Percent Depth Dose Calculator

Calculator Inputs

Enter measured beam data. The calculator determines percent depth dose, equivalent square, inverse square behavior, and optional Mayneord SSD correction.

Measured Dose At Depth

Dose measured at the selected depth.

Reference Dose At dmax

Usually the dose at maximum depth.

Depth d (cm)

Measurement depth inside the medium.

Reference Depth dmax (cm)

Depth used for normalization.

Original SSD (cm)

Source to surface distance for measured setup.

New SSD For Correction (cm)

Optional value for Mayneord correction.

Field Length (cm)

Beam field length at the surface.

Field Width (cm)

Beam field width at the surface.

Absolute Reference Dose

Optional absolute dose at reference depth.

Example Data Table

Depth (cm)	Measured Dose	Reference Dose At dmax	PDD (%)	Note
1.5	200	200	100.0	Reference maximum depth
5.0	174	200	87.0	Moderate attenuation
10.0	145	200	72.5	Useful for comparison checks
20.0	97	200	48.5	Large depth falloff

Formula Used

1) Percent Depth Dose
PDD = (Dose at depth d ÷ Dose at reference depth) × 100

2) Equivalent Square Field Size
Equivalent Square = 2LW ÷ (L + W)

3) Inverse Square Factor Between Depths
ISF = ((SSD + dmax) ÷ (SSD + d))²

4) Mayneord F Factor For SSD Change
F = ((SSD₂ + dmax) ÷ (SSD₁ + dmax))² × ((SSD₁ + d) ÷ (SSD₂ + d))²

5) Corrected Percent Depth Dose
Corrected PDD = Measured PDD × F

Percent depth dose is a fixed-SSD quantity. It compares the dose at depth with the dose at the normalization depth, often dmax. The optional correction section applies the Mayneord approach when the SSD changes between setups.

How To Use This Calculator

Enter the measured dose at the depth of interest.
Enter the reference dose at dmax or the chosen normalization depth.
Provide depth, reference depth, original SSD, and field dimensions.
Optionally add a new SSD to estimate a corrected PDD.
Submit the form to view results, graph the fitted depth response, and export the report as CSV or PDF.

Frequently Asked Questions

1) What does percent depth dose represent?

It shows how much dose remains at a selected depth relative to the dose at the reference depth, usually dmax, under fixed SSD conditions.

2) Why is dmax important?

dmax is commonly used as the normalization point because it marks the depth where dose peaks for a given beam quality, making comparison easier.

3) When should I use Mayneord correction?

Use it when you have a measured PDD at one SSD and need an approximate PDD at another SSD. It is an estimate, not a full recalculation.

4) Why are field dimensions included?

Field length and width allow the calculator to compute equivalent square size, which helps describe scatter behavior and compare non-square fields more clearly.

5) Can this calculator replace commissioning data?

No. It is useful for checks, education, and quick comparisons. Clinical implementation should always rely on validated beam data and local protocols.

6) What happens if depth is less than dmax?

That point lies in the build-up region. The calculator still reports ratios, but interpretation should consider electronic equilibrium and beam build-up effects.

7) Why does the graph say fitted visualization?

The curve is generated from your entered point and reference conditions. It helps visualize trend behavior, but it is not a substitute for full measured depth scans.

8) What units should I use for dose?

Use consistent units for both measured and reference dose values. The ratio remains valid whether you enter cGy, Gy, or another consistent dose unit.