Does a Maximum Phototherapy Dose Exist?
One of the most common questions in phototherapy is what maximum dose should patients be exposed to. Over the course of phototherapy treatments, patients photoadapt, which can be thought of as a form of tachyphylaxis where due to a variety of mechanisms (thickening of the stratum cornea, DNA repair enzyme upregulation, pigmentation), increasing doses are needed.
Generally, Daavlin’s answer to this question has been that a fixed maximum dose per Fitzpatrick skin type isn’t particularly useful, as a huge range of maximum tolerable doses are seen within each skin type.
An interesting way to look at maximum dose is to consider what multiple of a patient’s minimal erythemal dose (MED) occurs over treatment. In other words the final dose a patient can tolerate without erythema after a course of therapy, divided by the initial MED would be the patients adaptation factor (AF)
A recent study published in the British Journal of Dermatology attempts to address this question, and found that narrowband UVB AF after 20 treatments to range from 1.1 to 6.0, with the mean AF at 2.7. The extreme AFs were found in Fitzpatrick Skin Type III patients. Interestingly, although the initial MED per skin type varied greatly, there was no correlation between skin type and overall AF.
Another interesting result of this research is to note that broadband UVB adaptation factors have been reported to be much higher, even to the point of 17.5 in one study. Given Broadband UVB lamp’s much higher UVA content compared to Narrowband UVB lamps, it seems reasonable to suggest that some of this difference is related to increased pigmentation induction in Broadband UVB phototherapy.
Practically speaking, one could generally take this information to design protocols, but they would only be of value when starting from an accurate MED test, which in turn supports the position that a universal fixed maximum dose is generally unworkable when starting from a Fitzpatrick skin type derived starting dose.
 Darne, et al. Investigation of cutaneous photoadaptation to narrowband ultraviolet B.Br J Dermatol. 2014 Feb;170(2):392-7. doi: 10.1111/bjd.12662.
 Taylor CR, Stern RS. Magnitude and duration of UV-B induced tolerance.Arch Dermatol1991;127:673