01 Therapy

Monitor units — SAD technique

Isocentric MU calculation using TMR.

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Simulation

Monitor units — SAD technique — 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}{\dot{D}_{\mathrm{cal}}\cdot\mathrm{TMR}\cdot S_c\cdot S_p\cdot W_f\cdot T_f\cdot(\mathrm{SCD}/\mathrm{SAD})^2}

Variables

D · Prescribed dosecGyḊ_cal · Calibration outputcGy/MUTMR · Tissue-maximum ratioS_c · Collimator scatterS_p · Phantom scatterW_f · Wedge factorT_f · Tray factorSCD · Calibration distancecmSAD · Source-to-axis distancecm

Results

MU
Monitor units
223.15MU
(SCD/SAD)²
Inverse-square factor
1.0302

Explanation

\mathrm{MU} = \frac{D}{\dot{D}_{\mathrm{cal}}\cdot\mathrm{TMR}\cdot S_c\cdot S_p\cdot W_f\cdot T_f\cdot(\mathrm{SCD}/\mathrm{SAD})^2}

What it means

Isocentric MU uses TMR instead of PDD and an inverse-square from the calibration distance (SCD, often SSD_cal+dmax) to SAD. Most linacs treat isocentrically; this is the matching hand calculation. 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

Enter TMR at the calculation depth and field size at isocentre. SCD = 100 + dmax if calibrated at SSD 100. The (SCD/SAD)² term is slightly above 1 when SAD = 100 cm and dmax > 0. Change one input and watch the curve and the simulation follow.

Symbols

DPrescribed dose180 cGy
Ḋ_calCalibration output1 cGy/MU
TMRTissue-maximum ratio0.76
S_cCollimator scatter1.02
S_pPhantom scatter1.01
W_fWedge factor1
T_fTray factor1
SCDCalibration distance101.5 cm
SADSource-to-axis distance100 cm

Worked example

A typical case from the default values: D = 180 cGy (Prescribed dose); Ḋ_cal = 1 cGy/MU (Calibration output); TMR = 0.76 (Tissue-maximum ratio); S_c = 1.02 (Collimator scatter); S_p = 1.01 (Phantom scatter); W_f = 1 (Wedge factor); T_f = 1 (Tray factor); SCD = 101.5 cm (Calibration distance); SAD = 100 cm (Source-to-axis distance). Substituting into the relation gives MU = 223.15 MU; (SCD/SAD)² = 1.0302. These are teaching numbers — align them with your machine.

Typical values give

MU = 223.15MU
(SCD/SAD)² = 1.0302

Where it comes from

The displayed formula is the working relation. Isocentric MU calculation using TMR. Usual reference: Khan / clinical MU calc. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan / clinical MU calc

Assumptions & limits

Same caveats as the SSD formula. SAD MU does not replace a commissioned TPS for modulated, wedged-in-MLC, or heterogeneous plans.

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. Same caveats as the SSD formula. SAD MU does not replace a commissioned TPS for modulated, wedged-in-MLC, or heterogeneous plans.

Keep this

SCD is the calibration distance (often SSD_cal + dmax). SAD is typically 100 cm.

In this specialty

Radiotherapy