Present address: College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506, USA
Photodynamic therapy for the treatment of periocular squamous cell carcinoma in horses: a pilot study
Version of Record online: 19 AUG 2008
© 2008 American College of Veterinary Ophthalmologists
Volume 11, Issue Supplement s1, pages 27–34, September/October 2008
How to Cite
Giuliano, E. A., MacDonald, I., McCaw, D. L., Dougherty, T. J., Klauss, G., Ota, J., Pearce, J. W. and Johnson, P. J. (2008), Photodynamic therapy for the treatment of periocular squamous cell carcinoma in horses: a pilot study. Veterinary Ophthalmology, 11: 27–34. doi: 10.1111/j.1463-5224.2008.00643.x
- Issue online: 19 AUG 2008
- Version of Record online: 19 AUG 2008
- photodynamic therapy;
- squamous cell carcinoma
Objective Local photodynamic therapy (PDT) is a novel cancer therapy in veterinary ophthalmology. A prospective pilot study seeking to demonstrate proof of principle and safety for the treatment of equine periocular squamous cell carcinoma (PSCC) was therefore conducted. We hypothesized that surgical excision with adjunctive local PDT is an effective and safe treatment for equine PSCC.
Procedures Nine horses (10 eyes) with PSCC were treated with surgical resection, local infiltration of resulting wound beds with 2-[1-hexyloxyethyl]-2-devinylpyropheophorbide-a (HPPH) and irradiation with 665-nm wavelength diode laser. Regular follow-up ophthalmic examinations were performed.
Results Surgical resection and PDT yielded disease-free intervals of 25–68 months in our study horses as of January, 2008. These results were obtained following a single treatment in seven horses and two treatments in one horse. In one horse, carcinoma in situ developed 2.5 months after partial surgical excision and PDT, requiring local excision under standing sedation.
Conclusions Preliminary results suggest that surgical resection and adjunctive local PDT is a safe and effective novel treatment for PSCC in horses. More research is needed before PDT for the treatment of equine PSCC can be adequately compared with other current modalities. Important to future investigations regarding PDT, tumor recurrence rate, length of hospitalization, number of treatment episodes required to effect tumor remission, and total treatment costs should be examined in a controlled manner. Our present results and experiences suggest that this treatment may be useful in the treatment of equine PSCC.
Squamous cell carcinoma (SCC) is the most common neoplasm of the equine eye and ocular adnexa,1–3 and the second most common tumor of the horse overall.4 Draft breeds, Appaloosas, American Paint Horses, Thoroughbreds, and Quarter Horses are predisposed.1,5–8 Squamous cell carcinoma may involve the corneoconjunctiva, bulbar conjunctiva, third eyelid, and eyelids.1,2,9,10 Biological behavior of SCC is reported to differ depending on location,11,12 but is typically locally invasive and may result in blindness.13 Metastasis to local lymph nodes, salivary glands, and lungs can occur.10 A variety of treatment modalities for equine SCC have been reported in case reports or case series with variable success.7,14–21
Reported therapies for ocular and periocular SCC (PSCC) include surgical excision,10,14 cryotherapy,15,16 immunotherapy,22,23 radiofrequency hyperthermia,18,23 topical or intratumoral chemotherapy,19 carbon dioxide laser ablation,24 and several radiation therapies.8,17,20,21,23,25–30 Tumor recurrence is frequently reported following surgical excision and ancillary therapy because it is often not possible to achieve satisfactory excisional margins and the additional therapies fail to successfully eliminate residual tumor cells. Given the variety of treatment options currently available coupled with significant differences in reported recurrence rates there does not presently exist a uniformly satisfactory treatment for SCC in horses. Therefore, strong rationale exists for the development of more effective adjunctive therapies to enhance destruction of residual tumor cells beyond the surgical margins.
