Monitor Unit Calculator

Beam Input Form

Large screens use three columns, smaller screens use two, and mobile uses one.

Prescribed Dose (cGy)

Dose delivered by this beam or field.

Machine Output (cGy/MU)

Calibrated output at the reference condition.

TMR / PDD Factor

Depth-dependent attenuation or tissue factor.

Collimator Scatter Factor (Sc)

Head scatter correction for collimator setting.

Phantom Scatter Factor (Sp)

Scatter contribution from phantom conditions.

Wedge Factor (WF)

Use 1.00 when no wedge is present.

Tray Factor (TF)

Accessory transmission factor for tray use.

Off-Axis Ratio (OAR)

Use 1.00 for central axis estimates.

Calibration Correction

Additional machine or protocol correction factor.

Calibration Distance (cm)

Reference distance used during output calibration.

Treatment Distance (cm)

Distance from source to treatment point.

Fractions

Used to show MU per fraction.

Boost Dose Adjustment (%)

Optional dose escalation percentage.

Rounding Step (MU)

Rounds raw MU to a practical step.

Technique

For workflow labeling and documentation.

Depth Model

Choose the depth representation in use.

Apply boost adjustment

Notes

This field helps documentation but does not change the formula.

Example Data Table

Scenario	Dose (cGy)	Output	Depth Factor	Sc	Sp	WF	TF	OAR	Calc Dist	Treat Dist	Estimated MU
6 MV Open Field	200.00	1.000	0.790	1.020	1.010	1.000	1.000	1.000	100 cm	100 cm	245.74
6 MV Wedge Field	180.00	1.000	0.760	1.015	1.005	0.720	1.000	0.985	100 cm	100 cm	327.38
10 MV Tray Field	250.00	1.020	0.815	1.030	1.012	1.000	0.965	1.000	100 cm	105 cm	329.62

Formula Used

MU = Prescribed Dose × Boost Multiplier ÷ [ Output × Depth Factor × Sc × Sp × WF × TF × OAR × Calibration Correction × ISQ ] ISQ = (Calibration Distance ÷ Treatment Distance)²

Prescribed Dose is the intended beam dose in cGy.

Output is the calibrated dose delivered per MU.

Depth Factor represents TMR, TPR, PDD, or a custom attenuation term.

Sc and Sp account for collimator and phantom scatter.

WF, TF, and OAR handle wedge, tray, and off-axis corrections.

Calibration Correction applies any extra protocol-specific adjustment.

ISQ applies distance scaling using inverse square behavior.

How to Use This Calculator

Enter the prescribed dose in cGy for the beam.
Input machine output at the reference calibration condition.
Choose the depth model and enter its factor.
Fill in scatter, wedge, tray, and off-axis factors.
Enter calibration and treatment distances in centimeters.
Set fractions and optional boost adjustment if needed.
Pick a rounding step for operational MU rounding.
Submit the form to view results above the form.
Use the chart and table to review the breakdown.
Export CSV or PDF for records and checking.

Frequently Asked Questions

1. What does this calculator estimate?

It estimates monitor units from dose, output, depth, distance, and correction factors. It is useful for educational review, workflow documentation, and quick consistency checks.

2. Why are monitor units important?

Monitor units help translate planned dose into machine delivery settings. Accurate MU estimation supports consistent beam delivery and highlights the effect of correction factors.

3. What is the depth factor here?

The depth factor represents attenuation and scatter behavior at treatment depth. It may come from TMR, TPR, PDD, or another validated beam model term.

4. When should I change the inverse square setup?

Change the distances when calibration and treatment geometry differ. The calculator uses those values to compute an inverse square correction automatically.

5. Why can wedge or tray factors lower MU efficiency?

Accessories often reduce beam transmission. Lower transmission means the denominator becomes smaller, so more monitor units are needed to achieve the same dose.

6. What does the rounding step do?

It rounds the raw monitor unit value to a practical delivery increment. This helps operational workflows when fractional MU values are not preferred.

7. Can I export the results?

Yes. The page includes CSV export for spreadsheet review and a PDF-style print export for records, handoff, or planning documentation.

8. Is this calculator suitable for direct clinical decisions?

No. It is best used as an educational or administrative aid. Any real treatment workflow needs independent physics review, validated beam data, and institutional procedures.