Case study - using Ionactive free resources to determine skin dose from F-18 contamination

Case study - skin dose from F-18 contamination (scenario)

A technician is wearing a double pair of gloves and is working with F-18 during a quality assurance (QC) procedure. At some point during this procedure the index finger tip of the left hand glove becomes contaminated with F-18. Five minutes later, whilst checking their gloves on a CPS (counts per second) scaled radiation monitor, they realise their gloves are contaminated. [Ionactive comment: At this point the glove would probably be immediately removed and the hand monitored to ensure that contamination was only on the outer surface of the glove. For this scenario the glove remains on long enough to make the following evaluations. Alternatively, the same dose assessment could be performed on the glove once it had been removed. ]

Determine the likely contamination on the glove (where index finger is located)

F-18 is a positron emitter with the following radiological properties. 

• Positron emitter with a β⁺ energy of  0.6339 MeV (E_Max).
• Annihilation radiation (from β⁺ e^- interaction) yields two 0.511 MeV photons (in apposing directions).

The activity can be evaluated by taking CPS (counts per second) measurements or by dose rate measurements. The CPS method might be more 'precise' (better signal), but it requires that you know the F-18 activity to CPS response relationship (which will vary with monitor type, size of detector, distance and overall geometry etc). So for this scenario we will instead consider measuring the dose rate from the 0.511 MeV photons to calculate (estimate) the activity present. 

In calculating the dose rate (from a photon source) we need to be aware of the inverse square law (ISL) and how this is interpreted when taking the dose rate measurement. This law assumes a point source of radiation, so taking a dose rate measurement right on the gloved finger (or very close) may overestimate the actual dose rate at that point. In addition, we want an ISL relationship to reliably calculate an activity. 

We use the following rule of thumb 'the inverse square law will be valid, and approximate a point source, where the distance between the source is 10 times the longest dimension of the source'.  We have a lot of resource on our website to explore this concept such as : Inverse Square Law - when is a source a point source? For a more mathematically rigorous ISL analysis, the following resource may be a helpful read: When 1/d² breaks down - part 1: line source. 

Given it's a finger tip, it is reasonable to estimate that maximum dimension of the source is 1 cm (i.e. area is 1 cm²). This may be an oversimplification, but since IRR17 skin dose will be averaged over 1 cm (dose averaged over any area of 1 cm² regardless of the area exposed), then this is a valid assumption. Whilst it could be debated if this is an area source or a point source, for dose rate to activity assessments it is better to assume a point source. 

Using a suitable monitor it is found that the dose rate measured at 10 cm from the finger tip is 1 micro Sv/h. 

We can now use this dose rate and the Ionactive dose rate to radioactivity calculator to estimate the activity on the glove finger. 

Below is the live calculator - have a play! [Ionactive comment: If you prefer to try the calculator in its own page then go here: Ionactive Dose Rate to Radioactivity Calculator (will open in a new tab). ]

Below is a screen capture of what you should be seeing, using the above data. 

Using the calculator (and noting the screen capture) it can be seen that the output is Activity (Absorbed Dose in Air): 74.2390 kBq. You may prefer to use the Effective Dose Equivalent data, but given the unknowns we will stick with the higher value. With rounding we estimate we have 74 kBq of F-18 on the gloved finger. 

Given the discussion above (including how skin dose limits are expressed), and noting the average size of the tip of an index finger, it is valid to make a further assumption that the 74 kBq is spread over an area of 1cm²,  such that the skin dose rate (through the glove) arises from an activity which can be expressed as 74 kBq / cm². 

We can now introduce the Ionactive Beta Skin Dose Rate Calculator. Whilst F-18 is a positron emitter, and the dominant whole body (or extremity) radiation exposure is via the 0.511 MeV annihilation photons, the skin dose exposure is dominated by the positrons (β⁺ particles). They deliver a 0.07 HP skin dose before (or whilst) undergoing annihilation with an electron (e^-) in the skin or other material.  

Determine the likely skin exposure (dose) to the finger

The live beta skin dose calculator is featured below. Have a play and / or follow the screen capture below for the desired inputs and outputs. [Ionactive comment: If you prefer to try the calculator in its own page then go here: Ionactive Beta Emitter Skin Dose Rate Calculator (will open in a new tab). ]

Most of the inputs are obvious - and for the moment we will ignore airgap and the fact that gloves are being worn (later we can investigate the effect of both). 

The input to use is 74 kBq /cm² of F-18. 

Below is a screen capture of what you should be seeing, using the above data. 

The output from the skin beta dose rate calculator reveals 140.6 mSv/h (2.343 mSv/min)  - not trivial considering the activity is only 74 kBq. If we assume a 5 minute exposure, and ignore the decay over 5 mins, then the dose delivered could be in the region of 11.7 mSv. 

