Minimal erythema dose (Med) to narrow band ultraviolet - B (NB-UVB) broad band ultraviolet-B (BB-UVB) - A pilot study

Introduction

Ultraviolet B (UVB) is effective in the treatment of psoriasis. Photochemotherapy (PUVA) is effective in the treatment of psoriasis, vitiligo and atopic dermatitis. Narrow Band UVB (NB - UVB) 311 nm is reported to be effective in the treatment of psoriasis, vitiligo and atopic dermatitis. Although information regarding Minimum Erythema Dose (MED) and dosage schedule for broad band (BB) is available for Indian skin,[1] there are no Indian studies on MED to NB - UVB. We undertook this study to determine MED to NB- UVB and BB-UVB.

Materials and Methods

MED was determined by standard method.[1] A template with 20 apertures (10 on each side) of 1½ x 1½ cm2 was made over the back of a cotton suit used by operation theatre staff. Cotton flaps were made over the apertures enabling us to either shut or keep the apertures open by using Velcro [Figure - 1].

The source of NB - UVB was the whole body phototherapy unit with 24 Philips TL -01 bullbs. To determine MED a single panel in the whole body unit with 6 bulbs was used. BB- UVB phototherapy panel with 8 bulbs was used to determine MED to BB. The irradiance from the source was determined using photometer for BB. The same photometer was used for NB but the irradiance was calculated by using a correction factor of 0.74.[2]

MED was determined first for BB followed by NB by standard tecqnique. All the apertures were kept open and back was irradiated with 5mj of BB -UVB. One aperture was closed and remaining apertures were closed one after the other after delivering 5mj more than the previous aperture. Same procedure was repeated on the other half of the back starting with 50mj of NB - UVB and increasing the dose by 50m1 for each aperture. The dosage schedule for BB - UVB (in mj) was 5, 10, 15,20, 225, 30, 35, 40, 45, 50 and for NB - UVB (in mj) was 50, 100, 150,200,250, 300, 350, 400,450, 500. The readings were taken 24hrs after exposure.

Results

On the six volunteers tested the MED for NB - UVB ranged between 150 mj and 400 mj and for BB - UVB between 10 mj and 35 mi. The mean MED for NB was 300 mj and BB was 21 mi, the mean ratio was 1:15.

The MED to NB- UVB and the ratio is shown in [Table - 1].

Discussion

Determination of MED is essential for rational treatment with UV light. MED value is helpful for subsequent exposures and increments. The MED for BB - UVB was much lower compared to previous study.[1] This could be because of 1) higher irradiance 2) difference in recording of irradiance by photometer. We thus conclude that on Indian skin MED to NB and BB can be determined and rational treatment can be undertaken based on MED.