Photodynamic therapy (PDT) entails the use of light and light-sensitive compounds in an oxygen-rich environment to cause localized tissue necrosis.31–36 The application and photochemical principles of PDT have been recently reviewed.37 Briefly, a photosensitizer is administered to the patient, most commonly by intravenous injection. Tumor selectivity during treatment results from a combination of selective retention of the photoactive chemical by neoplastic cells and targeted delivery of light to the specific area of interest.38 Use of PDT in veterinary medicine has shown promise for the treatment of canine and feline SCC.32,39–41 Systemic injection of photosensitizing agents to horses is not practical at this time, as the volume of drug to be delivered would be excessive and pharmacokinetic, drug distribution, or toxicology studies have not been conducted using these agents in horses.
A pilot study was undertaken at the University of Missouri, Veterinary Medical Teaching Hospital (MU-VMTH) to demonstrate proof of principle and safety of a novel approach to the treatment of equine PSCC using PDT by injecting a photoactive agent (HPPH) locally into the tumor bed followed by localized laser irradiation. We hypothesized that local PDT would be an effective and safe treatment for equine PSCC.
MATERIALS AND METHODS
Selection of cases
Candidate equine patients presented to the veterinary ophthalmology service received complete physical and ophthalmic examinations by internal medicine and ophthalmology specialists (PJJ and EAG, respectively). Initial physical examinations evaluated the overall health of the patient and determined if any other significant disease (e.g. SCC in non-ocular locations) was present. Complete ophthalmic examinations included slit-lamp biomicroscopy, direct and indirect ophthalmoscopy, digital palpation of the upper and lower conjunctival fornix and orbital rim, and fluorescein staining when corneal ulceration was suspected. Intravenous sedation and auriculopalpebral nerve block were performed when necessary to facilitate ophthalmic examination. Lesions suspicious for eyelid SCC were first photographed (Figs. 1 and 2), measured using calipers (Table 1), and subsequently confirmed by biopsy and histopathology. Additional systemic workup to evaluate for metastatic disease included mandibular lymph node aspirates, screening blood work (complete blood count and a plasma biochemistry panel), and thoracic radiographs (as needed). All procedures were in accordance with institutional animal use and care guidelines. Horse owners were advised that the PDT protocol was experimental and were required to sign a clinical trial agreement and release form prior to initiation of treatment. Equine patients were included in the PDT pilot study providing they met the following criteria: Eyelid SCC confirmed by histopathology, tumor was localized to the eyelid with no extension into the bony orbit, mandibular lymph node aspirates were devoid of neoplastic cells, the horse's vaccination status for tetanus was current, and results of routine blood tests did not reveal evidence of other systemic disease.
|Case number and name||Signalment (age at initial presentation)||Tumor location and laterality||Tumor bed size (L × W)||Disease-free interval in months as of January, 2008|
|1||16 y.o. Tennessee Walking Horse Mare||Inferior middle to nasal eyelid OD||3 × 1.5 cm = 4.5 cm2||68*|
|2||13 y.o. Missouri Fox Trotter Gelding||Superior and inferior nasal canthus OD||(2 × 1)2 cm = 4 cm2||64*|
|3||9 y.o. American Paint Horse Mare||Superior eyelid OD||1 × 1 cm = 1 cm2||62|
|4||17 y.o. Appaloosa Mare||Superior nasal eyelid and canthus OD||2.5 × 1cm = 2.5 cm2||45|
|5||9 y.o. Missouri Fox Trotter Mare||Inferior eyelids OU||3 × 1 cm = 3 cm2 OD 0.5 × 0.5 cm = 0.25 cm2 OS||44 OU|
|6||6 y.o. Belgian Mare||Inferior nasal canthal eyelid OS||3 × 1.5 cm = 4.5 cm2||35|
|7||5 y.o. Missouri Fox Trotter Gelding||Temporal canthus OS||2 × 1.5 cm = 3 cm2||32|
|8||10 y.o. American Paint Horse Mare||Superior eyelid OS||5 × 4 cm = 20 cm2||31|
|9||17 y.o. American Paint Horse Gelding||Inferior eyelid OD||1.5 × 1 cm = 1.5 cm2||25|
