Getting the best out of your surgical biopsies – canine mast cell tumours

01 May 2020, at 7:50am

When presented with a skin mass, one of the main concerns is to rule out a possible mast cell tumour. If a mast cell tumour is present, then it is helpful to find out whether it is cutaneous or subcutaneous and to grade it in order to get prognostic information and help with further therapy.

The easiest and least invasive way of doing this is to take a fine-needle aspirate. Most mast cell tumours are easily recognised on fine needle aspirates; however, a few aggressive, poorly granulated tumours require biopsy. Two or three smears are useful. Try not to push the needle through the mass as this can implant neoplastic mast cells into deeper tissue.

Fine-needle aspirates can determine if a mast cell tumour is present and some recent literature has assisted in trying to grade fine-needle aspirates; however, histology is still the most useful method of grading.

For small tumours, an excisional biopsy is best. This can grade the tumour and identify margins. Incisional biopsies can be useful for grading but cannot determine the degree of infiltration by the tumour and may produce samples which are largely haemorrhagic or necrotic.

Mast cell tumours can also cause significant fibroplasia with most of the lesion demonstrating dense fibrous connective tissue with relatively few mast cells. This is a complication of both fine-needle aspirates (which do not produce a good yield of cells in these cases) and incisional biopsies.

Mast cell tumours can also contain large apocrine gland cysts. If the cysts rupture, there can be extensive surrounding inflammation. Inflammation is also a complication of ulcerated mast cell tumours and can complicate impression smears.

Mast cell tumours can be graded using two grading systems: the Kiupel (two tier) and the older Patnaik (three tier) grading systems. For the time being, pathologists like to grade using both grading systems. For Kiupel low-grade, Patnaik grade 1 tumours and Kiupel high-grade, Patnaik grade 3 tumours this provides clear and useful information about prognosis and treatment options. For those tumours that fall between classic low and high grade (Patnaik grade 2 tumours) the outlook is less clear. To assist with these tumours, we can use immunohistochemical prognostic markers Ki67 and c-kit to assist with prognosis and forward planning for therapy.

Both the Patnaik and newer Kiupel grading systems for cutaneous mast cell tumours do not apply to deeper subcutaneous tumours. Most subcutaneous mast cell tumours have a favourable prognosis and show extended survival times with low rates of recurrence and metastasis. The mitotic index (the number of mitotic figures per 10 high-power fields), infiltrative growth patterns and presence of multinucleate cells indicate a poorer prognosis.