Mammary tumors can occur commonly in intact cats and dogs or those spayed later in life. Malignancy is also common; therefore, early detection and surgical removal can be important in reducing morbidity and mortality. Understanding the biologic behavior of these tumors helps the clinician determine the ideal staging and surgical doses for each case. Anatomical review Dogs have eight to 12 mammary glands, with most having 10 arranged in pairs. The first two to four glands are usually smaller and are considered thoracic glands. Most dogs have four abdominal glands and two larger inguinal glands. Cats typically have eight glands, with the inguinal glands absent.1 Lymphatic drainage of the glands is clinically significant when considering staging and metastasis. Glands one and two may drain to the axillary, superficial cervical and ventral thoracic lymph centers. The abdominal glands, three and four, drain into the axillary and inguinofemoral lymph centers. Finally, glands four and five drain to the inguinofemoral, medial iliac, and popliteal lymph centers.2,3 Lymph node mapping should be considered when treating a malignant tumor. Evaluation and characterization The acronym CHANG is nice to use when evaluating a mass lesion and stands for cyst, hematoma, abscess, neoplasia, and granuloma. The differentials for mammary masses include cyst, abscess, lipoma, mast cell tumor, soft tissue sarcoma, and mammary gland disease. Mammary tissue is secretory and only functional and needed during pregnancy and lactation for nursing. Diseases affecting the mammary glands include mastitis, benign non- cancerous lesions, such as cysts, hyperplasia, fibrosis, and finally, benign and malignant neoplasms.History, physical exam characteristics, and fine needle aspiration (FNA) can differentiate these various conditions and establish biologic behavior. Important history questions include neuter status, lactation history, whelping history, duration of lesion, growth rate and changes over time, ulceration, and systemic abnormalities. Characterizing the mass can also help triage the differential list: size, number, location in the tissue (dermal, subcutaneous, deep), consistency (firm, fluctuant), pain, color/ appearance, ulceration, fixed or mobile. This is a three-year-old female intact doodle two weeks post-whelping, presentingwith fever, pain, and purulent discharge from the mammary skin. There is necroticskin and mammary tissue evident. The necrotic tissue was excised, and open wound management was initiated. Culture and sensitivity testing should be performed to prescribe appropriate antimicrobial therapy to eradicate the infection. Once the wound bed is healthy and the infection is resolved, wound closure can be performed. Photo courtesy Dr. Kathleen M. Ham Impact of spay/neuter It is reported mammary tumors are the most common tumor in intact female dogs, with toy and miniature poodles, Spaniel breeds, English setters, pointers, Yorkies, dachshunds, Dobermans, and German shepherd dogs being predisposed.4 The risk of mammary tumors increases in older and intact females. An older study found that spaying prior to the first heat decreases the risk of developing mammary tumors. Hormones, genetics, and growth factors are known risk factors for the development of mammary tumors.5 Although rare and usually benign, male dogs can develop mammary tumors.6 Mammary tumors usually arise from epithelial cells, but some arise from myoepithelial or mesenchymal cells. Therefore, there are many histologic subtypes of mammary tumors. Mammary tumors grow slowly, can be in the subcutaneous tissue, dermis, and may adhere to the body wall. Some tumors may grow large enough to become ulcerated and even abscessed. Mammary inflammatory carcinoma is a rare and highly malignant form of mammary cancer associated with the rapid development of redness, swelling, pain, and severe inflammation in the mammary skin and tissue. It is associated with a poor prognosis, and surgery is not recommended to treat inflammatory carcinoma.7 This is a 10-year-old female spayed spaniel presenting for multiple mammary gland tumors. The dog is positioned and draped for surgery, with the head towards the left and the vulva towards the right. A sterile marking pen has been used to outline the larger tumors, two to five mammary glands are included for