Equine Health Update Issue 1 Volume 16 - Page 27

techniques may include local excision, segmental posthioplasty (‘reefing’), partial phallectomy, and en bloc penile and preputial resection with penile retroversion (Scott 1976; Markel et al. 1988; Doles et al. 2001; Archer and Edwards 2004). Use of CO2 laser for surgical management of MC-SCC resulted in complete resection without recurrence in 78% of cases (McCauley et al. 2002), suggesting a viable method for MC-SCC removal. When MC-SCC metastasis to regional lymph nodes is detected, those nodes should also be excised. Cryotherapy Cryotherapy has been used successfully in the management of MC-SCC, often after surgical debulking (Joyce 1976; Hilbert et al. 1977; Harling et al. 1983). This involves application of liquid nitrogen at -196°C (by spray or probe) to lyse tumour cells through intracellular ice formation. The best results are often for treatment of early or precancerous MC-SCC, especially on the penis and prepuce (Bosch and Klein 2005; Stick 2006). Hyperthermia Hyperthermia has been used as an adjunctive therapy following surgical debulking for equine MC-SCC (Grier et al. 1980; King et al. 1991; Moore 1992). In one study, 8 horses with ocular MC-SCC were treated with hyperthermia, half of which had been treated previously with surgery, cryotherapy, radiotherapy or immunotherapy; 75% of these tumours regressed and, of the 25% that required additional hyperthermia, 66% regressed (Grier et al. 1980). Radiotherapy Radiotherapy uses ionising radiation to kill tumour cells by damaging nucleic acids and protein through free radical formation. Two forms of radiotherapy include teletherapy and brachytherapy. Teletherapy is high energy x-ray or g-ray radiation that is applied 80–100 cm from the tumour by a linear accelerator or cobalt-60 unit and thus requires multiple treatments under general anaesthesia. Teletherapy is used for advanced tumours that cannot be completely excised and, although it is often used palliatively, it has been curative for periorbital MC-SCC in ~50% of cases (Theon 1998).With respect to brachytherapy, success rates for treatment of periorbital MC-SCC using iridium-192 were 82% at one year post treatment and 64% at 5 years (Theon and Pascoe 1995). Strontium-90 is b radiation and is directly applied with a probe to the neoplastic tissue. Because the radiation only penetrates ~2 mm, strontium-90 should only be used for superficial lesions. In one study, 83% of limbus, cornea or third eyelid MC-SCC were successfully treated with strontium-90 radiation when combined with superficial keratectomy and permanent bulbar conjunctival grafts (Plummer et al. 2007). Periorbital MC-SCC treated with several adjunctive forms of radiation therapy had a significantly lower recurrence rate (12%) than those treated without adjunctive radiation therapy (44%; Mosunic et al. 2004), supporting its use as an effective treatment modality. To summarise, brachytherapy is often used as an adjunctive therapy following surgical debulking of MC-SCC but may also be used alone for small lesions that cannot be completely excised. Complications include loss of hair pigment, permanent epilation, palpebral fibrosis, and cataract and corneal ulceration (Theon and Pascoe 1995; Theon 1998). Some cutaneous tumours suitable for brachytherapy are also good candidates for intratumoural chemotherapy, and the choice between the 2 modalities is based on tumour type, size, degree of invasiveness, predicted cosmetic and functional outcome, access to a radiation therapy facility, and cost. Chemotherapy Intralesional and topical chemotherapy have been used with some success for equine MC-SCC. Intratumoural cisplatin, 5-fluorouracil (5-FU), mitomycin-C and bleomycin have been used to treat equine MC-SCC. Cisplatin (cis-diammine dichloroplatinum) inhibits DNA synthesis by binding to DNA. Two different approaches allow controlledrelease administration: intralesional injection of a viscous fluid and implantation of biodegradable beads. The first method involves reconstitution of cisplatin powder with sterile water at a concentration of 10 mg/ml, followed by mixing with sesame seed oil (60%) and sorbitan monooleate (7%) by use of the pumping method (Theon et al. 1997). This results in 3.3 mg cisplatin/ml, and the dosage is 1 mg cisplatin/cm3 of tissue. The treatment recommendation includes 4 intratumoural injections given at 2-week intervals (Theon et al. 2007) and is often initiated immediately following surgery (Theon et al. 1993, 1999, 2007). Cisplatin beads are typically implanted through stab incisions at 1.5 cm intervals in the tumour bed and surrounding skin. It is recommended that tumours >1.5 cm be debulked prior to implantation, while tumours <1.5 cm be treated with bead implantation alone (Hewes and Sullins 2006). Intratumoural cisplatin, in the injectable or bead formulation, yields similar success rates for the treatment of MC-SCC (Hewes and Sullins 2006; Theon et al. 2007). In horses with MC-SCC that were treated with cisplatin injections, one study reported a relapse-free rate of 65% (Theon et al. 1993), while a second study reported an 88% success rate (Theon et al. 2007). Similarly, approximately 85% of horses with MC-SCC treated with perioperative or post operative cisplatin injections were tumour-free after 4 years (Theon et al. 1999). • Volume 16 no 1 • March 2014 • 27