Mammary Glands

Introduction

Mastectomy, the removal of varying amounts of mammary tissue, is the primary method for treating tumors of the mammary gland in dogs and cats. However, the amount of tissue to remove is a subject of some controversy. Cure rates for patients with malignant mammary disease are still low even after massive amounts of tissue have been removed. The biologic behavior of the tumor, not the extent of treatment, determines the eventual fate of the patient. Nonetheless, properly performed surgical treatment of mammary tumors can modify disease progression, prolong comfortable survival, and be curative in some instances.

Surgical Anatomy

Mammary glands in the dog and cat are modified sudoriferous skin glands with an apocrine compound lobuloalveolar structure. Mammary glands are arranged in two parallel paramedian rows from the axillary to the inguinal regions. The glands are surrounded by subcutaneous adipose tissue, which is scant in the thoracic region and abundant in the inguinal region.

Individual glands are discerned by the corresponding teat, although mammary tissue can be confluent between adjacent cranial and caudal glands. The midline separation between mammary chains is distinct. Glands are signified by name (cranial and caudal thoracic, cranial and caudal abdominal and inguinal) or by number (1 through 5 cranial to caudal). Dogs usually have five pairs of mammary glands, and cats have four, although the number can range from four to six in either species.

Thoracic glands adhere directly to the underlying pectoral muscles with little intervening fat or areolar connective tissue. Abdominal glands are loosely attached to the external fascia of the rectus abdominus muscle by connective tissue and fat. Inguinal glands are suspended from the body wall by an extension of the cutaneous trunci muscle. Blood is supplied to the thoracic glands by perforating branches of the internal thoracic artery, by cutaneous branches of intercostal arteries, and by the lateral thoracic artery. Cranial abdominal glands receive blood predominately from the cranial superficial epigastric artery. Caudal abdominal and inguinal glands are supplied by the caudal superficial epigastric artery and by perivulvar branches of the external pudendal artery. Veins parallel arteries, except for numerous veins that traverse the midline, which enlarge during lactation.

Lymphatic drainage of the mammary glands is subject to individual variation and also is influenced by the stage of lactation and by the presence of space-occupying masses. Lymph generally flows from the cranial three pairs of mammary glands toward the axillary lymph nodes and from the caudal two pairs toward the inguinal lymph nodes. A lymphatic connection between the cranial and caudal abdominal glands is present in some bitches.

Mammary Gland Neoplasia: Incidence and Prognosis

Neoplasia is the major indication for mastectomy in dogs and cats. Mammary tumors are the most common type of neoplasia in dogs and the third most common type in cats. Mammary neoplasia affects middle-aged and older animals, with a median age of onset of 10 to 11 years.

Only about half of all canine mammary tumors are malignant, whereas most (86%) feline mammary tumors are malignant. Prognosis for both dogs and cats with malignant tumors is guarded to poor. Although length of survival is inversely correlated with the growth rate of the tumor, the extent of local infiltration, and the status of regional lymph nodes and lungs, the major statistically significant survival factor is tumor volume. Both dogs and cats with large (> 3 cm) malignant mammary tumors have significantly shorter survival times than those with small malignant tumors, emphasizing the importance of early diagnosis and treatment.

Treatment failure is represented by intractable local recurrence or, more commonly, by the development of metastatic disease. Because metastatic mammary cancer is found most frequently in the lungs, thoracic radiography is a common screening test before mastectomy. Dogs with mammary cancer affecting the caudal mammary glands, especially when the inguinal lymph nodes are palpably enlarged, should also be radiographically or ultrasonographically checked for enlarged sublumbar lymph nodes, because metastasis through sublumbar lymphatics is often detectable before the radiographic appearance of lung metastases. Enlarged lymph nodes should be excised at the time of surgery in dogs and the draining lymph nodes routinely removed in cats. Axillary lymph nodes are not routinely removed unless palpably enlarged while the inguinal lymph node is removed when the inguinal mammary gland (#5) is excised. Lymph node removal is regarded as a staging rather than therapeutic procedure in most dogs.

Mammary neoplasia can be prevented by ovariohysterectomy performed when the bitch or queen is young (i.e., before the first estrus). Ovariohysterectomy loses its protective effect after the 4th estrus in the canine. However, although estrogen, progesterone, and other receptors have been found in canine and feline mammary tumors, it is controversial as to whether ovariohysterectomy has any beneficial effect as a treatment for existing mammary neoplasia. The current recommendation is to spay the animal at the time of mammary tumor excision.

Selection of Surgical Procedure

The amount of mammary tissue to remove from a dog or cat with mammary neoplasia is influenced by several factors: the size, consistency, and location of the tumor; the size, age, and physiologic status of the patient; and the beliefs and prejudices of the surgeon. Unfortunately, subjective criteria still play a major role in the selection of a mastectomy procedure because objective data for choice are inconclusive.

