Calculator Inputs
Example Data Table
Sample inputs (photon beam), including inverse-square and scatter correction, showing a typical PDD → TMR conversion.
| PDD (%) | SSD (cm) | Depth (cm) | dmax (cm) | Field (cm) | Sp(d) | Sp(dmax) | Computed TMR |
|---|---|---|---|---|---|---|---|
| 66.5 | 100 | 10.0 | 1.5 | 10 × 10 | 1.00 | 1.02 | 0.796664 |
Formula Used
This calculator uses a widely taught conversion between percentage depth dose (PDD) and tissue maximum ratio (TMR), based on geometry and (optionally) phantom scatter.
- Field size at depth rd is computed by divergence: rd = r · (SSD + d)/SSD.
- Equivalent square for rectangles uses 2ab/(a+b).
- Scatter ratio is optional; set it to 1 if unavailable.
How to Use This Calculator
- Select PDD → TMR or TMR → PDD.
- Enter SSD, depth, and dmax values in centimeters.
- Choose field shape, then enter square or rectangular sizes.
- Enable inverse-square correction if converting between SSD-style and isocentric-style use cases.
- If you have Sp (or PSF) data, enable scatter correction and enter both Sp values.
- Press Compute. The result appears above the form with export buttons.
Notes and Best Practices
- Use consistent definitions for field size (surface-defined vs depth-defined) across your data tables.
- For very small stereotactic fields, commissioning-specific methods may be required.
- Always validate conversions against measured reference data for your beam energy.
Technical Article
1) Where PDD and TMR are used
PDD is measured in a fixed-SSD setup and reports dose at depth as a percentage of the dose at dmax. TMR is used for isocentric planning and is the dose at depth divided by the dose at dmax at the same geometry.
2) Why converting matters in clinical workflows
Commissioning data are often recorded as PDD tables, yet monitor unit checks and many beam models rely on TMR/TAR-style ratios. This conversion helps cross-check depth-dose curves and keep SSD and SAD techniques consistent across datasets.
3) Geometry factor behind the conversion
The geometry term comes from inverse-square change in distance between the point of interest and the reference depth. With SSD in centimeters, the factor ((SSD + d)/(SSD + dmax))² increases the ratio as depth grows. Disabling geometry is useful only when your inputs are already distance-corrected.
4) Field size at depth and divergence
Field size affects scatter and therefore depth-dose ratios. This calculator expands the surface-defined field to depth using rd = r·(SSD + d)/SSD. For rectangles, it uses the equivalent square 2ab/(a+b), which typically matches scatter behavior better than using the raw sides separately.
5) Optional phantom scatter (Sp/PSF) correction
Many clinics keep phantom scatter factors by field size and energy. If you provide Sp at depth and Sp at dmax, the ratio Sp(dmax)/Sp(d) adjusts the conversion to better reflect changing scatter conditions. If you do not have Sp values, set the ratio to 1 and rely on measured TMR data for final commissioning.
6) Typical values for quick plausibility checks
For 6 MV photons, SSD 100 cm, dmax 1.3–1.7 cm, and a 10×10 cm field are common. At 10 cm depth, PDD is often 65–75%, while TMR is about 0.70–0.80, varying with energy, field size, and scatter.
7) Practical QA use case with beam tables
A useful workflow is to take a measured PDD curve, convert selected depths (5, 10, 15, 20 cm) to TMR, and compare against independently measured TMR or TPS-exported ratios. Agreement within a few percent is expected when the same field definition, SSD, and scatter formalism are used throughout the dataset.
8) Reporting, traceability, and audit readiness
CSV supports spreadsheet audits, and the PDF report fits commissioning notes. Record energy, reference field, measurement geometry, and scatter assumptions with the output so reviewers can reproduce the numbers and track the correction pathway used.
FAQs
1) What is the main difference between PDD and TMR?
PDD is measured at fixed SSD and reports dose at depth relative to dmax at the surface setup. TMR is used at isocenter and compares doses at depth and dmax at the same SAD geometry.
2) When should I enable inverse-square correction?
Enable it when converting SSD-based PDD data into SAD-style ratios, or when you want the conversion to explicitly account for distance changes between d and dmax. Disable it only if your source data already includes that geometry correction.
3) Do I need scatter factors to get a usable result?
No. If you do not have Sp/PSF values, leave scatter correction off and the calculator assumes a ratio of 1. Scatter factors mainly improve consistency when you are matching a specific clinical formalism or measured dataset.
4) How does the calculator handle rectangular fields?
It converts the rectangle to an equivalent square using 2ab/(a+b). This is a common approximation for matching scatter behavior. The equivalent square is then expanded by divergence to estimate field size at depth and at dmax.
5) What ranges indicate likely input mistakes?
As a quick check, TMR should generally fall between 0 and 1 for typical photon depths, and PDD is commonly below 100% for megavoltage beams. Extremely large values often indicate unit mix-ups or swapped depths.
6) Why does field size at depth matter if I enter surface size?
Because the beam diverges, the field is larger at depth than at the surface. Scatter-dependent quantities, including Sp and depth-dose ratios, depend on the effective field size at the point of interest, not only the collimator setting.
7) Can I use this for electron beams?
This tool is intended for photon-beam style PDD↔TMR conversions. Electron dosimetry uses different depth-dose behavior and reference quantities. For electrons, rely on electron-specific protocols and measured data rather than applying photon-based TMR relationships.