Answer to Question #11688 Submitted to "Ask the Experts"

Why is the reference field size chosen as 10 centimeters (cm) by 10 cm in the International Atomic Energy Agency's (IAEA) protocol TRS 398 for output calibration of a cobalt-60 (⁶⁰Co) teletherapy beam?

The dose rate in a phantom (as well as the fluence rate from a treatment unit, ⁶⁰Co unit, or linear accelerator) varies with field size, although the function relating the relative dose rates in a phantom, or the fluence rates, as a function of field size always remains constant for a given treatment unit. The absolute dose rate for a ⁶⁰Co unit obviously changes over time due to radioactive decay, and many orthovoltage units have variations in fluence rates that vary slowly over time. Historically, rather than updating the whole table of dose rates for each field size each month, a table of dose rates relative to a reference field size, that never changed, was used in conjunction with the absolute dose rate, or fluence rate, for the reference field which was updated monthly. Your question essentially is why was the reference field chosen to be 10 cm by 10 cm.

First, whatever reference field size used would be totally arbitrary, so how to pick it is the real question. Second, at least at the time, 10 cm by 10 cm was a very typical size, between a large size used for a whole pelvis, for example, and a small size used for a larynx. Third, and maybe most importantly, it was a very round and even number. Between 5 cm by 5 cm (the smallest available) and 20 cm by 20 cm (the largest at the time on many units), 10 cm by 10 cm was a nice, comfortable number.

Once that became popular, the standards labs used 10 cm by 10 cm to calibrate chambers, thus "writing it in stone." Further, it then became a standard in other applications, such as environmental sampling and wipe tests for contaminated surfaces, where it was a convenient size. Therefore, it may have been chosen simply for ease or consistency across the health physics and medical physics fields of practice.

The US Nuclear Regulatory Commission (NRC) has no specific requirements relative to the 10 cm by 10 cm field size measurement for ⁶⁰Co teletherapy units. Also, none of the NRC guidance goes into that much detail. The requirements simply require licensees to do calibrations on the units "…in accordance with published protocols accepted by nationally recognized bodies," (10 CFR 35.632(e)), and all of those have used 10 cm by 10 cm, as discussed above.

If your question is intended to determine what to do if your beam size isn't 10 cm by 10 cm, then I would refer you to the IAEA publication itself. Table 10, Reference Conditions for the Determination of Absorbed Dose to Water in ⁶⁰Co Gamma Ray Beams (which can be found here), contains this footnote about the field size: "c: The field size is defined at the surface of the phantom for an SSD type set-up, whereas for an SAD type set-up it is defined at the plane of the detector, placed at the reference depth in the water phantom at the isocentre of the machine." The worksheet in Section 5.8 then uses the field size to calculate z_ref.

Bruce Thomadsen, CHP, PhD
Margaret Cervera