1.

Introduction

Port wine stain (PWS) is a congenital vascular malformation histologically characterized by ectatic capillaries in papillary layer of the dermis. It usually appears at birth and scarcely disappears spontaneously. PWS is usually classified as pink, red, or purple according to the color, as the classification of PWS has still not been unified. In most cases, PWS is pink or red at birth. As the age increases, the PWS color tends to become darker, and the involved skin may become thicker, even nodular changes. Histologically, the mean vessel area (dermal area composed of vessels) increased with age, as well as vessel number, mean wall thickness, and mean vessel depth also minimally increased.¹

Photodynamic therapy (PDT) with the advantage of dual selectivity has been applied in the treatment of PWS for 20 years.² Our clinical experiences showed that it is effective for all types of PWS. Photocarcinorin (PSD-007) combining 532 nm wavelength laser is the most commonly used strategy in China.^2–4 With the deep insight of PDT dose-response relationship, people recognized that it is very much worth making individual PDT scheme to meet needs of diversity of PWS histological structure. For example, laser wavelength 510 nm which has a lower absorption for hemoglobin and higher molar extinction coefficient for PSD-007 than 532 nm can be applied to the treatment of PWS with deeply located ecstatic vessels.⁵

As is known, PSD-007 has a higher molar extinction coefficient around 400 nm than at 532 nm. However, melanin and hemoglobin also has higher absorption around these wavelengths, and scatter coefficient around these wavelengths is higher than that at 532 nm. It suggested that these wavelengths may penetrate more superficially in skin than 532 nm, and the number of photon deposited in target vessels may be lower. Whether these wavelengths are suitable for PWS PDT or only suitable for certain types of PWS attracts wide notice, and needs further research.

Previous research has indicated that mathematical simulation can facilitate the comprehension of complicated interrelationship between laser, photosensitizer, and oxygen in PDT reaction.^6–9 In the present study, we choose 457 nm at which the excited efficiency of photosensitizer (PSD-007) is similar to 532 nm (Fig. 1). Firstly, the light distribution of 457 and 532 nm in infantile or adult PWS skin model was simulated, then singlet oxygen yield of 457 or 532 nm mediated PDT at the same photosensitizer dosage was compared. Finally, clinical experiment was done to compare the efficiency of 457 and 532 nm PDT.

Fig. 1

Schematic diagram of the absorption spectra of PSD-007, hemoglobin, and melanin in water. The molar extinction coefficient of PSD-007 at 457 nm is similar to that at 532 nm. The molar extinction coefficient of hemoglobin at 457 nm is slightly higher than that at 532 nm. The molar extinction coefficient of melanin at 457 nm is higher than that at 532 nm.

2.

Materials and Methods

2.1.

Mathematical Simulation

2.1.1.

Port wine stains skin geometry

The PWS skin geometry used in the present modeling was previously described.⁵ It consists of stratum corneum, epidermis above the basal melanin layer, basal melanin layer, and dermis embedded with ectatic vessels (Fig. 2). The total thickness of PWS skin model is 1300 μm, not including subcutaneous tissue below dermis. Vessels are parallel to $Y$ axis. Laser beam diameter is 1 mm at $x$, $y=0\mathrm{\mu m}$.

Fig. 2

Geometry model of PWS skin. It consisted of stratum corneum, epidermis above the basal melanin layer (with less melanin, $m\%=0.1\%$), basal melanin layer (containing lots of melanin) and dermis embedded with ectatic vessels. Infantile PWS model contains one layer vessels, and adult PWS contains three layer vessels. The separation between epidermal-dermal junction and the up wall of the first layer capillary was set at 100 μm. The vertical separation between two vessel layers is 100 μm, and the lateral separation between two capillaries were 60 μm. The total thickness of skin is 13000 μm, not including the subcutaneous tissue below the dermis.

As Ref. 10 reported, the thickness of stratum corneum and epiderm in infantile skin is similar to adult skin. The main difference is that the size and volume of stratum corneum cells in infantile skin is more uniform, and the number of melanosome in basal layer of epidermis is less. It can be supposed that the scatter of stratum corneum and the melanin content in epidermis is lower in infantile skin. The total epidermis melanin content ($m\%$ was set at 2% for adult PWS and 0.5% for infantile PWS.

2.1.2.

Skin optical properties

Skin optical properties of PWS skin were calculated according to Refs. 1112.–13 and were listed in Table 1.

Table 1

PWS skin optical properties used in the present study.

