Mammary gland tumors are the most common type of neoplasia in sexually intact female dogs, representing up to 70% of tumors in this group.1–4 Previous studies3–5 of dogs have shown that benign MGTs are 2 to 5 times as common as malignant ones, whereas another study6 revealed that the incidence of malignant MGTs among female dogs in Norway was 53.3%. Mammary gland tumors are most commonly identified in middle-aged to older dogs, with the peak prevalence observed in dogs between 7 and 11 years of age.7–12 Dogs with malignant MGTs have a higher mean age than dogs with benign tumors (9.5 and 8.5 years, respectively).13 Several breeds, including Poodle (Toy and Miniature), English Springer Spaniel, Brittany, Cocker Spaniel, English Setter, Pointer, Maltese, Yorkshire Terrier, and Dachshund, are reported to have a higher prevalence of MGTs than other breeds.14 Certain mammary gland regions appear more predisposed to tumors than others, with approximately 65% to 70% of identified tumors located in the fourth (caudal abdominal) and fifth (inguinal) glands.4,15
Female dogs neutered prior to the first estrous cycle have a lower risk of malignant MGTs than females that have never been neutered (relative risk, 0.005); those neutered between the first and second estrous cycles or after the second estrous cycle also have a lower risk (relative risk, 0.08 and 0.26, respectively).16 This reduction in risk is thought to be attributable to hormonal influence because both benign and malignant MGTs express estrogen receptors,17 and dogs exposed to progestin have an increased risk of developing mammary tumors.18 Additionally, the use of exogenous estrogen and progestin to prevent estrus in dogs is thought to contribute to an increased risk of malignant MGTs in dogs in many European countries.6,14
Benign MGTs include simple and complex adenomas, mixed tumors, ductal adenomas, fibroadenomas, and ductal papillomas. Additionally, nonneoplastic conditions such as ductal ectasia, cysts of the hair follicles, and tumors that occur in tissue overlying the mammary glands (ie, not of mammary gland origin) may resemble MGTs on physical examination. Malignant MGTs include various forms of carcinoma, fibro- and osteosarcoma, and carcinosarcoma.7,13,19 No differences in the gross characteristics among malignant or benign MGTs or nonneoplastic conditions of the mammary gland region have been reported, with the exception of inflammatory mammary carcinoma, an especially aggressive form of carcinoma, which is rapidly progressive with a high metastatic rate.20,21 Clinical signs of inflammatory mammary carcinoma indicate the presence of intense inflammation (eg, erythema, edema, pain, and warmth of the mammary glands). Cytologic or, more often, histologic examination is required for definitive diagnosis of masses in the mammary gland region; a diagnosis is often followed by treatment with lumpectomy, mammectomy, or radical chain mastectomy.4
Studies7,11 show that > 60% of dogs with MGTs have multiple tumors that often represent different histologic types, suggesting individual tumor formation rather than metastasis. If multiple MGTs arise independently, the presence of multiple tumors at the time of diagnosis should not increase the likelihood that an individual tumor is malignant but may increase a patient's overall risk of having a malignant tumor. To the authors' knowledge, a comparison of the prevalence of malignancy between dogs with a single mass and those with multiple masses in the mammary gland region has not been previously reported. In the study reported here, the prevalence of malignancy (determined by histologic examination of masses after surgical removal) was compared between dogs with single versus multiple masses in the mammary gland region. Our hypothesis was that dogs with multiple masses in the mammary gland region would not have a higher prevalence of malignancy than would dogs with single masses.
Mammary gland tumor
SAS, version 9.4, SAS Institute Inc, Cary, NC.
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