All horses received treatment using triple antibiotic ointment (Neomycin, Polymyxin B Sulfates, Bacitracin Zinc Ophthalmic Ointment U.S.P., Bausch & Lomb Pharmaceuticals, Inc., Tampa, FL) to the affected eye q 8 h beginning 24 h prior to surgery to minimize chances of secondary bacterial infection. Systemic flunixin meglumine (Banamine®, Schering-Plough Animal Health Corp., Union, NJ) at 1.1 mg/kg, IV was administered within 1–2 h prior to induction of general anesthesia to decrease postprocedural eyelid pain and inflammation. Horses were routinely placed under general anesthesia, positioned in lateral recumbency, and the affected eye and periorbital area was prepared for aseptic surgery. To calculate amount of drug to be administered locally to each horse after tumor resection, tumor bed size was calculated with the patients under general anesthesia to ensure measurement accuracy prior to any surgical manipulations being performed. Jamison calipers were used to measure the length and width of each tumor on gross physical examination at the beginning of surgery.
Eyelid tumors were resected using sharp dissection (Fig. 1) and the resultant wound bed was infiltrated with HPPH (provided by Dr Thomas J. Dougherty, Roswell Park Cancer Institute, Buffalo, NY). The HPPH was diluted with DMSO (Domoso®, Dimethyl Sulfoxide Veterinary Solution 90%, Fort Dodge, IA) to a final concentration of 2 mg/mL. The HPPH/DMSO solution was infiltrated into the resultant wound bed using a 5 mL syringe and 25-gauge needle. Treatment dose was 1 mg/cm2 of HPPH in tumor bed.42 Immediately following wound bed infiltration, a 665-nm wavelength diode laser (Coherent Laser Systems Innova, San Jose, CA) was used to irradiate the tumor bed using an incident light dose of 100 J/cm2 and a dose rate of 100 mW/cm2 (Fig. 1).32,41,43,44 Total treatment time was calculated as fluence/power density41 and typically ranged between 25 and 40 min depending on the original tumor dimensions.44 All surgical procedures and administration of local PDT were performed by one author (E.A.G.). Globes were protected during PDT using a light-impervious shield and fashioned from Styrofoam and duct tape (Fig. 1). A protective eye cup (Equine Eye Patch™, Jorgensen Laboratories, Inc., Loveland, CO) was placed under the halter prior to extubation to protect the globe and periocular tissues during both recovery from general anesthesia and in the postoperative period.
Postoperatively, triple antibiotic ointment was applied topically into the treated eye (q 8 h × 10 days) and phenylbutazone (Phenylbute® 1 g, Animal Health, Inc., St. Joseph, MO) (1 g P.O. SID–BID × 5–7 days) was administered to reduce inflammation and promote patient comfort. Daily ophthalmic examinations were performed for a minimum of 3 days immediately postoperatively to monitor for excessive eyelid swelling, tissue necrosis, eyelid function, and corneal ulceration. Recheck examinations were performed at 1, 3, 6, and 12 months and then annually unless the owner or referring veterinarian suspected tumor regrowth. If tumor regrowth was suspected, the horses were promptly re-examined at the MU-VMTH. If PSCC regrowth was considered likely, repeat biopsy was performed and additional PDT planned accordingly, with owner consent.
A total of 9 horses with PSCC were treated using the combination of surgical resection and local PDT. Affected horses presented at an average age of 11 years (range 5–17 years), a greater number of mares (n = 6) than geldings (n = 3) were present in this study population, and a variety of breeds were represented (Table 1). A greater number of right (5) vs. left (3) eyes were affected and bilateral PSCC was evident in only one horse at presentation (Fig. 2). Average tumor bed size was 4.4 cm2 and ranged in size from 0.25 to 20 cm2. Surgically resected eyelid tissue was submitted for histological diagnosis to the University of Missouri Veterinary Medical Diagnostic Laboratory. A diagnosis of SCC with neoplastic cells extending to the surgical margin in at least one plane was confirmed in all 9 horses. Average disease-free interval was 45 months (range 25–68 months) as of January, 2008.
