Irradiance-dependent photobleaching and pain in delta-aminolevulinic acid-photodynamic therapy of superficial basal cell carcinomas

Abstract

Purpose: In superficial basal cell carcinomas treated with photodynamic therapy with topical delta-aminolevulinic acid, we examined effects of light irradiance on photodynamic efficiency and pain. The rate of singlet-oxygen production depends on the product of irradiance and photosensitizer and oxygen concentrations. High irradiance and/or photosensitizer levels cause inefficient treatment from oxygen depletion in preclinical models.

Experimental design: Self-sensitized photobleaching of protoporphyrin IX (PpIX) fluorescence was used as a surrogate metric for photodynamic dose. We developed instrumentation measuring fluorescence and reflectance from lesions and margins during treatment at 633 nm with various irradiances. When PpIX was 90% bleached, irradiance was increased to 150 mW/cm(2) until 200 J/cm(2) were delivered. Pain was monitored.

Results: In 33 superficial basal cell carcinomas in 26 patients, photobleaching efficiency decreased with increasing irradiance above 20 mW/cm(2), consistent with oxygen depletion. Fluences bleaching PpIX fluorescence 80% (D80) were 5.7 +/- 1.6, 4.5 +/- 0.3, 7.5 +/- 0.8, 7.4 +/- 0.3, 12.4 +/- 0.3, and 28.7 +/- 7.1 J/cm(2), respectively, at 10, 20, 40, 50, 60 and 150 mW/cm(2). At 20-150 mW/cm(2), D80 doses required 2.5-3.5 min; times for the total 200 J/cm(2) were 22.2-25.3 min. No significant pain occurred up to 50 mW/cm(2); pain was not significant when irradiance then increased. Clinical responses were comparable to continuous 150 mW/cm(2) treatment.

Conclusions: Photodynamic therapy with topical delta-aminolevulinic acid using approximately 40 mW/cm(2) at 633 nm is photodynamically efficient with minimum pain. Once PpIX is largely photobleached, higher irradiances allow efficient, rapid delivery of additional light. Optimal fluence at a single low irradiance is yet to be determined.

Fig 1

Treatment and measurement cycles during system operation include (A) PDT delivery at 632.8 nm with fluorescence monitoring in the lesion, (B) PDT delivery with fluorescence monitoring in the perilesion margin, (C) broadband interrogation with reflectance monitoring in the lesion, and (D) broadband interrogation with reflectance monitoring in the margin. (E) Normalized fluorescence emission curves for PpIX and photoproduct I illustrating the relative location of the 632.8 nm treatment beam to PpIX and photoproduct I fluorescence and the detection window in which fluorescence was monitored with this instrument.

Fig 2

(A) Representative SVD-fit corrected fluorescence spectrum taken from sBCC lesion being treated with ALA-PDT at 50 mW cm⁻² with 632.8 nm light. The linear decomposition shows the fluorescence contribution from PpIX, autofluorescence, photoproduct I, and Fourier terms to the corrected fluorescence signal. Several data points have been removed for clarity. (B) PpIX and photoproduct I contributions to the corrected fluorescence from the lesion measurement field throughout ALA-PDT in the same lesion normalized to the initial PpIX contribution. For this sBCC lesion, initial rapid PpIX bleaching was accompanied by a corresponding increase in photoproduct I fluorescence, which turns over and was also bleached during the course of therapy. Error bars are smaller than the plot symbols. (C) The PpIX and photoproduct contributions to fluorescence from a lesion treated at 150 mW cm⁻² for 200 J cm⁻².

Fig 3

(A) Comparison of the averaged ± SEM, normalized PpIX contribution to the corrected fluorescence as a function of total fluence for lesion and margin fields treated with 10, 20, 40, 50, 60, and 150 mW cm⁻² irradiances. PpIX contribution shows fluorescence photobleaching of PpIX during irradiation and suggests comparable bleaching trends between margin and lesion in the lower irradiance groups as well as a clear separation between average bleaching curves at 150 mW cm⁻² .(B) Averaged ± SEM, normalized PpIX contribution to the corrected fluorescence as a function of delivered fluence from the lesion and margin fields during ALA-PDT of 33 sBCC lesions at 10, 20, 40, 50, 60, and 150 mW cm⁻². (C) D80 values for individual sBCC (△), and their normal skin margins (○) at each irradiance. The horizontal lines are the means. †At 150 mW cm⁻², three margins had not reached 80% photobleaching at the maximum light dose of 200 J cm⁻² and are not included in the mean.

Fig 4

Representative reflectance spectra taken from (A) a sBCC lesion and (B) adjacent margin taken prior to and after 1.0 J cm⁻² PDT delivered at 50 mW cm⁻². Data between 625 and 640 nm is excluded due to bleed-through of the treatment laser into the reflectance channel.

Fig 5

(A) VAS pain scores ± SEM experienced with ALA-PDT of sBCC measured in this study along with results from Wang et al.², Algermissen et al.¹¹, and Holmes et al.⁵, measured at higher irradiances. (B) VAS pain scores ± SEM during low-irradiance delivery and following changeover to 150 mW cm⁻² during ALA-PDT of sBCC in our study, showing only moderate increases. No pain intervention was required for patients treated at 10–50 mW cm⁻² before or after changeover. †Two individuals in group received 1% xylocaine injections after reporting a VAS > 4.