DOS 525 - Week 2 Discussion
Initial Post: Devices and Practices for Simulation and Verification
Exposure rate from a radioactive source is dependent on several factors including distance, time, shielding used, the type of radioactive material in the source, and the quantity of radioactive material in the source.1 Exposure rate can be measured in a variety of units, but a common way of expressing it is in roentgens per hour (R/h). Distance, quantity of material, and decay rate unique to a material can be wrapped up into a single constant set at a baseline of 1 mCi of material and 1 cm distance from the source in order to simplify the calculation of this rate. The unit for this new constant then becomes
In order to use this constant to derive exposure rate, distance and quantity of material just need to be multiplied into the equation. Since exposure follows an inverse square law with distance, the distance in question is squared and divided away, making the centimeters disappear from the equation, and the once the mCi of material in the sample is multiplied through, all that remains is R/h.
This can be further incorporated with time to cancel out the hours and simply leave roentgens.
The exposure rate constant is a useful shorthand tool that can precalculate the effects of shielding and material type, which are usually well characterized and fixed for a given type of brachytherapy source in clinical practice. This simplifies the math for the remaining variables (distance, quantity, time), making exposure and exposure rate quick to calculate.
- Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:311.