Sr-90 Ophthalmic Applicator: Activity in reb/sec or in mCi?
Have you ever seen radioactive material labeled with the units Roentgen – Equivalent – Beta rays/second (reb/sec)? A medical physicist recently told me he came across these units on a Strontium-90 source at his new job while he was taking inventory of radioactive materials. This particular source was labeled with its model and serial number, as usual, but its radioactivity (the strength of the source) was given in Roentgen – Equivalent – Beta rays/second (reb/sec) instead of millicuries (mCi). Since the convention is to use miC when recording source strength in the inventory log book, he was wondering how to convert these units to mCi. Since I had not worked with Sr-90, I didn’t know the answer myself. I spoke to a couple of experienced physicists I know, and surprisingly no one had the answer. Like any good scientist, this peaked my curiosity…so I did some research. I am guessing many physicists may not know the answer, so I am sharing the fruits of my labor and the result of my due diligence in this post. This is for those who, like me, are curious and are interested to learn:
The 1950s marked the first use of Sr-90/Y-90 sources for treating wedge-shaped benign ophthalmic lesions. Because of the long (~28.8 years) half-life of Sr-90, many of these ophthalmic applicators are still in use today. The US NRC now requires that the long lived radioactive sources have accredited calibration traceable to the NIST dose to water standard established in 1978 by Pruitt1 and revised by Soares2 in 1991.
Several different units have been used to describe source input of Sr-90/Y-90. Soares described these units and the historical progression of these values3:
1- Roentgen Equivalent Beta (reb)
2- Roentgen Equivalent Physical (rep)
So, what exactly is a “reb”? A reb is equal to “the amount of beta radiation that would produce ionization per unit mass in air equivalent to that produced by 1 R of photons.”4
Soares suggested using the conversion factors 0.0093 and 0.00982 (Gy/reb) to convert reb to Gy. For derivation of these conversion factors see Reference 3. According a to recent paper published in Medical Physics by the University of Wisconsin Accredited Dosimetry Laboratory (UWADCL), mCi (1 mCi = 37 MBq) is also used to describe nearly all planar Sr-90 sources4. Though for most applicators, these values represent estimated nominal contained activity rather than precisely known or determined values.
If a Sr-90 source is described with reb rather than nominal activity in mCi, one way to find out the activity in mCi is to calibrate the source at an Accredited Dosimetry Calibration Laboratory (ADCL), such as UWADCL, in Gy/sec. Then the activity can be estimated in mCi by using the conversion coefficients derived by Soares to convert reb to Gy1.
For more information see the following references:
1- J. S. Pruitt. “Calibration of beta-particle-emitting ophthalmic applicators.” NBC special publication No. 250-9, 1978.
2-C. G. Soares. “Comparison of NIST and manufacturer calibration of Sr90+Y90 ophthalmic applicators.” Med. Phys. 22, 1487-1493 (1995).
3- C. G. Soares. “Calibration of ophthalmic applicators at NIST: A revised approach.” Med. Phys. (18), 787-793 (1991).
4- Shannon M. Holmes, John A. Micka, and Larry DeWerd. “Ophthalmic applicators: A review of calibration following the change to SI units.” Med. Phys. 36 (5), 1473-1477, May 2009.