First, the concept of photodynamic therapy Photodynamic therapy is a new technology that utilizes photodynamic response for disease diagnosis and treatment. In clinical practice, photodynamic therapy usually refers only to photodynamic therapy, while photodynamic diagnosis is called fluorescence diagnosis. The basic process of photodynamic response is that endogenous or exogenous photosensitive substances in biological tissues are irradiated by corresponding wavelengths (visible, near-infrared or ultraviolet), absorb photon energy, and turn into excited state from the ground state, and its physical de-excitation process can produce fluorescence, which can be used to diagnose diseases through fluorescence spectroscopy; its chemical de-excitation process can produce a large amount of reactive oxygen species, the most important of which is the single linear oxygen species, which can interact with many biological macromolecules. The reactive oxygen species can interact with a variety of biological macromolecules to produce cytotoxic effects, which can lead to cell damage or even death, thus producing therapeutic effects. Therefore, the three main elements of PDT are: photosensitizer, light source and oxygen in the tissue. Second, the photosensitizer used in photodynamic therapy In 1993, hematoporphyrin was firstly used as photosensitizer for photodynamic therapy of bladder cancer in Canada, and later used in the treatment of esophageal cancer, lung cancer, gastric cancer and cervical cancer. Because derivatives of hematoporphyrin as photosensitizers accumulate at the skin site, the drug is slow to clear, and patients need to be protected from light for 4-6 weeks after the drug is administered, this method is difficult for patients to accept. In the 1990s, 5-amino ketoglutaric acid was used as a photosensitizer in the field of photodynamic therapy of tumors, and achieved good results, and is still the most widely used photosensitizer.ALA is the precursor of heme synthesis in vivo, so that the body’s normal endogenous substances, which is synthesized by glycine, succinate, coenzyme A in the ALA synthetase, and then in a series of enzyme such as ALA dehydrogenase, ALA conversion of intracellular to produce a strong photosensitizer with a strong light, which can be used to treat tumors. It is synthesized by glycine, succinate and coenzyme A under the action of ALA synthase, after which ALA is converted into protoporphyrin with strong photosensitizing effect in the cell under a series of enzymatic actions such as ALA dehydratase, in which protoporphyrin IX is the main body of strong photosensitization. Under normal circumstances, ALA in living cells is trace, itself has no photosensitivity, when given a large number of exogenous ALA, the body of some fast proliferating tissues or some metabolically vigorous tumor cells selective absorption of ALA significantly higher, and converted to excess protoporphyrin IX and other porphyrins, and accumulated in the corresponding tissue cells, was 630nm laser irradiation photodynamic reaction. One of the characteristics of 5-ALA is that the accumulation of photoactive porphyrin in tumor damage is much higher than in most normal tissues. Svanberg et al. treated human skin basal cell carcinoma and Bowen’s disease with local topical 20% ALA cream and found that the content of protoporphyrin IX in tumor tissues was 15 times that of the surrounding normal skin, which led to the photodestructive effect mainly in the tumor cells; the other characteristic is that the photodynamic reaction of 5-ALA formed during photodynamic therapy is not as strong as that of 5-ALA. Another feature is the short half-life of the photosensitizer formed in ALA-photodynamic therapy. Third, the characteristics of ALA-photodynamic therapy 1, a high degree of tissue selectivity: ALA-photodynamic therapy is the most important feature of selective killing of tumor cells and over-proliferation of cells, while the normal cells have no significant impact. This feature is obviously better than previous means such as surgery, laser, chemotherapy and radiotherapy, and it is much safer for normal cells. 2, minimal side effects: it has been confirmed that ALA is absorbed by the body 1 hour after the concentration of protoporphyrin IX reached its peak, 6 hours after the beginning of the decline, 24 hours after the undetectable, so patients receiving ALA-photochemotherapy do not need to avoid sunlight. Patients had no significant side effects during or after ALA-photodynamic therapy, and blood, urine, and liver functions were in the normal range. Only a few patients in the laser irradiation process have a tingling sensation, which is due to the 5-ALA is absorbed by the peripheral nerve endings, the general degree of light, without treatment, so the patient to accept the treatment of good tolerance and compliance. 