resection with 3 cm lateral margins. Following resection of the body wall fascia, the incision is closed by apposing the deep subcutaneous layer with simple interrupted sutures, careful to assess for tension. When positioning these patients on the table, it is beneficial to avoid pulling the limbs excessively. Photos courtesy Dr. Kathleen M. Ham Workup It is very important to palpate the entire mammary chain; 50- 63 percent of dogs have multiple tumors. Roughly 50 percent of canine mammary tumors are malignant, and since animals can have multiple tumors, a dog may have both benign and malignant tumors.8 Up to 70 percent of tumors are reported to affect glands four and five.9 There is a thought benign tumors can become malignant. Palpation characteristics of benign lesions include small (<1-2 cm), well-circumscribed, and firm. Palpation characteristics and history consistent with malignant tumors include rapid growth, poorly defined edges, deep and fixed, and ulceration. One study found 75 percent of tumors <2 cm are benign, and 86 percent of tumors >2 cm are malignant.5,10 In cats, malignant tumors are more common, with rates reported as high as 80-90 percent.11 This is an eight-year-old spayed female cat presenting for a staged bilateral mastectomy. The head is to the left and the vulva to the right. The left mammary chain was previously removed one month prior. A sterile marking pen outlined the mammary tumor and margins. The mammary chain is removed to the body wall fascia. Once the incision is closed, pain management is a significant consideration for these radical surgeries. Multimodal analgesia is recommended, combining local blocks with continuous rate infusions using opioids and NMDA antagonists. Photos courtesy Dr. Kathleen Ham Cytology When a mass is detected, cytology of the tumor can help determine if the mass is mammary versus another type of skin cancer, such as a mast cell tumor. Cytology can be helpful to determine the criteria of malignancy, but benign results cannot completely exclude malignancy.12 Staging All cats and dogs suspected of having a malignant mass based on the palpation characteristics and history should have preoperative staging. Staging involves evaluating lymph nodes, which may include palpable and non-palpable lymph centers. Lymph node evaluation can be performed with FNA or imaging using ultrasound or CT scan. Thoracic and abdominal imaging are used to assess distant metastasis. In dogs where a benign mammary tumor is suspected, staging can be skipped and performed if needed following histopathology results. This mass has multiple characteristics of being malignant, it is >3 cm, ulcerated,fast growing, and fixed. This patient should be staged prior to surgical removal as the mass is most likely malignant. Following staging, surgical removal of the mass and any affected lymph nodes would be recommended. Photo courtesy Dr. Kathleen Ham Treatment Surgery is the treatment of choice for both benign and malignant mammary tumors, excluding inflammatory carcinoma. In dogs, the surgery dose is based on benign/ malignant, size and palpation characteristics.13 For tumors <1 cm, a lumpectomy or mammectomy can be performed with 1-2 cm lateral margins dissecting to the body wall fascia. Tumors that are 1-3 cm and movable can be excised with 2-3 cm lateral margins, dissecting to the body wall fascia. A mammectomy or regional mastectomy is performed for tumors 1-3 cm or if there are multiple tumors within those glands. When the mass is fixed, the body wall needs to be excised with the mass. In all cats or in dogs, when there are multiple tumors or large tumors, radical mastectomy should be performed. In cases where a bilateral mastectomy is required, a staged unilateral mastectomy is recommended to reduce complications associated with tension. Animals that are intact should also be spayed to remove the hormones for future masses.14 When performing a regional or radical mastectomy, inguinal and/ or axillary lymph nodes can be removed with the same incision. If other lymph nodes were abnormal during staging, they should also be removed at the time of surgery. Following staging and histopathology, the Tumor Node Metastasis classification can be used to determine recommendations for adjunctive therapy.4 Chemotherapy is the most common adjunctive treatment used in