Two of 9 horses underwent prior treatment for PSCC before treatment with local PDT. Case 1 had previously received 2 treatments of surgical excision and cryotherapy, followed by 5 intralesional cisplatin treatments in the 10 months preceding referral. Case 2 had received 3 antecedent treatment episodes of surgical resection with ancillary cryotherapy in the 18 months preceding referral. Interestingly, cases 1 and 2 were also the only horses in which tumor recurrence after one episode of local PDT was observed.
In case 1, repeat biopsy at 6 weeks revealed PSCC regrowth at the medial margin of the initial tumor bed. This mare underwent repeat surgical resection and PDT at that time and remained tumor free with a cosmetic, functional lower eyelid OD for 68 months prior to losing her globe secondary to a corneal stromal abscess necessitating enucleation. Case 2 was noted 10 weeks postoperatively to have a 3 mm area of erythematous skin at the medial aspect of the original surgical site. This area was biopsied and read out as carcinoma in situ on histopathology. The affected area was removed under standing sedation with sharp dissection alone. Sixty-six and a-half months after surgery and local PDT, and 64 months after removal of the carcinoma in situ, case 2 remains comfortable, visual, with good eyelid function and no evidence of tumor regrowth as of January, 2008.
Other ophthalmic complications observed following local PDT and identified in all cases in this study were limited to severe eyelid swelling in the first week postoperatively (Fig. 1). Although all eyelids swelled considerably after surgery and developed a dark red to purplish discoloration, all horses were observed to hold their eyelids comfortably open and corneal ulceration did not occur in any horse. All owners were able to administer topical ophthalmic ointment at discharge and no horse required a subpalpebral lavage system to facilitate postoperative topical ophthalmic treatments. Long-term complications in this study were limited to cases 2 and 6 in which epiphora was reported when compared to the opposite eye. In these cases, epiphora was attributed to the fact that both the upper and lower eyelid puncta and canaliculi (case 2) or inferior puncta (case 6) were resected due to tumor involvement. In case 4 epiphora did not develop following resection of the upper puncta because the lower puncta was preserved. In all cases, eyelid function and cosmesis were preserved (Fig. 2).
Equine eyelid abnormalities present unique challenges to veterinary ophthalmologists. The periocular skin in horses is tightly adherent to the underlying fascia and bone, often precluding successful reconstructive eyelid surgery for extensive eyelid tumors. If surgical reconstruction of the eyelid is unsuccessful, it is likely that the globe will be lost to the consequence of inadequate tear film maintenance/distribution and exposure keratitis. Due to the potentially devastating visual consequences for a horse affected by an extensive periocular neoplasm, a variety of treatment modalities have been employed with varying reported success for PSCC. Caution should be exercised when attempting to make direct comparisons between published reports. Efforts aimed at comparing preliminary results reported in this manuscript with those in the literature are problematic. The extent of tumor involvement is not always well characterized, thus reported results may have been skewed toward a more favorable outcome in those cases with superficial tumors. Of particular significance, readers should be aware that marked disparity in the methods used to report treatment outcome (e.g. recurrence rate7,8,17,21,26 vs. overall survival rate6,23 vs. progression-free survival time30), as well a lack of published reports that specifically examine recurrence rates of adnexal (separate from either ocular or conjunctival) SCC with and without adjuvant therapy21 highlights the need for greater uniformity in evaluated parameters and data analysis by way of controlled clinical trials in veterinary ophthalmology. In addition to the aforementioned limitations, for direct comparisons between reports to be meaningful, additional parameters such as previous treatment modalities utilized on PSCC, length of hospitalization, number of treatments needed to affect resolution, and overall cost associated with treatment should be evaluated. Currently, this information is not uniformly available.
