Use of a reverse saphenous skin flap for the excision of a grade II mast cell tumor on the hind limb of a dog

Abstract

A 7-year-old, neutered, male Labrador retriever was diagnosed with a tarsometatarsal grade II mast cell tumor. Metastasis was identified in the popliteal lymph node. Amputation was not an option. A reverse saphenous skin flap was used to cover the skin defect caused by excision of the tumor. Surgery was followed with adjunctive chemotherapy.

A 7-year-old, neutered, male Labrador retriever was referred to the small animal hospital of the Université de Montréal for an ulcerated mass that had appeared recently on the left tarsus. The mass had been noticed by the owners a month earlier and had not changed in appearance since its discovery. Fine-needle aspirates of the mass and popliteal lymph node, obtained by the referring veterinarian, revealed a mast cell tumor (MCT) and eosinophilic inflammation of the node. The dog had worked for the past 5 y as an assistance dog for the owners, both of whom were in wheelchairs, and it had no previous history of medical problems.

Physical examination revealed a 3- to 4-cm, firm, erythematous, ulcerated, subcutaneous mass on the caudolateral aspect of the left tarsometatarsal region and mild left popliteal lymphadenopathy. A 3-cm, soft subcutaneous mass was also noted on the left side of the thorax. Thoracic and abdominal radiographs showed no evidence of metastasis. Results from a complete blood cell count and serum biochemical analysis were all within normal limits. A buffy coat analysis detected no evidence of metastatic mast cells. A wedge biopsy of the mass was obtained under local anesthesia in order to establish its histologic grade before any further treatment was prescribed. Diphenhydramine hydrochloride (Sabex, Boucherville, Quebec), 1 mg/kg body weight (BW), was administered 15 min prior to biopsy to avoid any anaphylactic reaction secondary to mast cell tumor degranulation (1). Microscopic examination of the biopsy identified a grade II MCT.

Surgical treatment options for a grade II MCT were discussed with the owners. Amputation or aggressive surgical excision of the mass combined with radiation therapy was proposed. Because of the dog's assistance functions, amputation was not considered as an option. Radiation therapy was not feasible, as it was not available in the province and the owners could not readily travel. A surgical excision with adequate margins and direct skin apposition was impossible due to location and size of the mass, so it was decided that an aggressive surgical excision with wide margins and a reverse saphenous axial skin flap (9) would be the best option for maximizing the chances of attaining adequate margins while preserving maximal locomotor function. The mass was injected with 1 mg triamcinolone (Kenalog; Westwood-Squibb, Montreal, Quebec) to achieve a reduction in its size prior to surgery, which was scheduled for the following week.

The dog was presented for surgery 1 wk following the triamcinolone injection. Upon inspection, the mass had not decreased in size or changed in appearance. The patient was anesthetized, placed in right lateral recumbency, and had its skin clipped and prepared aseptically for surgery. The mass was excised with a 3-cm margin proximally and 2.5-cm margins cranially, caudally, and distally. A maximum of deep subcutaneous tissue was removed en bloc along with the mass. The saphenous vein was preserved. The skin defect created by excision of the mass extended over approximately 65% of the circumference of the limb in the tarsal and metatarsal regions. A 10-cm transverse incision was made 5 cm proximal to the patella on the inner thigh. The incision was extended distally in a slightly converging fashion towards a 4- to 5-cm base. The saphenous artery and vein were ligated and transected at their junction with the femoral artery and vein proximally. After visualization of the cranial and caudal saphenous vessels, the undermining was continued towards the base of the flap. Attention was paid to avoid the tibial nerve and to include part of the fascia of the gastrocnemius muscle with the flap to avoid traumatizing the caudal branch of the distal saphenous vessels. The fibular artery and vein were ligated and transected in the middle of the flap to allow greater mobility for tension-free transposition of the flap. A bridge incision connecting the donor bed with the recipient bed was made craniomedially and the flap was rotated laterally to cover the skin defect. Great care was taken to minimize tension at the base of the pedicle. A wedge biopsy of the left popliteal lymph node, which was accessible from the inner thigh wound (donor site), was taken before closure of the donor bed and flap with several subcutaneous, simple interrupted sutures every 1 or 2 cm, using polyglecaprone 25 3-0. Afterwards, the flap was sutured in place by using a cruciate pattern with nylon 3-0. A noncompressive Robert-Jones bandage was applied on the entire limb to restrain movement without exerting pressure on the flap, to avoid impairing venous return. The tumor and biopsy of left popliteal lymph node were submitted for histopathological analysis.

The dog recovered uneventfully from surgery in the intensive care unit. A fentanyl infusion (Fentanyl Citrate; Abbott Laboratories, Toronto, Ontario), 2 μg/kg BW/h, was administered to provide adequate analgesia for the next 24 h. Postoperative recommendations included exercise restriction and daily bandage changes to monitor the surgical wounds and assess the viability of the flap. Upon removal of the bandage within the 1st wk, the distal part of the flap showed moderate venous congestion, which improved after a few minutes. No signs of necrosis or ischemia were noted, except for a superficial epithelial defect that developed at the most distal part of the flap. The affected area reepithelialized uneventfully.

