Radiotherapy Treatment of Canine Mast Cell Tumor: A Case Report

Hazal Öztürk Gürgen (1), Evrim Egeden (1, 2), Işıl Aytemiz Danyer (1), Özlem Calp Egeden (2), Züleyha Akgün (3), Aydin Gürel (1)
1. Istanbul University-Cerrahpasa, Faculty of Veterinary Medicine, Pathology Department
2. Ada Veterinary Policlinic Sulun Sokak No: 14 1. Levent 34330 Besiktas, İstanbul
3. Istanbul Bilgi University, department of Radiation Oncology

Mast cell tumors (MCTs) are common skin tumors in dogs. MCTs can occur anywhere on the body, however some dog breeds, such as English Setter, have high incidence of MCT on hindlegs (1). The aim of this case presentation is to present the recovery of the histopathologically MCT diagnosed dog by radiotherapy treatment.

An eight year-old, male, English Setter dog was admitted to the veterinary clinic with the complaint of the swelling in the inguinal area. The mass was completely removed and submitted to Pathology Department of IUC Veterinary Faculty for histopathological examination. The tissue sample was fixed in 10% formaldehyde solution, routinely processed and embedded in paraffin blocks. 4 -5 μm thickness of tissue sections were cut and stained with Hematoxylin & Eosin and Giemsa Staining methods. Furthermore, additional immunohistochemical staining using c-Kit and CD25 antibodies were applied on the tissue samples according to indirect staining method. In addition to the surgical excision, treatment of the dog was completed by radiotherapy with 250.0 cGy / fractions for 16 sessions in 7 weeks.

Macroscopic Findings: The patient had 8x10 cm in diameter solid, firm, white in color of the cut-surface, mass in the inguinal area. Histopathological Findings: Histopathological examination revealed rows and ribbons of neoplastic mast cells in dermis and subdermis, deeply in muscle tissue and surgical margin. They appeared as round to polygonal in shaped, most of them had abundant basophilic granules in their cytoplasm. Nuclei were found uniform and round, and mild degree of mitosis were observed. In addition, large numbers of eosinophilic leukocytes were accompanied with the neoplastic mast cells (Figs. 1, 2).

Histopathological grading was evaluated as high grade and grade 3 according to the grading system by Kuipel and Patnaik, respectively. And the tumor was considered as stage 3 because of its large infiltrative character. Giemsa staining: The mass were composed of neoplastic mast cells that are round, and have abundant basophilic granules in their cytoplasm. Nuclei are uniform and round, nucleoli are not visible, and mitoses are rare or absent (Figs. 3, 4).

Immunohistochemical Findings: Neoplastic mast cells showed weak diffuse positivity in 60 to 70% of the mast cells against antibody CD25 (Fig. 5) and C-kit staining was pattern III (4) (Fig 6).

Radiotherapy: Radiothreapy treatment provided increased survival rate. Figure 7 shows color isodose distribution of 6 MV photons with IMRT technique with bolus, dose prescribed to the %95 isodose line. Dose volume histogram of the treatment plan represents good conformality (Fig. 8).
The clinical history obtained from the patient was consistent with the literature reports about higher incidence of mast cell tumors on hindlegs in English Setters and a mean of involvement of 8.5 years (1). In the present case; the confirmed as MCT by histopathological examination and Giemsa staining revealed cytoplasmic metachromatic granules. Although the widely used method in MCT prognostication is histopathology, prognostication can be supported by immunohistochemical staining using CD25 and C-kit. C-kit staining pattern III is associated with decreased survival time and higher recurrence rate (2), in other words, with malignancy. Similarly, loss of CD25 expression is also associated with malignancy (3) which is compatible with weak CD25 staining in this case. 

Local treatment procedures, such as surgery and local radiation therapy, are possible for MCTs (1). Surgical excision is not sufficient, if there is ongoing tumor cell infiltration at the surgical margins (4). In such cases, radiotherapy can be considered as an effective adjunctive therapy, even in grade 3 tumors (5). It has also been reported that dogs with a tumor of high grade or in a particular body location, as inguinal region in this case have poor response to local-regional radiation therapy (5). However following surgical excision in this case, the patient has completely recovered with the radiotherapy treatment and the clinical condition remain stable. MCT free period has been about three years now and the patient still has no sign of MCT.

1) Miller, W. H., Griffin, C. E., Campbell, K. L. (2013). Muller and Kirk’s Small Animal Dermatology (7th ed.). St. Louis, Missouri: Elsevier.
2) Kiupel, M., Webster, J. D., Kaneene, J. B., Miller, R., Yuzbasiyan-Gurkan V. (2004). The use of KIT and Tryptase expression patterns as prognostic tools for canine cutaneous mast cell tumors. Vet Pathol, 41, 371-377.
3) Meyer, A., Gruber, A. D., Klopfleisch, R. (2012). CD25 is expressed by canine cutaneous mast cell tumors but not by cutaneous connective tissue mast cells. Vet Pathol, 49, 988-997.
4) Al-Sarraf R, Mauldin GN, Patnaik AK, et al. (1996). A prospective study of radiation therapy for the treatment of grade 2 mast cell tumors in 32 dogs. J Vet Intern Med;10:376–378.
5) Hahn, K. A,; King, G. K.; Carreras, J. K. (2004). Efficacy of radiation therapy for incompletely resected grade-III mast cell tumors in dogs: 31 cases (1987–1998). JAVMA, 224, 79-82.
Radiotherapy Treatment of Canine Mast Cell Tumor: A Case Report

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