A wide variety of skin tumors occur
in dogs and cats, both benign and malignant. An important principle of surgical management of these
tumors is to establish a diagnosis before the operation. Fine needle aspiration (FNA) is a practical
and reasonably accurate method to biopsy skin masses, and the results allow
clinicians to plan appropriate treatments. For example, benign skin tumors such as epidermal inclusion cysts
require only a marginal excision and routine skin closure. Malignant skin tumors, such as mast
cell tumor (MCT), require extensive tissue resection (e.g. removal of 2-3 cm of
normal tissue with the mass) followed by more complicated reconstruction in
some cases. An adequate deep
margin of normal tissue should be removed with the tumor as well as medial and
lateral margins. Excise a section of the tissue layer below the tumor to
achieve a complete resection. If the tumor is located in the subcutaneous space
remove the muscle or deep fascia below the tumor.
Preoperative Considerations
As is true for any animal with neoplasia,
tumor staging is done prior to surgery to establish the extent of disease.
Appropriate imaging, such as thoracic radiographs and abdominal ultrasound, is
used to examine for metastasis or other unrelated problems. Also carefully examine regional lymph nodes and if enlarged perform FNA. With MCT, administer
preoperative antihistamines such as diphenhdyramine to reduce the inflammation
associated with histamine release by the tumor. The drug can be given either
orally for 1 or more days preoperatively, or parenterally just before surgery. Avoid
excessive manipulation of the tumor before surgery to prevent degranulation of
mast cells and release of histamine.
Be sure to warn owners about
potential complications of surgical removal of MCT. Even with antihistamine
pre-treatment wound complications such as excessive inflammation, seroma, and
dehiscence are possible.
Surgical Technique
After placing the dog or cat under
general anesthesia, perform a wide aseptic preparation of the surgical site.
(Fig 1)
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| Fig. 1: Cutaneous mast cell tumor (circle) over the dorsal thoracic area in a spaniel. (note Figs 3-7 are the same dog as in this picture) |
Use a sterile ruler and marking pen to delineate the mass (Fig 2), then
draw a circle around the tumor that is 2-3 cm from the edge of the mass.(Fig 3)
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| Fig. 2: Sterile surgical marking pen and ruler to map surgical margins around skin tumors. |
 |
| Fig. 3: MCT (inner circle and X) delineated by an outer circle of 2cm margins of normal skin |
Draw lines that taper the incision on each end to make the incision an ellipse
and thus avoid having “dog ears” of skin on the ends.(Fig. 3) Be sure that the
long axis of the resultant incision is parallel to the tension lines in that
area of the body.
Make the incisions on the proposed
lines and continue the dissection into the deep tissues. Avoid dissecting
toward the tumor; as you proceed deeper in the tissues continue to honor the 2
or 3 cm margin originally mapped on the skin. Once the desired layer of deep
margin has been reached, incise the fascia or muscle, lift the tumor and
associated tissue (en bloc), and dissect the block of tissue completely free.(Fig.
4)
 |
| Fig. 4: Intraoperative picture with skin mass and underlying tissue being removed from right to left. Note underlying muscle being removed with the mass. |
Wide excision of skin or
subcutaneous masses frequently leaves large skin defects that can be difficult
to close. When primary closure cannot be obtained due to excessive skin
tension, consider either immediate or staged flap or graft reconstruction. (see
blogs from 3/20/14 on punch skin grafts and 4/1/14 on axial pattern skin flaps)
If local tissues are adequate for closure, use the “Rule of Halves” technique
(see blog from 11/3/14 on closure of elliptical skin incisions). A towel clamp
can be used to initially bring the skin together at the middle of the incision
to make suture placement easier.(Fig. 5)
 |
| Fig. 5: A towel clamp is used to temporarily relieve tension and allow suture closure the incision. |
Close deep tissues at the middle of
the incision first, then continue to place sutures in the rule of halves manner
to achieve complete closure.(Fig. 6, 7)
 |
| Fig. 6: Deep sutures have been placed in the middle of the incision; the next 2 deep sutures will be placed at the arrows in the "Rule of Halves" manner. |
 |
| Fig. 7: Final appearance of closed incision |
Place a closed suction drain if excessive dead space exists in the deep
tissue layers that cannot be closed (see blog from 3/15/14 on Jackson Pratt
drains)
After removal of the mass, ink the
tissue specimen with appropriate dye such as India ink to allow the pathologist
to identify the margins of the excision. Also, place a suture on either the
cranial or caudal aspect of the specimen to further orient the pathologist in
case one of the margins shows incomplete excision of the tumor.
Postoperative Care
Routine supportive care is
administered after removal of mast cell tumors. Avoid NSAIDS administration on
MCT dogs to prevent compounding the gastric irritation from the histamine. Tramadol
is a good alternative postoperative analgesic. Monitor the surgical incision
for swelling or bleeding, and instruct owners to limit exercise and monitor the
surgical site carefully at home. Although most dogs and cats recover without
major systemic complications after removal of MCT, systemic vascular collapse
is possible from massive histamine release in rare cases. Fluid resuscitation
and corticosteroid administration may be necessary to support and stabilize the
patient if this occurs.
Prognosis
Long-term outcome is dependent upon
the histopathologic classification of the MCT. There are 2 major classification
schemes now used by pathologists, i.e., Grade 1, 2 and 3 (1 is low grade, 3 is high grade and 2 is intermediate grade) or a simpler 2 tier system of low-grade vs. high
grade.(1) Regardless of the system used, the higher the grade of MCT the poorer
the prognosis.(1,2) Information on adjunctive therapy such as chemotherapy or
radiation therapy is readily available and may be useful in animals with incompletely
excised or metastatic tumors.(3)
References
1. Sabattini
S, Scarpa F, Berlato D, Bettini G. Histologic grading of canine mast cell tumor:
is 2 better than 3? Vet Pathol. 2015 Jan;52(1):70-3.
2. Donnelly
L, Mullin C, Balko J, et.al. Evaluation of histological
grade and histologically tumour-free margins as predictors of local recurrence
in completely excised canine mast cell tumours.Vet Comp Oncol. 2015 Mar;13(1):70-6.
3.
London CA, Thamm DH. Mast cell tumor. In: Small
Animal Clinical Oncology, eds: Withrow S, MacEwen G, Elsevier, 2013, pg.
335.