Phototesting based on a divergent beam – a study on normal subjects

Abstract

In a previous publication from our group, phototesting based on a single exposure to a divergent UVB beam with radially decreasing irradiance values was suggested. The aim of the present study was to evaluate technical, practical and biological aspects of the suggested method in normal subjects. Twenty healthy volunteers were provoked on the back with both a collimated beam (four fixed doses, in circular areas with a diameter of 1.5 cm) and the divergent beam (a continuous, radially attenuating dose spectrum covering an area with a diameter of 4.5 cm). Eleven of the subjects were subjected to double provocation with the divergent beam. Assessment was carried out at 6 and 24 h after exposure by measuring the diameter of the reactions both visually and by mapping the skin blood flow change with laser Doppler perfusion imaging (LDPI). Minimal erythemal dose (MED) was determined for both the collimated and the divergent provocation. The reaction diameters were used to decide MED by combination to a mm for mm mapped dose spectrum of the divergent beam profile. Dose-response curves were plotted using the quantitative response data of the LDPI-images against the corresponding dosimetry data. No systematic difference could be proven between LDPI and visual diameters and a 95% confidence interval for the mean difference was calculated to (-0.8, 2.0). Slightly greater diameters were found at the visual assessment performed at 6 h compared to 24 h (95% confidence interval (-0.1, 2.8)). Double provocation showed a good reproducibility both for the visual and the LDPI assessment (P<0.05). The divergent beam provocation allowed a more detailed discrimination of MED compared to the collimated beam provocation. The MED values determined with the divergent beam were, however, generally higher, especially in the lower range of MED values. Technical factors related to the beam divergence and the correct measurement of erythemal effective irradiance are believed to be the explanation for this phenomenon, which is thus correctable. In conclusion, the results from this study support our belief that the phototesting protocol based on a divergent beam constitutes a good opportunity for improved phototesting, since MED and dose-response characteristics may be extracted in more detail from a single UV exposure.

Inter‐observer variability in reading of phototest reactions with sharply or diffusely delineated borders

“…In the original publications on the divergent beam method, reproducibility in naked‐eye reading by a skilled observer was found to be good (11). The results of the present study demonstrate that inter‐observer variability in reading the diameter of the divergent beam reaction, for ‘unskilled’ (i.e.…”

“…LDPI enables a quantitative estimation of the tissue blood perfusion, by scanning a laser beam over the skin area of interest. The measurement results in a two‐dimensional data set representing the spatial variations of the skin blood perfusion (10–12, 16, 17). The instrumental settings used for this study were a scanner‐head to skin distance of 9 cm, 40 × 40 measurement sites, low resolution.…”

“…If performed in a standardised manner, however, there is a lower risk for observer subjectivity and inter‐observer variability. In the divergent beam method, LDPI has been used to demonstrate skin blood perfusion by the two‐dimensional mapping of the erythema, from which the MED can be calculated (11). Data can also be used to calculate a dose‐response curve (12).…”

Phototesting with a divergent UVB beam in the investigation of anti‐inflammatory effects of topically applied substances

“…Visual (naked eye) assessment of the MED contains an element of subjectivity. By introducing laser Doppler perfusion imaging (LDPI) (3), the threshold dose for hyperperfusion (which corresponds to the naked eye assessed MED (2)) as well as the individual dose–response data for doses above and below the MED can be obtained. This method has thus, compared with traditional phototesting, the potential to increase the amount of information generated during the characterisation of an individual's light sensitivity or the investigation of a patient with suspected light dermatosis (4–7).…”

“…The dose-response approach seems to be applicable even in diagnostic testing of an individual patient's response to UVB. I n recently published studies, a phototesting method using radially attenuating ultraviolet B (UVB) doses has been presented (1,2). The provocation produces a circular, radially fading, skin erythema, the diameter of which corresponds to the minimal erythema dose (MED).…”