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FNA cytology is an inexpensive diagnostic test that can be done in‐house to confirm the diagnosis of cutaneous MCTs in approximately 90% of cases. Cytologically, MCTs consist of large round cells with central nuclei and abundant cytoplasm. The cytoplasm contains intracytoplasmic granules that stain purple with methanolic Romanowsky stains (e.g. May‐Grunwald‐Giemsa, Wrights) (Figure 4.6).

Figure 4.4 Range of cutaneous mast cell tumor appearances. (a) Ulcerated grade III MCT. (b) Grade II MCT over the mandible area.

Figure 4.5 Darier’s sign secondary to vasoactive substance release of an MCT.

In clinical practice, rapid aqueous Romanowsky stains (e.g. Diff Quik) are commonly used, but these stains may not adequately stain mast cell granules.

Other inflammatory cells such as eosinophils and neutrophils are frequently observed mixed with the MCT cells on cytologic examination.

Historically, cytology was not able to predict histological grade. Cytological grading systems have now been developed and evaluated to determine if FNA cytology can provide accurate information regarding MCT grade prior to definitive excision (Camus et al. 2016; Hergt et al. 2016; Scarpa et al. 2016). The cytograding system correctly predicted the histological grade with an accuracy of 94%, sensitivity of 84.6–88%, and specificity of 94–97.3%.

Incisional or excisional biopsy is required to provide enough tissue to determine the histologic grade of cutaneous MCTs. An incisional biopsy, obtained via wedge, skin punch, or needle core techniques can be done to establish MCT grade if negative prognostic factors are present or the surgical site is not amenable to wide surgical resection (e.g. distal extremity) to determine if a more conservative marginal excision is appropriate (e.g. for a low‐grade tumor) or if or more radical surgery or other adjunctive therapies (e.g. radiation and/or chemotherapy) are indicated for higher grade MCTs. Incisional biopsy grade has been shown to have a high concordance (92–96%) to definitive excisional grade. and are sufficiently accurate for differentiating low‐grade from high‐grade MCTs (Shaw et al. 2018).