Wavelength (nm)	Optical properties	Sc	Epidermis	Basal layer	Dermis	Blood
457	${\mu }_a({\mathrm{cm}}^{-1})$	202	1.98	50 ($m\%=2\%$)	3.1	200
				△ 10 ($m\%=0.5\%$)
	${\mu }_s({\mathrm{cm}}^{-1})$	2200	263	263	263	500
		△ 1000
	$g$	0.95	0.75	0.75	0.75	0.98
	$n$	1.45	1.4	1.4	1.4	1.33
532	${\mu }_a({\mathrm{cm}}^{-1})$	181	1.1	34 ($m\%=2\%$)	2.6	206
				△ 8 ($m\%=0.5\%$)
	${\mu }_s({\mathrm{cm}}^{-1})$	2200	184.4	184.4	184.4	500
		△ 1000
	$g$	0.93	0.774	0.774	0.774	0.98
	$n$	1.45	1.4	1.4	1.4	1.33

stratum corneum, the unit of μa and μs is cm−1, m%: percentage content of melanin in the whole epidermis. △ is the optical properties of infant which is different from adult, the other is similar to adult.

2.1.3.

Simulation of PDT singlet oxygen yield

The mechanism of PDT mediated vascular effect is based on the dual selectivity distribution of photosensitizer and laser in target vessels. At a short drug-light interval, photosensitizer accumulated predominantly in vascular. Highly reactive oxygen species (such as singlet oxygen) generated through the reaction between light and photosensitizer oxidize the membrane of vascular endothelial cell, leading to vascular stasis, thrombus formation or permanent vessel occlusion. The simulation model of singlet oxygen yield in PWS tissue and simulation method based on photochemical reaction of PDT was previously published.⁵ In the present study, the singlet oxygen yield in infantile PWS skin and adult PWS skin was simulated respectively.

3.

Results

3.1.

Light Distribution and Singlet Oxygen Yield of 457 nm PDT and 532 nm PDT in Infantile PWS Tissue

Light fluence rate of 457 nm laser at epidermis of infantile PWS was higher than that of 532 nm laser. As the 457 nm laser has larger melanin absorption and back scatter, more 457 nm light was back scattered to the superficial layer of skin. Light fluence rate of 457 nm laser at the top side of the blood vessels close to skin surface was slightly higher than 532 nm, and slightly lower than 532 nm at the inferior wall of vessels [Fig. 3(a) and 3(b)]. Light fluence rate of two wavelength laser was similar in the center part of vessels. In deep dermis, light fluence rate of 457 nm laser is lower than 532 nm. This result showed the penetration depth of 457 nm laser in infantile PWS is slightly shallower than 532 nm laser, more 457 nm laser was deposited in superficial layer of PWS skin.

Fig. 3

Spatial profiles of light distribution in PWS skin model. (a) 532 nm PDT for infantile PWS. (b) 457 nm PDT for infantile PWS. Light fluence rate of 457 nm laser at epidermis is higher than 532 nm. Light fluence rate of 457 nm laser at vessels is slightly lower than 532 nm, especially at the inferior wall of vessels. (c) 532 nm PDT for adult PWS. (d) 457 nm PDT for adult PWS. Light fluence rate of 457 nm laser at epidermis is similar to 532 nm. Light fluence rate of 457 nm laser at vessels is lower than 532 nm. Light fluence rate of 457 nm laser at vessels is lower than 532 nm, especially at the second and third layer vessels.

The feature of singlet oxygen yield in PWS vessels under 457 nm PDT and 532 nm PDT is similar. Singlet oxygen is mainly generated in PWS vessels. With the same light dose ($100\mathrm{mW}/{\mathrm{cm}}^2$, 40 min), 457 nm PDT generate a slightly lower singlet oxygen yield in PWS vessels than 532 nm PDT [Fig. 4(a) and 4(b)].

Fig. 4

Spatial profiles of singlet oxygen yield in PWS skin model. (a) 532 nm PDT for infantile PWS. The singlet oxygen yield in the superior wall and the inferior wall of PWS vessels were $5\times {10}^4\mathrm{\mu mol}/\mathrm{L}$ and $2.6\times {10}^4\mathrm{\mu mol}/\mathrm{L}$, respectively. (b) 457 nm PDT for infantile PWS. The singlet oxygen yield in the superior wall and the inferior wall of PWS vessels were $4.2\times {10}^4\mathrm{\mu mol}/\mathrm{L}$ and $2\times {10}^4\mathrm{\mu mol}/\mathrm{L}$, respectively. (c) 532 nm PDT for adult PWS. The singlet oxygen yield in the superior wall and the inferior wall of the first layer PWS vessels were $5\times {10}^4\mathrm{\mu mol}/\mathrm{L}$ and $2\times {10}^4\mathrm{\mu mol}/\mathrm{L}$, respectively. (d) 457 nm PDT for adult PWS. The singlet oxygen yield in the superior wall and the inferior wall of the first layer PWS vessels were $3.8\times {10}^4\mathrm{\mu mol}/\mathrm{L}$ and $1.8\times {10}^4\mathrm{\mu mol}/\mathrm{L}$, respectively.

