Mayneord Factor Calculator

Guide to Using the Mayneord Factor

1) What the calculator estimates

This tool estimates the Mayneord Factor (MF), an inverse-square correction used to approximate how central-axis depth dose changes when the source-to-surface distance (SSD) changes. It can also estimate PDD₂ from PDD₁ and suggest a quick MU scaling using MF. These outputs are intended for checks, education, and rough comparisons.

2) Typical SSD and depth ranges

In many photon-beam workflows, SSD values are commonly around 80–120 cm, while calculation depths often fall within 0–30 cm along the central axis. The calculator flags unusually low SSD values (for example, below 50 cm) and very large depths because the approximation can drift as geometry and scatter conditions change.

3) The data behind the correction

MF is built from two squared ratios. The first ratio compares the effective source-to-point distances (SSD + depth) between the new and original setups. The second ratio compares the SSDs directly. The calculator shows intermediate values (A, A², B, B²) so you can audit inputs and reproduce results by hand.

4) Example: changing SSD by 10 cm

Suppose SSD₁=100 cm and SSD₂=110 cm at the same depth d=10 cm. The geometry terms become (110+10)/(100+10)=120/110 and 100/110. Combining the squared terms yields MF≈0.992. That means PDD₂ is estimated to be about 0.8% lower than PDD₁, a small but measurable shift in some contexts.

5) Optional PDD and MU outputs

If you enter PDD₁, the calculator reports PDD₂≈PDD₁×MF and can cap results to 0–100% to prevent nonsensical outputs. If you enter planned MU, it can suggest MU₂≈MU₁/MF as a quick scaling idea. Use these as starting points, not as final clinical values.

6) Field size and energy context

You may record field size and nominal energy for documentation. MF itself does not model scatter dependence on field size, energy spectrum changes, wedges, blocks, or inhomogeneities. Larger fields and deeper points are more sensitive to scatter conditions, so direct measurement or TPS data is typically preferred for accuracy.

7) Sensitivity check for depth entry

To help detect input mistakes, the calculator reports MF at d₂±Δ (default Δ=0.5 cm). If a small depth change causes an unexpectedly large MF swing, review SSD and depth units, confirm decimal placement, and ensure you did not mix cm and mm.

8) Batch calculations and reporting

Batch mode accepts multiple rows (SSD₁, d₁, SSD₂, d₂, optional PDD₁, optional MU). Each row returns MF and the chosen derived outputs. CSV export is ideal for spreadsheet review, while the PDF report captures the key inputs, computed MF, and limitation notes for recordkeeping.

FAQs

1) When is Mayneord factor appropriate?

It is mainly a quick inverse-square check when SSD changes and setup remains comparable. It is less reliable for large depth differences, large field-size changes, modifiers, and heterogeneous media.

2) Does MF depend on field size?

The MF formula does not. However, real depth dose depends on scatter, which varies with field size, depth, and energy. Treat field-size entries as documentation and a reminder of this limitation.

3) Why can PDD₂ exceed 100%?

If PDD₁ is high or MF is greater than 1, multiplication can exceed 100%. The calculator can cap PDD₂ to 0–100% to keep outputs realistic, but you should still verify the underlying assumptions.

4) Is MU₂≈MU₁/MF always valid?

It is a rough scaling idea tied to the same approximation. MU depends on calibration conditions, output factors, scatter, and modifiers. Use it for a sanity check and confirm with your clinical workflow.

5) What depth should I enter?

Enter central-axis depth from the surface in the chosen unit. If you work with water-equivalent depth, use that value. For non-water media, MF alone does not capture heterogeneity effects.

6) How should I interpret the sensitivity results?

MF(d₂±Δ) helps you see how much MF changes with small depth differences. If results are overly sensitive, check units, SSD values, and whether the calculation point is unusually deep.

7) What is included in the exports?

CSV exports inputs and computed outputs per calculation or per batch row. PDF exports a compact text report with timestamps, key inputs, MF, optional derived values, and limitation notes.