In our study, the overall recurrence rate with no horse lost to follow-up over the study period was 22% (2 of 9 horses); and for those horses where local PDT was the first and only treatment modality used, the recurrence rate was 0% (0 of 7 horses). Reported recurrence rates for equine ocular and periocular SCC within 1 year of treatment have been reported between 50% and 67% with surgery alone, and range from 25% to 67% when surgery is performed in conjunction with ancillary hyperthermia/radiation or cryotherapy.7,18,28 Many of the reportedly more effective treatments entail the use of radioactive substances. Radiotherapy is not readily available in general veterinary practice because it requires special training, facilities, and licensure. A recent, extensive retrospective study by Mosunic et al. examined the recurrence rate of equine SCC that underwent surgery with and without a variety of adjuvant radiations therapies.21 In that study, a total of 19 horses with eyelid SCC received surgery and one of the following: strontium 90 (n = 6, where 3 of 6 horses also received cryotherapy), iridium 192 (n = 12), or cobalt 60 (n = 1). The recurrence rate for eyelid SCC horses studied was 0%; however, 14 of 19 horses (73.7%) were lost to follow-up. Of the 5 of 19 eyelid SCC cases with follow-up, 2 tumors treated with iridium 192 developed secondary infection, blepharitis, and skin depigmentation.21 Walker et al. reported on 17 horses with ocular and adnexal SCC following radiotherapy.8 In that study, only 3 horses met the anatomical localization criteria consistent with our pilot study (SCC limited to the eyelid). Size of original eyelid tumors was not specified; 1 of 3 horses had previous surgery and 2 of 3 horses had previous radiation therapy. Of the 3 horses with eyelid SCC only, 2 were treated with iridium 192 with reported remission rates of 4 and 18 months, respectively. One horse treated with strontium 90 had a remission rate of 6 months. Additionally, one horse was noted to have an eyelid infection after treatment with iridium 192. β-radiation is limited for use with very superficial lesions as 75% of β-rays are absorbed by the most superficial 2 mm of tissue and most of the remainder is absorbed by the next 1 mm of tissue.25 Walker states that strontium 90 is probably best reserved for use with thin lesions of the cornea, sclera, and small surgically cytoreduced tumor beds.8
No treatment modality for PSCC is without potential complication. Radiation treatment for ophthalmic neoplasia is sometimes complicated by the development of keratitis and uveitis.7,25,45 It has been speculated in veterinary medicine that interstitial gamma radiation (Cobalt-60, radon-222, radium, cesium-137 and gold-198) may increase the risk of metastasis by promoting neoplastic cell movement into vascular and lymphatic vessels7,20 and neoplastic seeding has been documented to occur in the physician literature.46,47 Cryotherapy, while inexpensive, is associated with development of local edema and depigmentation of hair and skin, may lead to an unacceptable cosmetic outcome.12 Radiofrequency hyperthermia carries the risk of ulcerative keratitis with scar formation and conjunctivitis.18 Immunotherapy and intratumoral chemotherapy treatments can be lengthy, as both necessitate serial treatments over the course of several weeks and often cause necrosis and suppuration at drug injection sites.1,19 In one report where intratumoral chemotherapy with cisplatin was used on a total of 20 horses but only 3 horses with PSCC, 2 of 3 horses (66%) demonstrated recurrence at 5 and 10 months, respectively.19 More recently, a larger retrospective study examined the effectiveness of intratumoral chemotherapy with cisplatin in sesame oil.48 Treatment consisted of a series of 4 intratumoral administrations of cisplatin given at 2-week intervals and 151 SCCs (103 periorbital SCCs) were treated. The overall control rate at 2 years after treatment, calculated as the ratio of disease-free horses to the total number of horses, was 88%; however, the control rate for periorbital SCC alone was not reported. In our study, the complications associated with local PDT included eyelid swelling and initial eyelid discoloration (Fig. 1).
