Firstly, vivid red nevus, also known as wine-like nevus, is often seen after birth or shortly after birth. The damage is a light red or purplish-red patch, not protruding from the skin, with smooth surface and fading when pressed, and irregular shape, commonly found on the face, neck and scalp, but also on other parts of the body such as upper and lower limbs, front chest or palms and back of hands. With age, the color of erythema will deepen and the area will increase, but the ratio with the face often does not change, and there may be nodules or warty growths on the surface. Erythema nodosum may also be accompanied by thickening of the lips or damage to the lateral side of the lesion that is significantly higher than the contralateral side or to certain other organs, such as Sturge-Weber syndrome with glaucoma and epilepsy and Klipple-Trenaunay syndrome with thick and long limbs on the affected side. The main histopathological findings are abnormal dilation of capillaries in the superficial dermis with weak walls, but no proliferation of endothelial cells. In the past, various treatment methods such as surgical implant, X-ray, isotope, freezing, electrocautery, various topical drugs, magnetic therapy and many kinds of laser treatments (CO2, YAG, argon ion, copper vapor, KTP) were used, but all of them were difficult to be accepted by patients because they could damage the skin and leave scars or could not eliminate the lesions. Until the advent of adjustable pulsed dye laser in recent years, on the one hand, the output wavelength of the laser can be absorbed by hemoglobin, on the other hand, because of selective photothermolysis, avoiding the concentration of heat, thus reducing the damage to the skin, and can achieve a more desirable effect of fading without scars. However, this equipment is expensive and needs to be imported, and the clinical practice of some domestic medical institutions in recent years has found that adjustable pulsed dye laser is only effective for some types of nevus, but not for more serious lesions, and scarring can still be left behind if the operation is not careful. Therefore, it is not yet ideal. Photodynamic therapy has been used to treat malignant tumors for more than 20 years. In the study of the mechanism of PDT for cancer treatment, it was found that in addition to the destruction of tumor cells, the blockage of blood vessels supplying tumor tissues can also cause photosensitive damage to tumor microvessels. Clinical classification of erythema nodosum According to the lesions of erythema nodosum and the need of PDT treatment, it can be divided into three types and six grades. (1) Pink erythema is pink in color, which is mostly seen in children, and the degree of lesion belongs to grade 1. If it is dark red, it is grade 2, and if it is dark red, it is grade 3; (2) Violet erythema is purple in color, which is mostly seen in adults, and it can be divided into light purple and dark purple according to the color shades, and the degree of lesion belongs to grade 4 and 5; (3) Thickened erythema is mostly dark purple, and the surface is thickened, which can slightly protrude from the skin, and in serious cases, there can be warty nodules growing on the surface. It is easy to bleed after touching, and often feels itchy. It is usually seen in older people, and the lesions are graded as grade 6. The correct lesion typing and grading is an important basis for determining the dose of irradiation and the amount of photosensitizer to be administered during treatment.