01 Therapy

Off-axis dose

Dose off axis from CAX dose and the off-axis ratio.

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Simulation

Off-axis dose — 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

D_{\mathrm{off}}=D_{\mathrm{CAX}}\cdot\mathrm{OAR}(x,d)

Variables

D_CAX · Central-axis dosecGyOAR · Off-axis ratio

Results

D_off
Off-axis dose
185.4cGy

Explanation

D_{\mathrm{off}}=D_{\mathrm{CAX}}\cdot\mathrm{OAR}(x,d)

What it means

Off-axis ratio is the dose at a lateral distance x relative to the central axis, at the same depth. Horns in flattened beams can push OAR slightly above 1 just inside the field; it then falls through the penumbra. 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 machine

How to use it

Multiply CAX dose by tabulated OAR (or a TPS profile). FFF beams have OAR < 1 off axis by design. Change one input and watch the curve and the simulation follow.

Symbols

D_CAXCentral-axis dose180 cGy
OAROff-axis ratio1.03

Worked example

A typical case from the default values: D_CAX = 180 cGy (Central-axis dose); OAR = 1.03 (Off-axis ratio). Substituting into the relation gives D_off = 185.4 cGy. These are teaching numbers — align them with your machine.

Typical values give

D_off = 185.4cGy

Where it comes from

The displayed formula is the working relation. Dose off axis from CAX dose and the off-axis ratio. Usual reference: Khan. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan

Assumptions & limits

OAR is energy-, depth- and field-size-specific. Do not reuse a 10 cm depth OAR at dmax.

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. OAR is energy-, depth- and field-size-specific. Do not reuse a 10 cm depth OAR at dmax.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. OAR is energy-, depth- and field-size-specific. Do not reuse a 10 cm depth OAR at dmax.

In this specialty

Radiotherapy