tumor with planned reconstructive skin fold transposition flap."/>
Figure 4.3 Surgical en bloc resection of cutaneous tumor with planned reconstructive skin fold transposition flap. (a) Preoperative skin marking of proposed en bloc tumor excision and axial skin fold transposition flap. (b) En bloc tumor excision. (c) En bloc tumor excision completed with deep margins. (d) Flank fold transposition flap raised to close tumor resection site. (e) Completed tumor resection and reconstructive procedure closure.
Source: Images courtesy of Dr. Julius Liptak.
The majority (65–80%) of cutaneous MCTs is solitary. Approximately, 50–60% of canine cutaneous MCTs occur on the trunk, with 25% occurring on the limbs and the remainder on the head and neck areas. All MCTs are locally aggressive but low‐ and intermediate‐grade MCTs have a lower metastatic potential than high‐grade MCTs. The gross appearance of cutaneous MCTs is very variable, ranging from raised hairless masses to aggressive, invasive ulcerated lesions (Figure 4.4).
Owners may report that MCT masses fluctuate in size over a short period of time. Local and paraneoplastic effects can result from release of inflammatory mediators contained within the MCT cytoplasmic granules.
MCT palpation or manipulation may cause release of histamine, leading to local redness, swelling, and pruritis. This can progress to degranulation of mast cells, producing erythema, edema, and wheal formation (Darier’s sign) (Figure 4.5).
Histamine release can cause gastrointestinal ulceration by stimulating H2 receptors on parietal cells of the stomach and increased secretion of hydrochloric acid.
Diagnosis of MCTs
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).
Clinical Staging
A modified version of the WHO TNM clinical staging scheme is used to stage canine cutaneous MCTs into one of four stages (Table 4.2) (Turrel et al. 1988).
Preoperative staging is important to determine prognosis and appropriate treatment options, including surgical dose, radiation, and chemotherapy. A minimum database of a complete blood count, serum biochemistry, and urinalysis is indicated as part of a presurgical workup for any patient with cancer, including MCTs.
Preoperative imaging of the cutaneous MCT mass with ultrasonography, computer tomography, or MRI can facilitate definition of anatomical margins, especially deep margins, prior to surgery and assist with surgical planning if reconstructive procedures are planned. Assessment of size and shape of intracavitary lymph nodes can also be obtained from CT or MRI imaging.
The metastatic pathway for cutaneous MCTs is first to the sentinel and regional lymph nodes and then to distant sites of spleen, liver, or bone marrow (Pizzoni et al. 2018). Complete staging for cutaneous MCT should include FNA cytology of the sentinel or other regional lymph nodes, abdominal ultrasound and FNA cytology of the spleen and liver and (in some cases) bone marrow cytology (Warland et al. 2014).
Figure 4.6 Cytological appearance of canine cutaneous MCT showing degranulation and intracytoplasmic granules.
Table 4.2 WHO Clinical staging system for canine mast cell tumors.
Adapted from: London, C.A. and B. Seguin. 2003. Mast cell tumors in the dog. Vet Clin N Am Small Anim Pract 33(3):473–489.
| Clinical stage | Description |
|---|---|
| 0 | Single tumor, incompletely excised from dermis |
| I | Single tumor confined to dermis without regional lymph node involvement |
| II | Single tumor confined to dermis with regional lymph node involvement |
| III | Multiple dermal tumors or large infiltrating tumors, with or without regional lymph node involvement |
| IV | Any tumor with distant metastases or recurrence with metastases (including blood or bone marrow involvement) |
Substage