For this prospective pilot study, HPPH was chosen as the photoactive agent due to its desirable properties: short photosensitivity period of only a few days; strong absorption peak at 665 nm giving better depth of light penetration in tissue compared to Photofrin®;49,50 our prior experience using this photoactive agent in small animal patients.32,43 Evidence that local PDT is efficacious as an adjunct to surgical resection for treatment of equine PSCC was supported by histopathology: all eyelid tissue submitted from the 9 study horses demonstrated neoplastic cells extending to the surgical margins in at least one plane (i.e. ‘dirty’ surgical margins). Cases 1 and 2 both demonstrated tumor regrowth after the initial PDT treatment episode at 6 and 10 weeks, respectively. Cases 1 and 2 were also the only study horses that had demonstrated repeated tumor regrowth after multiple failed treatment attempts prior to referral.
Treatment failure in these first 2 cases might have been attributable to inexperience using this modality (e.g. unsatisfactory drug infiltration). Complete and homogeneous distribution of photosensitizer in the tumor bed is essential for effective tumor destruction with local PDT because singlet oxygen has a short radius of action (0.01–0.02 µm).34,51 Studies have demonstrated that porfimer sodium, another PDT agent, injected directly into the center of subcutaneous gliomas and some bladder tumors was retained and distributed throughout the entire tumor.52,53 Moreover, intratumoral administration yielded a much higher concentration of photosensitizer in tumor tissues compared with normal tissues (including the skin and other organs).54
In this pilot study, DMSO was used as a solvent for HPPH. DMSO is readily permeable to biological barriers including mucous membranes, skin, and phospholipid membranes. It is widely used in veterinary medicine as an agent that enhances drug absorption across the skin.55–57 We theorized that DMSO would increase local diffusion of HPPH into the equine eyelid tumor bed, thereby resulting in more cell death of residual eyelid SCC tumor cells. However, recent experiments examining the effects of solvent in local PDT in a murine model have demonstrated no difference in tumor response using DMSO vs. using D5W as the solvent.58 Results of spectroscopic analysis showed that DMSO did not cause dissipation of Photofrin® from the injection site in normal healthy periocular equine skin.59 Therefore, the use of DMSO as a solvent in this study likely did not significantly affect the local tissue distribution of the photosensitizing agent.
Other reasons for which cases 1 and 2 might have developed early tumor regrowth compared to the other 7 cases are inherent tumor factors including initial tumor size and histologic features, with larger tumors and those possessing more malignant features carrying a more guarded prognosis. Of note, the first 2 cases were among the 4 largest tumors in this pilot study, with cases 6 and 8 being of comparable or substantially larger size (Table 1). However, the largest tumor treated in this study (case 8) measured 20 cm3 (tumor as measured in 3 dimensions at initial presentation: 5 × 4 × 1 cm) and this horse has sustained a 31 month disease-free interval after only one treatment with surgical resection and PDT. Cellular features of PSCC such as histologic grade, proliferation rate [which may be measured by proliferating cell nuclear antigen (PCNA), Ki-67, silver staining of nucleolar organizer region (AgNOR)], vascular endothelial growth factor expression, apoptotic rate, and nuclear mophometry have not yet been examined in the tumor sections removed from these horses. It is possible that the results of one or more of these factors may be predictive of treatment outcome. Finally, scar tissue formation secondary to prior surgery and ancillary therapies performed in cases 1 and 2 may have altered tumor vascular supply and tissue oxygenation. These tissue changes likely compromised drug diffusion and prevented uniform distribution of HPPH in the tumor bed inhibiting PDT efficacy.35,37,48 Cases 1 and 2 illustrate the importance of using effective treatment modalities as a ‘first-line defense’ and support the negative association between tumor recurrence and prognosis.48
The authors acknowledge the inherent limitations of this prospective pilot study. First, equine SCC is a spontaneously occurring disease, contributing to a relatively small number of study cases. Furthermore, this study's inclusion criteria were additionally restrictive as only horses with SCC limited to the eyelid were included, also leading to a low case recruitment (n = 9). Although follow-up periods were longer than many other reports in the literature, the lack of an in-hospital control population of eyelid SCC in which a different uniformly applied ancillary therapy was used represents another limitation. Finally, the extent of eyelid tumor involvement ranged from extensive (case 8) to minimal (case 5–OS). It is possible that small, superficial tumors would have demonstrated equally favorable long-term results irrespective of the therapy elected.
