## 3 PDT in Veterinary Oncology  
Until now, several photosensitizers have been applied in veterinary oncology, including porfimer sodium [78; 79; 80; 81; 82], phthalocyanines [83; 84], and 5-ALA [85; 86; 87], among others. The use of third-generation approaches was also reported [18]. Besides treatment, 5-ALA was also used for the photodynamic detection of tumors [68; 71]. The main chemical structures of photosensitizers used to treat veterinary neoplasms are presented in Figure 3.  
Photosensitizers can be administered by various routes. Systemic [81; 82] and oral [82] administrations are common in dogs and cats. Nonetheless, topical administration might have the advantage of avoiding generalized photosensitization. Other administration routes, such as intralesional, were reported less frequently [9]. It should be considered that the administration route can affect the photosensitizer biodistribution, influencing the action at the target tissue [88; 89; 90]. The chemical features and concentration of the photosensitizer, light dosimetry and target tissue may influence side effects [91]. However, severe adverse effects were rarely observed, even with systemic administration [89; 92; 93]. The most commonly reported side effects were itching and local discomfort when PDT was performed topically [91]. WhenPDT was performed systemically, there were reports of sneezing, oedema, alopecia, emesis, thrombocytopenia, anorexia, elevated body temperature during treatment, and prolonged photosensitization [9, 94, 95, 96]. Light sources are also a relevant component of PDT. Red or near-infrared light sources are preferred due to longer wavelengths, higher tissue penetration, and appropriate energy (Figure 2A). Thus, wavelengths between 600 and 1200 nm are considered the optical window for PDT [97]. The primary light sources include lasers, light-emitting diodes (LEDs), and lamps [33]. Lasers and light-emitting diodes are the most commonly used [9, 10]. Nevertheless, some reports suggest that sunlight has the potential for superficial skin cancer therapy in animals [10]. The light source selection depends on the photosensitizer to be used and the location and characteristics of the tumor tissue to be treated [32, 71]. Light for superficial neoplasms (superficial PDT) involves treatments with low penetration depth (e.g., skin, typically <2 mm, with the use of lasers, LEDs, and broadband lamps). Light sources for interstitial PDT can treat tumors beyond 1 cm, assisted by using needles, catheters, and optical fibers, but conventional light sources have light penetration similar to superficial PDT. For deep PDT, light sources being developed include a wide variety of technologies, including multifunctional nanoparticles (near-infrared light-excited nanomaterials, X-ray/Cherenkov excited scintillating/afterglow nanoparticles, and self-illuminated nanoconjugates) [22]. Veterinary biosafety regarding PDT comprises measures the professional, animal species, and waste disposal [9]. Animals are always subjected to sedation and/or an anesthesia; in some patients, antibiotic therapy is also provided [64, 100]. Treated areas must be protected with bandages [9], and the post-treatment use of the Elizabethan collar may be essential to avoid likcing and local mutilation [9]. Personal protective equipment must include appropriate clothing, gloves, masks, and glasses to protect from light sources [9]. Photosemsitizers should be protected from light and handled with sterile syringes for safe practice [7]. The placement of a protective physical barrier on the areas not receiving treatment is also recommended (e.g., gauze bandage or similar). All equipment and cables associated with the light source must be cleaned and disinfected before and after treatment [9]. PDT was investigated in various neoplasms in veterinary medicine [9, 10] despite not being actively used in clinical practice, as presented in Figure 4. Several of these studies point to PDT's potential to be used in veterinary oncology.