3. Cosmetic effect: ALA-photodynamic therapy is very suitable for facial tumors, such as tumors around the eyes, eye canthus, eyelids, ears, nose, lips and other parts of the face, which can get good cosmetic effect. In addition, it is also a good choice for patients who are old and weak and not suitable for surgery and other treatments. 4. It is especially suitable for tumors in the early stage of skin, especially those with diameter less than 1cm. For example, superficial or solid basal cell carcinoma can get 100% cure rate after 1~4 treatments; squamous cell carcinoma with a diameter of 1~1.5cm can be cured after 3~6 treatments; Bowen’s disease can be cured after 1~4 treatments. The application progress of ALA-photodynamic therapy in dermatology The application of ALA-photodynamic therapy in dermatology began at the beginning of the 20th century, and the earliest one was only used for the treatment of tumor. Nowadays, the spectrum of diseases treated by photodynamic methods has changed greatly, from inflammatory diseases, viral infectious diseases, skin beauty, to skin tumors, including acne, condyloma acuminatum, skin photoaging, nevus, limited sclerosis, annular granuloma, psoriasis, sebaceous gland hyperplasia, actinic keratosis, Bowen’s disease, in situ, squamous carcinoma, basal cell carcinoma, and so on. Acne: Photodynamic therapy for acne is based on the theory that endogenous porphyrins, especially fecal porphyrins, can be produced in Propionibacterium acnes. The porphyrin is absorbed by the epithelial cells and sebaceous glands, and then metabolized by the heme synthesis pathway to produce the photosensitive substance protoporphyrin IX, which accumulates in the sebaceous glands and epithelial cells, and reacts with oxygen to produce singlet oxygen after receiving external light irradiation, resulting in the destruction of the cell membranes and the death of the organisms. Zhang Linglin et al. randomly divided 70 patients with moderate and severe acne into two groups. Thirty-five cases in the treatment group were given ALA-PDT treatment once every 2 weeks for a total of 1 to 3 times; 35 cases in the control group were treated with oral isotretinoin capsules for 6 weeks. The efficacy of the two groups was judged and compared in the 2nd, 4th and 6th weeks of treatment. Results 35 patients treated with ALA-PDT after 1-3 times of treatment, the total effective rate reached 97.1%; the control group at 6 weeks the total effective rate was 80.0%, the efficacy of the treatment group was significantly better than the control group. In addition, the degree of recurrence in the AIJA-PDT group was significantly lighter than that in the control group, and the time of disease control was significantly prolonged.There were some patients in the ALA-PDT group who had temporary localized hyperpigmentation, but no scarring occurred. Wang Xiuli et al. further concluded through experimental research that ALA-PDT is suitable for the treatment of common acne with inflammatory papules, pustules and cysts as the main manifestations, and it is appropriate to use 3% ALA concentration and 3h dressing time. Lin Mengying et al. randomized and single-blind divided 30 cases of facial acne into two groups: one group applied 5% 5-aminolevulinic acid (ALA) externally on the right side of the facial lesions, and a placebo externally on the left side of the face; the other group applied 5% ALA externally on the left side of the facial lesions, and a placebo externally on the right side of the face. All of them used red light irradiation on the whole face once a week for 4 times. At the end of the second week of treatment, 25.9% of the patients on the test side showed an improvement in lesions of more than 60%, whereas the improvement in lesions on the control side was less than 60%, and the efficacy of the test side was better than that of the control side. PDT was more effective than red light alone, and the adverse effects were less severe. Condyloma acuminatum: ChenMK et al. applied photodynamic therapy to treat 48 patients with cervical condyloma acuminatum. After 1 treatment, 62.5% of patients’ warts were cleared, and 95.8% of patients’ warts were cleared after three treatments. The recurrence rate was 4.4% at 12-month follow-up. The total effective rate was 85.7%, and the total recurrence rate was 11.4%. The total effective rate was 85.7%, and the total recurrence rate was 11.4%. The clinical efficacy of the treatment of urethral warts is significantly better than that of other parts. Xu Peihong et al. used 5-aminolevulinic acid (ALA) photodynamic therapy (PDT) on 30 patients with condyloma acuminatum