An update on the treatment of cutaneous tumours

Sarcoids are the most common equine cutaneous tumour and continue to provide a clinical challenge. The majority of sarcoids are diagnosed based on their characteristic clinical appearance and locations, which is probably appropriate in most cases. Common locations for sarcoid lesions include the periocular region, axilla, inguinum and sheath, although they can occur anywhere, especially following a wound. However, it is important to recognise that other tumour types exist and can imitate sarcoids, especially nodular lesions. Where single nodular lesions are present and where lesions are in atypical locations, biopsy is strongly recommended to enable accurate diagnosis and appropriate treatment selection.

Benign neglect is most commonly employed with small occult or verrucose lesions. However, benign neglect may be a misnomer, as withholding treatment is not always a benign procedure. In a series of 42 periocular lesions that were not treated at the time of examination by a veterinary surgeon, all required treatment at a later stage and 64% of the horses were euthanised because the lesions were too extensive to treat at the time of referral (Knottenbelt and Kelly, 2000). However, in a series of 31 horses where some (but not all) of their sarcoid lesions were treated, some (but not all) of the untreated lesions spontaneously resolved regardless of the treatment used (Martens et al, 2001). Interestingly, in the same study, new lesions developed in 6 of the horses during the same time period (Martens et al, 2001). In a longitudinal study of 61 Franches-Montagnes horses, 38 horses had sarcoids documented at 3 years of age, but spontaneous regression occurred without any treatment in 65% of occult lesions and 32% of verrucose lesions (Berreux et al, 2016). It is not known if the same could be applied to other breeds, but the high prevalence of sarcoids in the Berreux et al (2016) study is surprising compared to the known owner-reported prevalence of 5.8% (Ireland et al, 2013). Although it is conceivable that many lesions are missed by owners, it is difficult to explain such a big discrepancy and to draw definitive conclusions. In one small placebo-controlled study, a complete or partial regression rate of 14% was reported in the placebo arm (Christen-Clottu et al, 2010), and in another study, 1.9% of small lesions spontaneously regressed (Pettersson et al, 2020), supporting the theory that some lesions may regress without specific therapy, albeit at a far smaller frequency than those reported in the Berreux et al (2016) study. Based on these data, benign neglect may be viable in a small number of early, small, occult or verrucose lesions, but regular monitoring is essential to ensure that lesions are not allowed to progress to the stage of being difficult to treat.