cats and dogs. Prognosis Size has been found to be prognostic. Bigger tumors are more likely to be malignant, and animals with tumors <3 cm had better survival times. Duration is also prognostic, with animals having a tumor longer having reduced survival times and increased rates of metastasis. Interestingly, a recent study found that most mammary gland tumors diagnosed incidentally are benign.15 Those animals with lymph node or distant metastasis had reduced survival times. Survival rates are lower for animals with higher Tumor Node Metastasis stages.5 Kathleen M. Ham, DVM, MS, DACVS (SA), graduated from Michigan State University College of Veterinary Medicine in 2004 and then completed a rotating internship, surgery residency, and master’s degree at the University of Illinois. After spending several years in specialty practice, she joined the surgery team at Ohio State University, was an associate professor, and won the Sydney Award for patient care. Dr. Ham moved back to Michigan to work in specialty practice at Michigan State University. In 2020, she transitioned to the University of Florida CVM and became the section chief of Soft Tissue Surgery. Ham’s areas of expertise and research include SmallAnimal parathyroid disease, wounds and reconstruction, surgical oncology, and upper airway disease, including BOAS. References Miller's anatomy of the dog: 5th Edition. Miller and Evan's anatomy of the dog. 2020:1004 pp. Collivignarelli F, Tamburro R, Aste G, et al. Lymphatic Drainage Mapping with Indirect Lymphography for Canine Mammary Tumors. Animals (Basel). Apr 13 2021;11(4)doi:10.3390/ani11041115 Pimentel PAB, da Silva PHS, de Sena BV, Flecher MC, Cassali GD, Horta RDS. The role of lymph nodes and their drainage in canine mammary gland tumours: Systematic review. Res Vet Sci. Mar 2024;168:105139. doi:10.1016/j.rvsc.2024.105139 Withrow & MacEwen's Small Animal Clinical Oncology, 6th edition. vol 81. 2020:391-391. Sleeckx N, de Rooster H, Veldhuis Kroeze EJ, Van Ginneken C, Van Brantegem L. Canine mammary tumours, an overview. Reprod Domest Anim. Dec 2011;46(6):1112-31. doi:10.1111/j.1439-0531.2011.01816.x Miranda FF, Borges LAB, Nakagaki KYR, Abreu CC, Cassali GD. Mammary neoplasms in male dogs: A 24-year descriptive study. Top Companion Anim Med. May 13 2025;67:100980. doi:10.1016/j.tcam.2025.100980 Silva DMD, Kluthcovsky LC, Jensen de Morais H, et al. Inflammatory Mammary Carcinoma in a Male Dog-Case Report. Top Companion Anim Med. Dec 2019;37:100357. doi:10.1016/j.tcam.2019.100357 Pecile A, Groppetti D, Ferrari R, et al. Solitary and multiple simultaneous malignant epithelial mammary tumours in dogs: An explorative retrospective study. Res Vet Sci. Mar 2021;135:153-161. doi:10.1016/j.rvsc.2021.01.014 Vazquez E, Lipovka Y, Cervantes-Arias A, et al. Canine Mammary Cancer: State of the Art and Future Perspectives. Animals (Basel). Oct 09 2023;13(19)doi:10.3390/ani13193147 Rasotto R, Berlato D, Goldschmidt MH, Zappulli V. Prognostic Significance of Canine Mammary Tumor Histologic Subtypes: An Observational Cohort Study of 229 Cases. Vet Pathol. Jul 2017;54(4):571-578. doi:10.1177/0300985817698208 Morris J. Mammary tumours in the cat: size matters, so early intervention saves lives. J Feline Med Surg. May 2013;15(5):391-400. doi:10.1177/1098612X13483237 Simon D, Schoenrock D, Nolte I, Baumgärtner W, Barron R, Mischke R. Cytologic examination of fine-needle aspirates from mammary gland tumors in the dog: diagnostic accuracy with comparison to histopathology and association with postoperative outcome. Vet Clin Pathol. Dec 2009;38(4):521-8. doi:10.1111/j.1939-165X.2009.00150.x Hörnfeldt MB, Mortensen JK. Surgical dose and the clinical outcome in the treatment of mammary gland tumours in female dogs: a literature review. Acta Vet Scand. Mar 11 2023;65(1):12. doi:10.1186/s13028-023-00674-1 Kristiansen VM, Nødtvedt A, Breen AM, et al. Effect of ovariohysterectomy at the time of tumor removal in dogs with benign mammary tumors and hyperplastic lesions: a randomized controlled clinical trial. J Vet Intern Med. 2013;27(4):935-42. doi:10.1111/jvim.12110 Murphy CB, Hoelzler MG, Newgent AR, Botchway A. Incidentally diagnosed mammary gland tumors are less likely to be malignant than nonincidental mammary gland tumors. J Am Vet Med Assoc. Oct 01 2023;261(10):1-6. doi:10.2460/javma.23.03.0133
