Hematoporphyrin monomethyl ether is primarily used in photodynamic therapy (PDT) for the treatment of various cancers, including skin, bladder, and esophageal cancers. In this application, it acts as a photosensitizer, which, when exposed to specific wavelengths of light, generates reactive oxygen species that destroy cancer cells. Its selective accumulation in malignant tissues makes it effective in targeting tumors while minimizing damage to healthy cells. Additionally, it is utilized in diagnostic imaging to identify and locate cancerous tissues due to its fluorescence properties under certain light conditions. This dual role in both therapy and diagnosis highlights its significance in oncology.