01 Therapy

Electron monitor units

MU = D / [(D/MU)_ref × cutout × insert × SSD factor].

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Simulation

Electron monitor units — Change the numbers; the scene follows.

Where it works

Linear accelerator

Treatment head

Inside the treatment head: monitor chambers measure output; wedges and leakage live here.

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Formula

\mathrm{MU}=\frac{D}{(D/\mathrm{MU})_{\mathrm{ref}}\cdot C_{\mathrm{cut}}\cdot C_{\mathrm{ins}}\cdot F_{\mathrm{SSD}}}

Variables

D · Prescribed dosecGyD/MU · CalibrationcGy/MUC_cut · Cutout factorC_ins · Cone / insert factorF_SSD · Effective-SSD factor

Results

MU
Monitor units
210.5263MU

Explanation

\mathrm{MU}=\frac{D}{(D/\mathrm{MU})_{\mathrm{ref}}\cdot C_{\mathrm{cut}}\cdot C_{\mathrm{ins}}\cdot F_{\mathrm{SSD}}}

What it means

Electron output is calibrated at a reference cone (often 10×10 or 15×15) and dmax, 1 cGy/MU. End-user cutouts change output (small cuts under-scatter and can drop 10–20%). An insert/cone factor accounts for the applicator. Extended SSD uses an effective-SSD inverse-square factor, not the nominal 100 cm, because of scatter from the cone. This is a working relation in Radiotherapy.

Where it is used

Clinically it sits on the Linear accelerator — Treatment head. Inside the treatment head: monitor chambers measure output; wedges and leakage live here. 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

Set unused factors to 1. Example: 200 cGy, cal 1 cGy/MU, cutout 0.92, cone 1.00, F_SSD 0.96 → MU ≈ 226. Verify against the departmental cutout-factor curve. Change one input and watch the curve and the simulation follow.

Symbols

DPrescribed dose200 cGy
D/MUCalibration1 cGy/MU
C_cutCutout factor0.95
C_insCone / insert factor1
F_SSDEffective-SSD factor1

Worked example

A typical case from the default values: D = 200 cGy (Prescribed dose); D/MU = 1 cGy/MU (Calibration); C_cut = 0.95 (Cutout factor); C_ins = 1 (Cone / insert factor); F_SSD = 1 (Effective-SSD factor). Substituting into the relation gives MU = 210.5263 MU. These are teaching numbers — align them with your machine.

Typical values give

MU = 210.5263MU

Where it comes from

The displayed formula is the working relation. MU = D / [(D/MU)_ref × cutout × insert × SSD factor]. Usual reference: Khan Ch. 14 / AAPM TG-70. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan Ch. 14 / AAPM TG-70

Assumptions & limits

Does not compute the cutout factor (needs measured data or a sector-integration / VMC model). Bolus, bone heterogeneity and obliquity are extra. Never use photon PDD/TMR for electrons.

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. Does not compute the cutout factor (needs measured data or a sector-integration / VMC model). Bolus, bone heterogeneity and obliquity are extra. Never use photon PDD/TMR for electrons.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Does not compute the cutout factor (needs measured data or a sector-integration / VMC model). Bolus, bone heterogeneity and obliquity are extra. Never use photon PDD/TMR for electrons.

In this specialty

Radiotherapy