If you start looking into the calibration of whole-body UV cabins, you rapidly open a huge can of worms.
The simple answer to your question "maybe it has to be done for each individual cabinet, a different conversion factor for UVA and UVB", is that YES, you need to measure each wavelength seperately for each cabin, and if you are issuing factors (rather than tweaking the machine), you need to ensure that the operator is applying the correct factors to the correct treatment unit.
The biggest problem is your phrase "Define an accurate calibration factor". We accept the cabin factors as "Arbitry units", and keep our calibration measurements to ourselves. If a transfer record is required, we can generate transfer records with absolute values rapidly.
If your center does not use prescriptive dosimetry (i.e. the Dr says - "Give this person 40J over 16 fractions"), but instead the operator increases the dose in line with the patients reactions to the treatment, then I do not believe absolute calibration is essential.
Waldmann and Cosmedico (the 2 big players at the moment) lock the calibration factors of the sensors into the comuter memory at the factory and you cannot change that. I have just completed the factory training course for service engineers at Cosmedico, and they won't even let their engineers change it!
As to dosimetry & calibration - well, where would you like to start. Some folk belive it is best to measure at 20cm out, some at 2cm out, some use a full size body "simulator" made from foam ... there really are as many different techniques as there are centers providing the service. I don't want to teach you to suck eggs, but technique and repeatability of technique is really important. If you cannot accurately repeat your measurements, then there is no point in doing them in the first place. Also, always document what you do - we are in a very litigatious society and given you are directly affecting a patients treatment, documentation will save you neck in court. I was always told - "It's the work that gets you paid, it's the paperwork that keeps you out of jail".
Anyhow, to try to answer your question. I would always consider it prudent to consider calibrations on a like for like basis. i.e. if the cabin's dosimetry is calibrated with a body in-situ, then that is how you should measure the UV intensity in the cabin. It is true that a body present in the cabin will attenuate the readings on the sensors, but then again, most sensors only look at one (or at most 2) banks of lamps.
Just for illustration, the Cosmedico caibration procedure, calls for the engineer to enter the cabin, wearing a white Tyvek jumpsuit (with hood - great fun in July, when it was 39ÂșC in the factory and then being sat in the cabin with all the lamps on for 3 minutes!!), and to measure the output along the mid-line of each bank of lamps, 2cm out from the clear, protective guard. The maximum value obtained (irrespective of lamp bank) is used. Only the factory specified dosemeter is approved for this. Personally, I measure the output using an IL1400A (with appropriate sensor) held on a tripod, at midline, mid height, 20cm out from each bank with a lab-coat hanging on the tripod to provide attenuation. I measure both intensity and calculated dose for a given exposure. These values are compared with the set/displayed values, and provided the % error does not change (significantly), the unit is deemed fit for use. At present. this has the advantage of permitting the patient to swap from one machine to another without correction factors - the factory calibration is still valid (and is still within acceptable error)
I would be careful in issuing new calibrations/correction factors on a regular basis - is the cabin really changing that much?
I believe it is really important that should you decide to change your calibration technique, you cross check with your old technique to ensure silly mistakes haven't been made (I nearly let one by some time ago when doing just that, but a double check caught it!).
There is a (what I hope to be good) meeting in December at Gloucester, targeted at those providing a QA/Service to W/B UV users. I shall be there and I hope to gain a lot from it.
Feel free to contact me at david.richardson@essexrivers.nhs.uk if you want any more info, or failing that, catch up with me at Gloucester in December.