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 25-28 September, 2018 | Singapore
Non-resectable primary tumours:
· Radiation therapy (RT): Palliative or definitive RT may be considered. There are no large prospective studies, but it appears that the addition of prednisolone +/- toceranib may improve response (approximately 80% versus approximately 50%) and control duration over palliative RT alone.
· Chemotherapy: Many different chemotherapy drugs alone and in combination (+/- corticosteroids) have been studied,
including vinblastine, CCNU, TKIs, paclitaxel, cyclophosphamide and hydroxyurea, and metronomic chlorambucil. Response rates range from approximately 20% to 80% and response durations from weeks to many months. In general, combination therapies seem more effective. I usually offer three options for chemotherapy for non-resectable MCT: 1) ‘traditional’ chemotherapy with vinblastine and/or CCNU along with prednisolone 2) toceranib with prednisolone or 3) metronomic chlorambucil and prednisolone
· Alternative local therapies: Electrochemotherapy results in response rates of 60-80% in the small studies published to date, and is generally well-tolerated. Intra-lesional triamcinolone may be an effective option in some cases, though responses are generally relatively short-lived.
· Treatment of metastatic MCT: In cases of MCTs
with LN but no distant metastasis (stage 2), aggressive local therapy, often along with adjuvant chemotherapy is generally recommended. For low- grade stage 2 MCT aggressive local therapy (surgery +/- radiation therapy) may be sufficient for long term control. In distant metastasis, systemic treatment options as for non-resectable MCT are typically attempted, with some retrospective data suggesting that using vinblastine and CCNU is more effective than toceranib.
· Supportive care: Anti-histamines +/- antacids are recommended in all dogs with gross MCT disease
Feline MCTs
· Cutaneous: The majority are benign, but a subset are more aggressive and identifying those is challenging. The 3-tier grading system for canine MCT is not useful, the 2-tier system has not been evaluated. Features that have been assessed for impact on prognosis include:
· Histologic subtype
· Mastocytic - most common, further divided into compact and diffuse (pleomorphic/anaplastic) forms. Diffuse MCT tend to have a higher
mitotic index and may be associated with more aggressive behaviour, however prediction of behaviour based on histologic subtype alone is difficult. A more recent study described a subset of well-differentiated tumours with prominent multinucleated cells which appeared to have aggressive behaviour.
· Histiocytic - more likely to affect young cats, with Siamese being most affected in some studies. Generally benign, may spontaneously resolve.
· Proliferation indices: Higher mitotic index (per 10 hpf) is associated with outcome, though the appropriate
· Surgery: For MCT with distant (beyond LN) metastasis, removal of the primary tumour is
not likely to impact prognosis, though it may improve quality of life in painful/ulcerated tumours. General oncologic principles of local en bloc resection with a margin of normal tissue around the tumour apply for cutaneous MCT. MCT should be minimally manipulated during preparation and surgery to decrease degranulation. Many MCT
will have definitive excisional surgery on the basis
of a cytology result. As a general rule, 1cm lateral margins are adequate for Grade I MCT, 2cm lateral margins for Grade 2 MCT and 3cm lateral margins for Grade 3 MCT. These have not been re-evaluated for the 2-tier grading scheme. If the grade is not known, then a minimum of 2cm lateral margins is recommended. For cytologically diagnosed MCT where definitive surgery is feasible based on size and location, biopsy for pre-operative grading is not usually required. Deep margins are more qualitative than quantitative. The deep margin should comprise fascia or muscle. Careful pre-operative palpation and imaging (Ultrasound, CT or MRI) are useful to determine degree of fixation to deeper structures
in planning the deep margin. The excised tumour specimen should be inked to facilitate histological margin assessment. If the MCT is greater than 1cm in diameter, then the skin surface should be cut at 1cm intervals to allow appropriate fixation.
· Adjuvant local therapy: In cases of incomplete
or narrow histological margins (i.e. risk of local recurrence), additional local therapy with revision surgery or radiation therapy is recommended. Electrochemotherapy is a newer modality which may also be an option. It is important to remember that the ‘safe’ histological margin (i.e. sufficient to prevent local recurrence) has not been determined for MCT and is influenced by grade. Many incompletely
or narrowly excised low-grade MCT do not recur, however additional local therapy with re-excision
or radiation therapy has been shown to improve survival and so should be considered in all cases (3).
· Adjuvant systemic therapy: Chemotherapy is recommended following surgery in high grade
MCT, even without visible metastasis, as adjuvant chemotherapy appears to improve survival over surgery alone. The ideal protocol for adjuvant chemotherapy in high grade/ high risk MCT is not fully determined. Vinblastine and prednisolone appears to be most commonly used, but protocols including CCNU alone or alternating with vinblastine are also reported. The adjuvant use of TKIs such
as toceranib (Palladia) or masitinib (Kinavet/Masivet) is not well studied, though one retrospective
study reported improved outcome in dogs with
high risk MCT receiving adjuvant vinblastine and prednisolone compared to those receiving masitinib (4). The other major challenge with adjuvant use
of TKIs is determining appropriate duration of treatment. The use of adjuvant corticosteroids or anti-histamines has not been studied and is not generally recommended.

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