Introduction

Canine MCT is one of the most common skin cancers in veterinary oncology. Multiple studies evaluating prognostic factors, biomarkers, a multitude of single and multi-agent chemotherapy protocols, multimodality treatment approaches, novel therapeutic approaches as well as novel information on advanced imaging procedures in the diagnosis of canine and feline mast cell tumor patients have been reported. Surgery is still considered the number one choice of therapy for local control of MCT, but if local control cannot be achieved, incomplete margins are reported on histopathology after surgical excision, or there is regional lymph node involvement, radiation therapy is indicated.

Radiation Therapy as Part of Standard of Care?

European Standard of Care (SOC) for diagnosis, staging, treatment and follow up management has been published in the European consensus document on mast cell tumors in dogs and cats (Blackwood et al. VCO 2012). The authors supply evidence based medicine grades (EBM Grade I–IV according to Roudebusch et al. JAVMA 2004) in connection with the recommendations. The recommendations regarding management of canine and feline MCT with radiation therapy are briefly outlined below.

Radiation Therapy

Radiation therapy is recommended as postoperative therapy of incomplete margins. If radiation therapy is contemplated post-surgery, tumor margins should be marked i.e., with metal surgical clips or photographs during surgery to aid in the planning of an appropriate radiation therapy field especially if post-surgery tissue migration is expected. Radiation therapy should be avoided as the sole therapy of larger tumors as shorter disease free intervals have been reported. One and 2-year disease free intervals have been reported in several studies in up to 95% of cases in patients with intermediate grade tumors. Importantly, many grade I and II MCT do not regrow despite microscopic incomplete margins.

In dogs with regional lymph node metastasis and grade 1/11 tumors, postsurgical radiation therapy including the lymph node has reported disease free intervals of up to more than 3 years.

A fairly recent study (Kry et al. Vet Surg. 2014) supported primary re-excision of incomplete margins/close surgery margins or radiation therapy. Aggressive local therapy resulted in significantly increased median survival times for the re-excision (2930 days) and radiation therapy (2194 days) groups compared to the surgery alone group (710 days).

There have been no randomized trials deciding the optimal radiation therapy protocol.

On the Horizon

The future management of canine and feline mast cell tumors with radiation therapy may include radio sensitisers or combinations with new treatment modalities such as tyrosine kinase inhibitors or immunotherapies. Therapy guidance and prognostication may be guided by molecular diagnostic techniques, molecular imaging techniques (PET-CT, PET-MRI) as well as species specific immune therapeutic approaches. Randomized controlled trials are needed to substantiate the role and protocols for radiation therapy in mast cell disease.

References

1.  Blackwood L, Murphy S, et al. European consensus document on mast cell tumours in dogs and cats. Vet Comp Oncol. 2012;10(3):e1–e29.

2.  Roudebush P, Allen TA, Dodd CE, Novotny BJ. Application of evidence-based medicine to veterinary clinical nutrition. J Am Vet Med Assoc. 2004;224:1765–1771.

3.  Poirier VJ, Adams WM, Forrest LJ, Green EM, Dubielzig RR, Vail DM. Radiation therapy for incompletely excised grade II canine mast cell tumours. J Am Anim Hosp Assoc. 2006;42:430–434.

4.  LaDue T, Price GS, et al. Radiation therapy for incompletely resected canine mast cell tumours. Vet Rad Ultrasound. 1998;39:57–62.

5.  Al Sarraf R, Mauldin GN, Patnaik AK, Meleo KA. A prospective study of radiation therapy for the treatment of grade 2 mast cell tumours in 32 dogs. J Vet Intern Med. 1996;10:376–378.

6.  Kry KL, Boston SE. Additional local therapy with primary re-excision or radiation therapy improves survival and local control after incomplete or close surgical excision of mast cell tumors in dogs. Vet Surg. 2014;43(2):182–189.