Photodynamic therapy for the treatment of periocular squamous cell carcinoma in horses

Abstract

Abstract: : Purpose: Photodynamic therapy (PDT) is an evolving new treatment modality. A pilot study seeking to demonstrate proof of principle and safety is being conducted. We hypothesize that surgical excision with adjunctive local PDT is an effective and safe treatment for equine periocular squamous cell carcinoma (PSCC). Methods: Three horses with naturally occurring PSCC were identified. Tumors were characterized by complete ophthalmic examination, 3–dimensional tumor measurement, extraocular photography, and histopathology. With patients under general anesthesia, PSCC tumors were treated with surgical debulking and infiltration of the resulting wound bed with pyropheophorbide–a–hexyl–ether (HPPH, Photochlor^TM) in dimethylsulfoxide (final concentration: 2 mg/ml; treatment dose = 1 mg/cm² of tumor bed). Surgical beds were subsequently irradiated using a 665–nm wavelength diode laser (incident light dose=100 J/cm²; fluence rate=100 mW/cm² ) Duration of treatment for each 3 cm–diameter treatment area was 16 min 40 sec. Post–treatment progress of horses was regularly evaluated by complete ophthalmic examination and repeated biopsy. Results: Refractory PSCC had been unsuccessfully treated (using surgical excision with cryotherapy and/or intralesional cisplatin) in 2 of 3 horses prior to PDT. Partial surgical excision and PDT has subsequently yielded disease–free intervals of 16, 12, and 11 months in our study horses. These results were obtained following a single treatment in 2 horses and 2 treatments in 1 horse. In one horse, carcinoma in situ developed 2 months after partial surgical excision and PDT, requiring local excision under standing sedation. Other minor post–operative complications included mild–to–moderate eyelid swelling and regional tissue necrosis, typical of PDT. Conclusions: Preliminary results suggest that surgical excision and adjunctive PDT may represent a safe and useful novel treatment modality for PSCC in horses. Providing these initial favorable results are supported by results of on–going studies, additional advantages of PDT in the treatment of equine PSCC compared with current methods include improved non–recurrence rates, shorter hospital periods, reduced overall cost, good cosmesis and excellent preservation of eyelid function. Future studies are underway to further evaluate the cellular and biochemical effects of PDT on equine PSCC.

Keywords: photodynamic therapy • oncology • tumors