AAPM TG-51 Reference Dosimetry Calculator

Complete implementation of the AAPM TG-51 protocol for reference dosimetry of high-energy photon and electron beams. Calculate all correction factors and determine absorbed dose to water at the reference point.

Chamber & Beam Parameters

For 10×10 cm² field
Optional (for FFF beams)
Typical Farmer chamber ≈ 0.054 Gy/nC (5.4×10⁷ Gy/C)

Environmental Conditions

For information only (not used in P_TP)

P_TP Calculation:

P_TP = (273.2 + T) / 295.2 × 101.33 / P
P_TP = 1.000

Charge Measurements

P_ion - Recombination Correction

Voltage (V) M_raw⁺ (nC) M_raw⁻ (nC) M_raw (avg)
300 (V_H) 9.998
150 (V_L) 9.948
P_ion (Two-Voltage)
1.0025
P_pol (Polarity)
1.0003

P_elec - Electrometer Correction

From calibration certificate
Usually 1.000 for Farmer chambers

Electron Beam Parameters

50% depth from I₅₀
0.6×R₅₀ - 0.1 cm

Electron Specific Corrections

From Table VII or measurements

k'_R50 Calculation:

k'_R50 = 0.9905 + 0.0710×e^(-R₅₀/3.67)
k'_R50 = 1.010

Note on kQ for electrons (simplification)

  • In TG-51, the full electron beam-quality factor is kQ = PgrQ · k'R50 · kecal.
  • This tool applies k'R50 (the beam-quality dependence) and the gradient correction Pgr, but omits kecal, the chamber-specific photon-to-electron conversion factor (e.g. ≈ 0.897 for an NE2571, ≈ 0.906 for an Exradin A12 — see TG-51 Table III).
  • As a result the electron output here is for teaching the R₅₀ → k'R50 workflow; the absolute dose is approximate. Multiply by the chamber's kecal for a complete TG-51 electron calculation.

TG-51 Calculation Results

k_Q (Beam Quality Factor)
0.990
M_corrected
10.05nC
D_w (Dose to Water)
1.000cGy/MU
Dose per nC
0.0995cGy/nC

TG-51 Dose Equation:

D_w^Q = M × k_Q × N_D,w × P_ion × P_TP × P_elec × P_pol
Full calculation: D_w = ...

Important Notes:

  • Measurement depth: 10 cm for photons, d_ref for electrons
  • Field size: 10×10 cm² at phantom surface
  • SSD: 100 cm to phantom surface (photons)
  • Chamber positioned with effective point of measurement at depth
  • Allow sufficient time for temperature equilibration
  • Verify chamber waterproofing before measurements

k_Q Reference Table (Photons)

%dd(10)_x PTW 30013 Exradin A12 NE 2571 Exradin A19
58-60 1.003 1.002 1.004 1.004
63-65 0.994 0.993 0.995 0.995
68-70 0.985 0.984 0.986 0.986
73-75 0.975 0.974 0.976 0.975
78-80 0.965 0.964 0.966 0.965
⚠️ Educational Purpose Only: This TG-51 calculator is for educational purposes. Clinical reference dosimetry must be performed with calibrated equipment and verified procedures. Always refer to the official AAPM TG-51 protocol document for clinical use.