Aside: Is it necessary to decay correct over this short period of 5 minutes (?) - not really. The activity might have been up to 76.4 kBq, 5 minutes before the measurements were made - the difference is not worth worrying about.  The decay over 5 minutes (back in time) was calculated using the Ionactive Radioactive Half-Life Calculator (opens in a new window).  The live half life calculator is shown below. 

Below is a screen capture of what you should be seeing, using the above decay data. 

Discussion and extension

We set out to determine the dose to the finger from F-18 contamination over an exposure period of 5 minutes (11.7 mSv equivalent dose). This has been achieved by using a dose rate to activity calculator, considering validity of the inverse square law approximating a point source, using a beta skin dose calculator to estimate dose to the finger, and considering the effect of half-life over a 5 minute period by using the half life and decay calculator. 

Note that the calculation assumes direct skin contamination (and if you want to play safe you might want to leave it at that). However, whilst no one should cheat or fix dose calculations, over estimating serves no valid purpose and could be just as bad as underestimating. This might especially be the case if the dose calculated looks like being near a dose investigation level (DIL) or a legal dose limit. That fact here is that two pairs of gloves were being worn and they should absolutely be accounted for.

We will therefore consider the effect of two pairs of gloves on the dose calculation. If you want to follow on with the skin dose calculator it might be easier to use the given link and have it open in a separate window. Will assume that:

• Two pairs of nitrile gloves are being worn.
• The glove thickness at the finger tip is 5.6 mil (where 1 mil = 0.0254mm).
• Glove thickness (finger tip) in mm  = 0.142mm
• Airgap between finger and glove material assumed zero. 

The above data can be entered into the skin dose calculator by changing the 'Add Cover (material layer)' to yes.

Note that the dose rate reduces to 0.755 mSv/min (from 2.343 mSv/min) and so a more realistic assessment of dose over 5 minutes is now 3.8 mSv.  Imagine if we were talking about 7400 kBq (7.4 MBq) rather than 74 kBq. The calculated total dose over 5 minutes would then be:

• 1170 mSv (based on a calculation where no gloves are accounted for).
• 380 mSv (based on wearing gloves). 

Neither exposure is great, but calculations  show that if gloves were not be worn, exposures are  twice the UK legal limit (500 mSv/year equivalent dose to the skin). 

Wearing gloves for contamination control (avoiding contaminated skin) is a given. Wearing gloves for reducing skin exposure by attenuation of the F-18 positrons is particularly advantageous (it is a mistake to assume that gloves offer no attenuation due to the 511 keV photons being present - the photons are  not the issue when considering HP0.07, the positrons are the issue and they will be attenuated). 

We could take the assessment one stage further by considering the potential airgap between the inner surface of the inner most pair of gloves, and the surface of the skin. The validity of this will very much depend on the location of the the contamination on the glove in relation to the finger. If the contamination is on the finger tip then a 2mm gap (example) is probably valid (put a glove on and see why this is true). However, if the contamination is on the finger pad (i.e. finger print position) of the glove then the airgap is likely to be negligible as there will be a tight fit at this position.  For the purposes of demonstrating the skin dose calculator we will assume a 2 mm airgap - this has been added to the calculator as shown in the screen capture below. 

As noted above, the airgap reduces the calculated skin dose rate by a modest amount, down to 0.511 mSv/min (from 0.755 mSv/min). If the airgap is a reasonable assumption (or demonstrated  by inspection whilst wearing a clean glove), then the exposure of 5 minutes will now be: 2.6 mSv. You have to decide if you are fixing the calculation here!

It is very clear from the analysis in this blog article that gloves are important in two ways :

• They avoid direct skin contamination, and their swift removal will minimise dose to the skin of the hands.
• In the case of energetic beta or positron emitters, gloves can provide meaningful attenuation  - up to a point.  

How do other beta emitters compare?

We will finish by comparing F-18 with a few beta / positron emitters of note. We will use 74 kBq/cm² and compare skin dose rates assuming direct skin contamination, and then with one contaminated glove, then two gloves, and finally two gloves with a 2 mm airgap. 

The table provides some interesting comparisons. It shows that the headline beta E_max and E_ave do not tell the whole story, the beta spectrum matters too. For example, note that direct skin contamination from F-18 or P-32 is very similar per unit activity/cm². However, the benefits of using gloves and air gaps is much reduced with P-32 as compared to F-18. With Sr-90/Y-90 you are getting two high energy beta particles for the price of one decay - and this is reflected in the much higher dose rate. It must also be remembered that the dose calculation is for 0.07mm (HP0.07), in the case of Sr-90/Y-90 the dose distribution is over a much more extensive depth.