What is photodynamic therapy? This concept seems difficult to understand for the general public, but in fact Photodynamic Therapy (PDT) is a new method of treating tumors using photosensitizing drugs (referred to as photosensitizers) and corresponding wavelengths of laser light. Currently, it is mainly used for the radical treatment of precancerous lesions and early stage cancer and the palliative treatment of advanced solid malignant tumors. How does photodynamic therapy treat tumors? The photosensitizer itself does not have tumor-killing effect, but only with the participation of oxygen and after laser excitation will it trigger cytotoxicity to kill tumor. The specific process is as follows: the photosensitizer is injected into human body intravenously first, due to the high absorption and low metabolism of tumor tissues, the photosensitizer is specifically retained in tumor tissues, and after 48 hours, the high concentration of photosensitizer in tumor is formed, while the content in normal tissues is minimal, thus forming the selective retention of photosensitizer in tumor, at this time, the laser irradiation with specific wavelength makes the photosensitizer in tumor tissues excited, and the photosensitizer in the excited state is then stimulated. The excited photosensitizer transfers energy to the surrounding oxygen molecules and generates highly reactive singlet oxygen. The monomorphic oxygen reacts with adjacent biological macromolecules to cause irreversible damage to cell membranes, mitochondria, lysosomes and other subcellular structures, which eventually leads to cell death. At the same time, the concentration of photosensitizers in the endothelium of neovascularized tumor tissues is also very high. The vascular damage caused by photodynamic action and the resulting local ischemia and hypoxia of the lesioned tissue play a key role in the clinical treatment mechanism of PDT. In addition, the acute inflammation and antigen presentation after photodynamic therapy can prompt the immune cells to treat distant tumor lesions through immunotherapy. What is the process of photodynamic therapy? The top row of images is a gastroscopic picture, which shows the process of tumor necrosis before, during and after the treatment; the middle row of images shows the process of selective retention of photosensitizer in human body, laser irradiation and tumor necrosis animation simulation; the bottom row of images shows the process from injection of photosensitizer to laser irradiation after 48 hours and tumor necrosis after the treatment. The bottom row of images shows the process of laser irradiation and tumor necrosis after treatment. What are the advantages of photodynamic therapy? What are the indications? Photodynamic therapy is highly selective, does not damage normal tissues, has a wide anti-tumor spectrum, is non-resistant, can be repeated, and is minimally invasive, simple to operate, and has the effect of preserving organ function and cosmetic appearance, so it has good practicality. It is mostly used for the radical treatment of precancerous lesions and early stage tumors, or the palliative treatment of advanced tumors. Precancerous lesions include mucosal leukoplakia (leukoplakia of the vocal cords and leukoplakia of the vulva), cervical erosion, gastric ulcer, Barret’s esophagus, etc. Early tumors include esophageal cancer, gastric cancer, bronchial cancer, laryngeal cancer, rectal cancer, bladder cancer, skin cancer (PAGET’S disease, basal cell carcinoma, squamous cell carcinoma), etc. It is particularly effective for recurrence of limited-stage recalcitrant tumors such as nasopharyngeal, tongue, and oropharyngeal cancers after radiotherapy. Currently, the commonly used photosensitizer is hematoporphyrin derivative, which is injected intravenously with 2mg/kg and irradiated with 630nm laser 48 hours later. The imported PHOTOFRIN is produced in Canada, and the domestic one is Chongqing Huading’s Xiboxian, both of them are hematoporphyrin derivatives, and their effective killing depth is 5-7mm. The laser wavelength of European new generation photosensitizer FOSCAN is 652nm, and its effective killing depth is about 10mm, which is mainly used for recalcitrant head and neck tumors. The biggest side effect of photodynamic therapy is photoallergy, because the photosensitizer stays in the skin stratum corneum for a long time, so patients need to avoid light for a long time after photodynamic therapy, and once photoallergy occurs, it can be treated according to conventional allergy treatment. The figure below is a typical clinical example of photodynamic therapy for laryngeal cancer. Before treatment, the patient’s voice was hoarse and could not be preserved if surgically removed, but after photodynamic therapy, the vocal cords were restored to normal and the patient retained normal speech function. Photodynamic therapy is a new technology of minimally invasive tumor treatment. Unlike other tumor treatments, it is a selective tumor treatment with the effect of preserving organ functions (e.g. tongue cancer laryngeal cancer can preserve voice function, cervical cancer can preserve fertility function) and cosmetic effects (skin cancer does not need skin implants). Early stage tumors can be treated with photodynamic therapy to achieve radical effect. For elderly tumor patients who are not suitable for surgery or radiotherapy or recurrent tumor patients after radiotherapy, photodynamic therapy is more suitable.