The extent of tissue removal with various mastectomy procedures is illustrated in Figure 40-1. For the purposes of this chapter, these procedures are defined as follows:

Lumpectomy (nodulectomy): Removal of the tumor only with 1 to 2 cm of surrounding normal tissue. Generally, lumpectomy is used when a tumor is small, encapsulated, and noninvasive, thus requiring a minimum of surgical dissection for removal.

Partial mammectomy: Removal of the tumor and a surrounding margin of mammary tissue. This procedure usually is indicated for tumors that are small to moderate in size (up to 2 cm in diameter) and occupy only a portion of an individual gland. The tumor may be suspected to be invasive and may or may not have palpable distinct margins.

Simple mastectomy: Removal of the entire mammary gland containing the tumor.

Regional mastectomy (modified radical mastectomy): Removal of groups of mammary glands depending on which glands contain tumor. The rationale for regional mastectomy depends on the presumed anatomy of mammary gland lymphatic drainage and the assumption that mammary cancer spreads from one gland to another along lymphatic pathways, which are not altered by space-occupying masses.

Complete unilateral mastectomy (radical mastectomy): Removal of all ipsilateral mammary glands, intervening tissues, and regional lymphatics.

Complete bilateral mastectomy (bilateral radical mastectomy): Removal of both entire mammary chains, intervening tissues, and regional lymphatics. If performed, a 3 to 4 week interval between sides is recommended to reduce skin tension and postoperative complications.

Available data indicate that the extent of surgery had little influence on either the survival time or the rate of recurrence of mammary cancer in dogs. In other words, no evidence indicates that complete unilateral mastectomy (radical mastectomy) is any more beneficial for treating a 2 cm tumor in the fourth mammary gland of a dog than is a simple mastectomy. Until further data is available, selection of a surgical procedure in dogs is dictated by what is most efficient with the goal of attaining clean surgical margins by complete removal of the tumor. Good oncologic surgical principles still apply, however, regardless of the procedure used, invasive tumor should be widely resected with deep and centrifugal 2 cm en bloc margins of normal tissue with early ligation of blood vessels performed.

In cats, complete unilateral mastectomy is the surgical procedure of choice for all mammary tumors. This approach has been recommended by veterinary oncologists because most feline mammary tumors are highly malignant. The 10 to 15% of cats with benign mammary nodules are overtreated by this philosophy.

Surgery is contraindicated for inflammatory carcinoma of the mammary gland. Inflammatory carcinoma of the mammary gland is a fulminant and aggressive malignant disease. Affected tissues are diffusely thickened, inflamed, painful, and frequently ulcerated. A space-occupying mammary mass may or may not be obvious. Commonly, the tissues are so diffusely thickened that discrete tumors are not apparent. The condition closely resembles severe mastitis and is frequently misdiagnosed as such. Surgery is unrewarding because it is virtually impossible to remove the affected tissues completely, and, more important, because most affected dogs also suffer from disseminated intravascular coagulation. Attempts at extensive surgical therapy often result in severe, intractable bleeding from the incision, deterioration of the patient over 12 to 24 hours, and death. Inflammatory mammary carcinoma is invariably fatal, usually within a month after clinical signs are obvious. Treatment is strictly palliative and consists of antiinflammatory drugs, analgesics, and antibiotics.
 

Surgical Techniques

Mastectomy procedures are performed similarly in cats and dogs, although the laxity of feline skin generally makes surgery easier in cats.

Lumpectomy

A lumpectomy is initiated by making a skin incision directly over the tumor. The mammary tissue overlying the tumor is bluntly separated. The periphery of the tumor is grasped with forceps, and the natural tissue planes adjacent to the isolated tumor are defined by blunt dissection with mosquito hemostats or by wiping the tissues away from the tumor with a sponge (sponge dissection). The tumor is removed, partially sectioned, and placed in 10% buffered formalin. After hemorrhage is controlled, the wound is closed by approximating mammary tissue with fine (4-0) absorbable suture. Skin is closed with suture of the surgeon’s choice.

Partial Mammectomy

A liberal incision is made over the tumor. If the tumor contacts or is adherent to the skin or subcutaneous tissue (i.e., if the tumor is “fixed” to skin), an elliptic incision is made that encompasses both the tumor and the affected skin. An artificial plane of dissection is developed in normal mammary tissue surrounding the tumor. A liberal amount of tissue, often approaching one-third to one-half of the affected gland, is removed. Closure of the defect in the gland is by direct apposition of tissue if possible. Subcutaneous tissue apposition with 4-0 or 3-0 absorbable suture is performed to reduce tension on the skin closure. The skin is closed routinely.

