01 Therapy

SSD ↔ SAD output conversion

Convert dose rate between SSD and SAD calibrations at dmax.

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Simulation

SSD ↔ SAD output conversion — Change the numbers; the scene follows.

Where it works

Linear accelerator

Treatment couch

On the couch: changing SSD versus SAD is a setup change, not a new beam model.

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Formula

\dot{D}_{\mathrm{SAD}} = \dot{D}_{\mathrm{SSD}}\left(\frac{\mathrm{SSD}+d_{\max}}{\mathrm{SAD}}\right)^2

Variables

Ḋ_SSD · Output at SSDcGy/MUSSD · Calibration SSDcmdₘₐₓ · Depth of dose maximumcmSAD · Source-to-axis distancecm

Results

Ḋ_SAD
Output at SAD
1.0302cGy/MU
F
Conversion factor
1.0302

Explanation

\dot{D}_{\mathrm{SAD}} = \dot{D}_{\mathrm{SSD}}\left(\frac{\mathrm{SSD}+d_{\max}}{\mathrm{SAD}}\right)^2

What it means

Calibration at SSD (dose to dmax at the surface distance) and at SAD (dose to dmax at isocentre) differ by the inverse-square between (SSD+dmax) and SAD. Converting lets you compare outputs quoted in different protocols. This is a working relation in Radiotherapy.

Where it is used

Clinically it sits on the Linear accelerator — Treatment couch. On the couch: changing SSD versus SAD is a setup change, not a new beam model. 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 machine

How to use it

If Ḋ_SSD = 1 cGy/MU at 100 cm + 1.5 cm and SAD = 100 cm, Ḋ_SAD = 1 × (101.5/100)² ≈ 1.03 cGy/MU. Insert your centre’s numbers. Change one input and watch the curve and the simulation follow.

Symbols

Ḋ_SSDOutput at SSD1 cGy/MU
SSDCalibration SSD100 cm
dₘₐₓDepth of dose maximum1.5 cm
SADSource-to-axis distance100 cm

Worked example

A typical case from the default values: Ḋ_SSD = 1 cGy/MU (Output at SSD); SSD = 100 cm (Calibration SSD); dₘₐₓ = 1.5 cm (Depth of dose maximum); SAD = 100 cm (Source-to-axis distance). Substituting into the relation gives Ḋ_SAD = 1.0302 cGy/MU; F = 1.0302. These are teaching numbers — align them with your machine.

Typical values give

Ḋ_SAD = 1.0302cGy/MU
F = 1.0302

Where it comes from

The displayed formula is the working relation. Convert dose rate between SSD and SAD calibrations at dmax. Usual reference: Khan, output calibration. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan, output calibration

Assumptions & limits

Applies at dmax in water on central axis. Scatter differences between SSD and SAD setups are ignored in this pure inverse-square conversion.

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. Applies at dmax in water on central axis. Scatter differences between SSD and SAD setups are ignored in this pure inverse-square conversion.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Applies at dmax in water on central axis. Scatter differences between SSD and SAD setups are ignored in this pure inverse-square conversion.

In this specialty

Radiotherapy