Results of the biopsies confirmed a grade II MCT with lymph node metastasis. Clean margins had been achieved, but the deep margin was, as expected, only a few millimeters wide. An abdominal ultrasonograph and fine-needle aspirates of the liver and spleen were taken and showed no evidence of metastasis. The clinical stage was classified as grade II due to the metastatic disease in the lymph node.

Fourteen days after surgery, the dog was discharged, with exercise restriction recommended for another week. He was scheduled to return in 7 d for his first chemotherapy treatment and suture removal. Chemotherapy with chloroethylcyclohexylnitrosourea (CCNU) (Lomustine, CeeNU; Bristol-Myers Squibb, Princeton, New Jersey, USA), 60 mg/m², PO, q3wk, was initiated to prevent local recurrence and treat undetected metastasis.

Mass cell tumors represent 20% to 25% of skin tumors in dogs. Their clinical behavior ranges from benign to highly aggressive (1). The most important factor in determining prognosis and treatment is the histological grade (1). Grade I MCTs are composed of well-differentiated and mature cells. They tend to have a low potential for metastatis and systemic dissemination. Grade II MCTs are composed of intermediately differentiated cells. They can either be locally invasive or have moderate metastatic behavior. Grade III MCTs are composed of undifferentiated, immature cells. They are usually highly metastatic (2).

Another important factor in determining prognosis and treatment of MCTs is the clinical stage. Metastasis commonly occurs to regional lymph nodes. Systemic dissemination can also occur; in such cases, splenomegaly and hepatomegaly can be seen (1). Pulmonary metastases are uncommon. In stage I MCTs, a single dermal mass is present without lymph node involvement. A single dermal mass with regional lymph node involvement corresponds to a stage II. Multiple masses or large infiltrating neoplasms correspond to a stage III. When distant metastases are present, the clinical stage is IV. In Grade 0 MCTs, there is no macroscopic mass, but the disease is present microscopically (2).

Treatment for MCTs includes surgery, radiation therapy, chemotherapy, or a combination of these. In cases where the mass can be excised with clean margins and metastatic disease is not present, surgery alone is usually curative. In cases where complete excision cannot be achieved or metastases are present, surgery followed by radiation therapy is the best option. Treatment with surgery, radiation therapy, or both has proven to be curative in some cases (2). When surgery or radiation therapy has not been successful or is unavailable, chemotherapy is recommended to slow systemic metastasis and increase survival time (3). Drugs used in traditional chemotherapy protocols for MCTs include prednisone (4), vincristine (5), vinblastine (6), or a combination of these. Treatment with CCNU is a newer protocol. Rassnick and al (3) reported 42% of measurable response in dogs with high grade MCTs that were treated with CCNU. The response rate is similar to those obtained with prednisone and vinblastine protocols. The same study reported neutropenia as the main toxic effect associated with CCNU treatment.

The excision of a malignant tumor with wide margins on lower extremities often represents a challenge for the surgeon. An axial pattern flap allows immediate skin closure and avoids numerous bandage changes, as well as the complications sometimes associated with 2nd intention healing, such as excessive scarring, contracture, and delayed healing. Axial pattern flaps are composed of the epidermis and the dermis, and also contain the subdermal vascular plexus and nerves. They can be designed to incorporate a direct cutaneous vessel (7). Axial pattern flaps incorporating a direct cutaneous vessel have a better blood supply and, therefore, a survival rate twice that of random subdermal plexus pedicles (8).

The reverse saphenous flap is an axial pattern flap incorporating the craniomedial and caudomedial saphenous vessels. The flap usually extends from the distal third of the inner thigh to a line slightly proximal to the anastomosis between the cranial branch of the medial saphenous vein and the cranial branch of the lateral saphenous vein. The width of the flap must extend beyond the cranial and caudal branches of the saphenous artery for 0.5 to 1.0 cm. To achieve transfer to the donor bed, the flap can be rotated and sutured to a bridge incision or tubed.

Survival of a skin flap depends mostly on the preservation of the vascular tree and on tension-free closure (9). The reverse saphenous flap relies on anastomotic connections between the saphenous vasculature and the surrounding vessels to maintain reverse blood flow after division of the vascular connections with the femoral artery and vein (8). Main anastomoses include the cranial branch of the medial saphenous artery with the superficial branch of the cranial tibial artery, the caudal branch of the medial saphenous artery with the perforating metatarsal artery, and the cranial branch of the medial saphenous vein with the cranial branch of the lateral saphenous vein (9).

Flap survival rates are high with the reverse saphenous flap (9). Possible complications include wound drainage, incision dehiscence, wound infection, and distal flap necrosis. Postoperative care should include frequent bandage changes and movement restriction. The flap should be monitored closely for at least 1 wk before concluding that successful transfer has taken place. Antiobiotic drug administration should be restricted to those animals with a documented wound infection (10).

Despite an increased risk for recurrence when compared with amputation, a possible return to normal function made the reverse saphenous skin flap a valuable treatment option in this case of a grade II stage II MCT involving the tarsus. With metastatic disease present, the addition of CCNU chemotherapy could prolong the survival time.

Twenty-one days after surgery, the dog was back to his normal assistance functions. At his 2nd chemotherapy, 45 d after surgery, no side effects or recurrence of the mass was observed.

Figure 1. Skin flap undermined from the medial thigh.

Figure 2. Donor bed and skin flap sutured.

Figure 3. Donor bed and skin flap wound on day 20.