Simple Mastectomy, Regional Mastectomy, and Complete Unilateral Mastectomy

The basic technique for simple mastectomy, regional mastectomy, and complete unilateral mastectomy is the same. All these procedures involve removal of the skin segment that encompasses the affected mammary gland or glands. Surgery is initiated by making an elliptic incision with 2 cm margins around the mammary gland or glands to be removed. The incision is extended sharply through the subcutaneous tissue to the body wall. In the thoracic region, the body wall is represented by the pectoral muscle and in the abdominal region by the external rectus fascia. A plane of dissection that allows the skin segment and associated mammary tissues to be cleanly stripped from the body wall is then established. The proper plane of dissection is deep to the adipose tissue and directly on the muscle fascia.

In the abdominal and inguinal regions, the glands are loosely adherent and can be stripped from the underlying fascia with a sponge (Figure 40-2A). In the thoracic region, the glands adhere to the underlying muscle, and the plane of dissection must be developed by a combination of sharp and blunt dissection with scissors (Figure 40-2B). The proper thoracic plane is represented by lacy but tough strands of fibrous connective tissue. Traction on the rostral portion of the skin segment facilitates dissection. Dissection proceeds from cranial to caudal without, in most cases, the need to damage underlying muscle. When removal of tissues is completed, intact muscle should be clearly visible in the thoracic region and rectus fascia should be seen in the abdominal region.

Invasion of underlying tissue by tumor, whether pectoral muscle in the thoracic region or rectus fascia in the abdominal region, requires en bloc resection of the affected body wall tissue with the tumor. In extreme cases, full-thickness resection of the body wall must be done to remove all visible tumor, even though body wall invasion by tumor is a grave prognostic sign, and even massive surgical resection is seldom curative.

Inguinal gland removal entails en bloc removal of the inguinal fat. Care must be taken to isolate and ligate the caudal superficial epigastric artery and vein, which emerge from the inguinal canal (Figure 40-3). The vaginal process, the finger-shaped protrusion of fat extending through the inguinal canal, along with the artery, vein, and vaginal ligament, may be bluntly separated from the inguinal fat and left behind or ligated and removed. Inguinal lymph nodes are removed along with the skin segment, mammae, and inguinal fat when the dissection is done correctly. Arteries and veins from the pudendal vessels enter the inguinal glands caudally from the tissues around the vulva and may require ligation or cauterization depending on their size.
 

Closure of the tissue defect left after a simple, regional, or complete unilateral mastectomy must account for the considerable dead space created. In most instances, drains, stents, bandages or reconstructive procedures are not necessary. Even large defects can be closed by initially apposing skin edges with subcutaneous absorbable suture. An interrupted pattern is preferred. The subcutaneous tissue may be tacked to the underlying body wall to reduce dead space. Skin is then closed according to the surgeon’s preference. An interrupted cruciate suture pattern of 2-0 or 3-0 monofilament nylon can be placed quickly and distributes tension well (Figure 40-4). This pattern has the advantage of being an interrupted pattern, but one that spans a longer segment of incision per suture than a simple interrupted pattern. Regardless of the suture pattern used for skin closure, the ultimate success of skin apposition depends on placement of a proper subcutaneous suture line.
 

Postoperative Care and Complications

Postoperative complications associated with mastectomy include seroma formation, wound dehiscence, and edema of one or both rear limbs. Seroma formation is most common in the inguinal region and may be treated by the use of warm, moist compresses. Drainage by aspiration helps temporarily but increases the risk of infection. Wound dehiscences, if not extensive, are best left to heal by second intention. Extensive dehiscences may require debridement and delayed or secondary closure.

Rear limb edema may occur because of the surgical procedure or because of lymphatic invasion by the tumor. Removal of inguinal mammary tumors temporarily interrupts lymphatic drainage by removing lymphatic vessels and nodes. Moderate exercise, warm compresses, and time usually result in edema resolution. More ominous is the edema that results from tumor emboli in lymphatic vessels between the mammary glands and the sublumbar nodes and from tumor metastasis to sublumbar lymph nodes.

The latter situations are grave prognostic indications, and whereas edema may subside as potential lymphatic pathways become established, the chances for complete return to normal tissue fluid homeostasis are remote. Blockage of lymphatic vessels or nodes by tumor infiltration sometimes results in “retrograde metastasis.” For example, inguinal tumor may extend distally in a string of nodules on the medial aspect of the hind leg.

Ovariohysterectomy, may improve survival time and may be desired at the time of mastectomy for other reasons. Spaying should performed before the mastectomy, taking care to not contaminate or introduce tumor cells into the abdomen. After the ovariohysterectomy and closure of the linea alba are completed, the skin incision forms part of the medial border of the mastectomy incision and is extended as needed to encompass the mammary gland or glands to be removed. Mammary tumors that extend across the midline should be removed before the ovariohysterectomy.

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