 cm (Depth); dₘₐₓ = 1.5 cm (Depth of dose maximum). Substituting into the relation gives TMR = 0.7869. These are teaching numbers — align them with your machine.

Typical values give

TMR = 0.7869

Where it comes from

The displayed formula is the working relation. Convert percent depth dose to tissue-maximum ratio. Usual reference: Khan, Ch. 10. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan, Ch. 10

Assumptions & limits

Omits phantom-scatter ratio Sp. Field size at depth differs from the surface; Clarkson or equivalent-square interpolation is still required for blocked fields.

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. Omits phantom-scatter ratio Sp. Field size at depth differs from the surface; Clarkson or equivalent-square interpolation is still required for blocked fields.

Keep this

Simplified form without Sp correction. Field size at depth differs from the surface.

In this specialty

Radiotherapy