In addition, the singlet oxygen yield of 457 nm PDT in PWS with deeply located vessels (500 μm below the epidermal-dermal junction) was simulated. The result showed that singlet oxygen yield of 457 nm PDT in PWS vessels is much lower than 532 nm PDT. This suggested that 457 nm PDT is not suitable for PWS with deeply located vessels (Fig. 5).

Fig. 5

Spatial profiles of singlet oxygen yield in infantile PWS skin model with deeply located vessels. (A) 532 nm PDT. (B) 457 nm PDT. Vessels were located in 500 μm below the epidermal-dermal junction. Singlet oxygen yield of 457 nm PDT in vessels is significantly lower than 532 nm PDT.

4.

Discussion

Individuation of PDT dosimetry is a developing trend for PDT clinical application.¹⁷ According to the diversity of PWS histological structure (with different vascular depth or vascular area), more alternative laser wavelengths should be available. Infantile skin had less melanin and better translucency, lower light dose is needed, as there are only 532, 578, and 510 nm laser wavelengths available in clinical for PDT of PWS. Light dose for infantile PWS was adjusted by changing laser power density from the adult used $100\mathrm{mW}/{\mathrm{cm}}^2$ to $80\mathrm{mW}/{\mathrm{cm}}^2$ (Ref. 4) or shortening irradiation time. Moreover, infantile PWS has shallower located vessels and less vascular area. Shorter laser wavelength may have the potential to be used in infantile PWS.

In this study, the main difference between infantile PWS and adult PWS was taken into consideration, such as melanin content in epidermis, vascular area, and vascular depth in dermis. The simulation results and clinical outcome of this study demonstrated the feasibility of applying 457 nm to the PDT of infantile PWS. Theoretically, melanin absorption at 457 nm is higher than at 532 nm, nevertheless, there is less melanin in infantile epidermis, the effect of melanin absorption on the difference of 457 nm and 532 nm laser distribution in infantile epidermis might be ignored. The treatment outcome of 39 spots in infantile PWS patients indicated that there is no significant difference between 457 nm PDT and 532 nm PDT. 457 nm PDT with a lower occurrence of scab may reduce the risk of tiny scar left by careless tearing of the scab, and simplify the postoperative care.

As adult skin has a more rough stratum corneum and more melanin in epidermis, the transparency of adult skin is not as good as infant’s. Notwithstanding, the photosensitizer excited efficiency at 457 nm is similar to 532 nm, less light is deposited in PWS vessels at 457 nm. Singlet oxygen yield of 457 nm PDT in adult PWS is lower than 532 nm PDT. In the initial stage of this study, four adult PWS patients received 457 nm PDT (data was not displayed here). For three adult PWS patients, no significant color blanching was obtained. Nevertheless, a $50\%\sim 75\%$ clear of PWS color was obtained in one fair-skined adult female patient (18-years-old) with PWS pink color, when she was treated for the first time. In the following treatment, 532 nm PDT was used, and good treatment outcome was also obtained. It suggested that 457 nm PDT might also be effective for mild type PWS in fair-skinned adult in the initial PDT courses. There is also severe type PWS in infancy (extensive macula in dark purple color or macula with increased thickness). For such patients, neither PDT nor pulsed laser treatment can gain complete response. Such PWS was not taken into consideration in the present study. As only a few adult patients were recruited in this study, a bigger sample size of adult PWS may prove the above hypothesis. In view of the poor therapeutic outcome predicted for adult, infantile patients were mainly recruited in the present study.

Previous animal experiment and clinical experience have shown that laser wavelength at green and yellow bands are suitable for porphyrin-based photosensitizer mediated PDT of PWS. However, laser wavelength at red band is not suitable.^7,18 Currently, laser wavelengths 532, 510, and 578 nm are usually used as light source for PDT of PWS. Blue light is seldom used. This study give us a hint that red-shifting of the max absorption peak from 400 nm to 500 nm or enhancing the absorption around 500 nm by modifying the chemical structure of porphyrin-based photosensitizer has the possibility of increasing PDT efficiency for PWS, especially for PWS with superficially located vessels.

5.

Conclusion

457 nm PDT is as effective as 532 nm PDT for pink or red flat PWS in infant. 457 nm wavelength laser has the potential to be applied in PDT for infantile PWS and pink PWS in fair-skinned adult. Since the small sample size in the present study, a further large-scale study is required to confirm the efficiency of 457 nm PDT for PWS and its indications.