after carbon dioxide (CO2) laser treatment 7d photodynamic therapy, to the same period of CO2 laser treatment of 116 cases of condyloma acuminatum patients as a control. Results: 26 cases were cured after one photodynamic therapy, and the recurrence rate was 13.3% (4/30), while the recurrence rate of the control group was 54.3%, which confirmed that ALA photodynamic therapy could reduce the recurrence rate of condyloma acuminatum. et al. applied photodynamic therapy to treat 14 miles of patients with intra-anal condyloma acuminatum, resulting in 13 cases cured, and concluded that the application of 16-20% ALA concentration, light energy density of 100-150J/cm2 is the best effect. Chen Wei et al. 213 cases of urethral condyloma acuminatum patients with photodynamic therapy, 1 time per week, up to 3 times of treatment, warts subside and then consolidate the treatment for 1 time. The result was that 208 patients were cured after 3 treatments, with a cure rate of 97.7%. After 6 months of follow-up, there were 17 cases of recurrence, with a recurrence rate of 8.2%. Adverse reactions were slight. Skin precancerous lesions and skin cancer: For skin precancerous lesions and a considerable portion of skin cancer, photodynamic therapy has become one of the preferred methods, which has been confirmed in many large-scale observations.Tschen et al. treated 110 patients with actinic keratoses with photodynamic therapy for 12 months, and the complete clearance rate reached 76% and 72% in the first and second months, respectively, and the clearance rate was 86% in the fourth month. The clearance rate at month 1 and month 2 was 76% and 72%, respectively, and at month 4 was 86%. Cai Mei et al. observed the efficacy of photodynamic therapy in the treatment of some skin cancers and pre-cancerous lesions. 50 patients with pre-cancerous skin lesions and skin cancers were treated with photodynamic therapy for the first time, including 25 cases of solar keratosis, 16 cases of Bowen’s papulosis, 4 cases of Bowen’s disease, 4 cases of basal cell carcinoma, and 1 case of squamous cell carcinoma of the oral cavity, and the results showed that 20 cases of solar keratosis were cured after 2-6 photodynamic treatments, and 8 cases of Bowen’s papulosis. After 2-6 times of photodynamic therapy, 20 cases of solar keratosis were cured, 8 cases of Bowen’s papulosis were cured, 3 out of 4 cases of Bowen’s disease were cured, 3 cases of BCC were cured, and the area of lesions of patients with oral squamous cell carcinoma was reduced by 70%. Yi Yunlian et al. treated 6 cases of basal cell carcinoma, 7 cases of Paget’s disease, 2 cases of squamous cell carcinoma, 3 cases of Bowen’s disease and 1 case of solar keratosis with photodynamic therapy for 2-5 times. Results: 6 cases of basal cell carcinoma were cured in 4 cases and improved in 2 cases; 7 cases of paget’s disease were cured in 4 cases, improved in 3 cases and recurred in 1 case; 2 cases of squamous cell carcinoma were improved; 3 cases of Bowen’s papulosis were cured; and 1 case of actinic keratosis was cured. Wang Hongwei et al. carried out photodynamic therapy on the skin lesions of 8 patients with Bowen’s disease, resulting in partial remission in 1 case after 4 treatments, and complete remission in another 7 cases after 4-8 treatments, and only 1 case relapsed at 4 months after 8-32 months of follow-up, and complete remission was achieved after re-treatment. Tang Zengqi et al. observed the efficacy of photodynamic therapy in the treatment of sclerosing atrophic moss of the vulva. The treatment was carried out once every 2 weeks for a total of 3 times, and was followed up for 6 months after the end of the treatment to observe the changes in itching and skin lesions. Results after treatment, local itching disappeared in all 3 patients; skin sclerosis improved to different degrees in 3 patients, and skin elasticity recovered more obviously in 1 case; the extent of white spots was reduced in 1 patient. Others. In addition to the above diseases are the main indications for photodynamic therapy, photodynamic therapy can also be used for the treatment of skin photodamage, multiple sebaceous nevi, senile sebaceous hyperplasia, follicular keratosis, psoriasis, limited scleroderma, chronic radiodermatitis, and other diseases reports have emerged, have obtained better results. In conclusion, ALA-photodynamic therapy is more and more widely used in the treatment of dermatologic diseases, and its advantages of effectiveness, safety and convenience are becoming more and more obvious. With the further improvement of photosensitizer and light source as well as the deepening and expanding of clinical application, ALA-photodynamic therapy will have a broader application prospect.