01 Therapy

Percent depth dose

Dose at depth d relative to dose at dmax on the beam axis.

Play lecture Cast to TV

Listen

Listen · English

Simulation

Percent depth 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).

Open this machine

Formula

\mathrm{PDD}(d) = 100 \cdot \frac{D_d}{D_{d_{\max}}}

Variables

D_d · Dose at depthcGyD_dmax · Dose at dmaxcGy

Results

PDD
Percent depth dose
90%

Explanation

\mathrm{PDD}(d) = 100 \cdot \frac{D_d}{D_{d_{\max}}}

What it means

Percent depth dose is the central-axis dose at depth d expressed as a percentage of the dose at dmax, measured at a fixed SSD. PDD increases with energy and field size and falls with depth beyond dmax. 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

Enter measured (or TPS) doses at depth and at dmax. A 6 MV 10×10 field at 10 cm is typically ~67% PDD at SSD 100 cm. Combine with Mayneord F when SSD changes. Change one input and watch the curve and the simulation follow.

Symbols

D_dDose at depth180 cGy
D_dmaxDose at dmax200 cGy

Worked example

A typical case from the default values: D_d = 180 cGy (Dose at depth); D_dmax = 200 cGy (Dose at dmax). Substituting into the relation gives PDD = 90 %. These are teaching numbers — align them with your machine.

Typical values give

PDD = 90%

Where it comes from

The displayed formula is the working relation. Dose at depth d relative to dose at dmax on the beam axis. Usual reference: Khan, Ch. 9. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan, Ch. 9

Assumptions & limits

Defined at a stated SSD, energy, and field size. Electron contamination and dmax shift with field size are not modelled. Do not mix PDD from different SSDs without Mayneord or TMR 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. Defined at a stated SSD, energy, and field size. Electron contamination and dmax shift with field size are not modelled. Do not mix PDD from different SSDs without Mayneord or TMR conversion.

Keep this

PDD depends on energy, SSD, and field size. Usually measured at fixed SSD.

In this specialty

Radiotherapy