01 Therapy
Ir-192 HDR decay
HDR source strength with T½ = 73.83 days.
Listen
Listen · English
Simulation
Ir-192 HDR decay — Change the numbers; the scene follows.
Where it works
Brachytherapy suite
Afterloader
In the afterloader safe: Ir-192 source strength and the dwell-time calculation start here.
Open this machineFormula
Variables
Results
S_K
Current strength
30,181.4U
DF
Decay factor
0.7545
per week
Weekly loss
6.36%
Explanation
What it means
Ir-192 (HDR) has T½ = 73.83 days, so source strength drops ~6.4% per week. After-loaders require a decay table or live calculation before every fraction. Air-kerma strength S_K is in U (cGy cm² h⁻¹). This is a working relation in Radiotherapy.
Where it is used
Clinically it sits on the Brachytherapy suite — Afterloader. In the afterloader safe: Ir-192 source strength and the dwell-time calculation start here. Radiotherapy equations sit at the console and in the bunker: output, depth dose, equivalent square, and the monitor units that treat the patient. Hand-calc them beside the TPS, never instead of a commissioned plan.
Brachytherapy suite · Open this machineHow to use it
Enter S_K at a reference date and elapsed days. A 40 kU source is ~33 kU after 30 days. Pair with the TG-43 point-source calculator for a dwell dose. Change one input and watch the curve and the simulation follow.
Symbols
- S_K0Air-kerma strength40,000 U
- tElapsed time30 d
Worked example
A typical case from the default values: S_K0 = 40,000 U (Air-kerma strength); t = 30 d (Elapsed time). Substituting into the relation gives S_K = 30,181.4 U; DF = 0.7545; per week = 6.36 %. These are teaching numbers — align them with your machine.
Typical values give
- S_K = 30,181.4U
- DF = 0.7545
- per week = 6.36%
Where it comes from
The displayed formula is the working relation. HDR source strength with T½ = 73.83 days. Usual reference: NNDC / TG-43. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: NNDC / TG-43
Assumptions & limits
Single exponential of Ir-192. Anisotropy, applicator attenuation and patient scatter belong in TG-43, not here.
Pitfalls
Never mix PDD from one SSD with TMR from another without converting. Field size at the surface is not the size at isocentre. A hand MU is a check, not a treatment. Single exponential of Ir-192. Anisotropy, applicator attenuation and patient scatter belong in TG-43, not here.
Keep this
Name the SSD, energy, and field size with every PDD or TMR you quote. Single exponential of Ir-192. Anisotropy, applicator attenuation and patient scatter belong in TG-43, not here.
In this specialty