The results reported in this manuscript are preliminary. A second, controlled, prospective clinical trial is currently underway at our teaching hospital using a commercially available photodynamic agent (Visudyne®) and the aforementioned parameters are being recorded in addition to PSCC recurrence rate. It should be emphasized that the photodynamic agent used (HPPH) in the current study is not commercially available. Additionally, the length of time horses remained in the hospital varied greatly as the primary author's experience using this treatment approach matured. Initially, the first horses were accommodated at the teaching hospital and monitored daily for 7–10 days after surgery. It became clear that postoperative complications were minimal after treating 5 horses and subsequent horses were discharged on the third postoperative day.
Our present results and experiences using local PDT suggest that it may be useful in the treatment of equine PSCC and may have advantages over other currently used treatment approaches. PDT appears to require fewer treatment episodes than intralesional cisplatin and shorter hospital stays than some radiation therapies, while affording excellent preservation of eyelid function and cosmesis, especially in cases of moderate to extensive eyelid tumor involvement. In addition, PDT does not require that the horse be placed in isolation, as is the case for some radiation therapies, and does not require special institutional licensure or risk to operating personnel. Of particular importance to veterinary medicine is the advent of smaller, more affordable diode lasers which allow for convenient mobility and affordability with respect to clinical applications to the general practitioner. The authors feel that a direct comparison of the preliminary results given here with other current treatment methods should be interpreted with caution. More research is needed to further develop and evaluate PDT using different photosensitizers and in different clinical situations. Research efforts aimed at determining optimal clinical treatment parameters such as photodynamic agent and laser light dosages are on-going. Local PDT may prove to be an effective new modality in the treatment of other periocular neoplasms, minimizing the number of treatments required to effect remission while preserving eyelid cosmesis and function.
The authors express their sincere gratitude to the following individuals: (i) the University of Missouri, Veterinary Medical Teaching Hospital equine house officers and clinicians for assisting with the management of all cases in this study, (ii) Mr Howard Wilson and Mr Don Connor, multimedia specialists, for their technical expertise in the area of digital imaging.
- 11Tumors of the eye. In: Tumors in Domestic Animals, 4th edn. (ed. ) Iowa State Press, Ames, IA, 2002; 739–754..
- 12Eyelid tumors. In: Ocular Tumors in Animals and Humans, 1st edn. (eds , ) Iowa State Press, Ames, IA, 2002; 25–86., , .
- 25Treatment of equine squamous cell carcinoma of the conjunctiva using strontium 90 applicator. Equine Veterinary Journal 1983; 2: 125–126., .
- 41Applications of photodynamic therapy. Advances in Small Animal Medicine and Surgery 2002; 15: 1–3., .
- 42Preliminary clinical data on a new photodynamic photosensitizer: 2-[hexyloxyethyl]-2-devinylpyropheophorbide-a. Progress in Biomedical Optics 2000; 1: 25–27., , et al .
- 51Photodynamic therapy for the treatment of periocular squamous cell carcinoma in horses. Investigative Ophthalmology and Visual Science 2004; 45: E–5072., , et al .
- 59Spectroscopic determination of photofrin® following periocular subcutaneous injection in horses. Veterinary Ophthalmology 2004; 7: 443., , et al .