Page 38 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 1 The Etiology of Cancer 17

Surgery and Implanted Devices TABLE 1.1 Tumors That May Be Influenced by Spay/
Neuter Status
The development of sarcomas at the site of metallic implants
VetBooks.ir has been reported in people, dogs, and laboratory animal mod- Tumor Types Possibly Influenced Concerning Breeds
; however, it often is difficult to discern whether sarcoma
219–221
els
development is related to implants or to other factors, including Tumor with Increased Risk After Castration
Cardiac tumors
All
wound healing complications and osteomyelitis. Three large vet-
erinary studies have examined the relationship between surgical Osteosarcoma All, purebred dogs, rottweilers
implants and tumor development in dogs. The first reported on (<1 yr of age at castration)
222 dogs that developed tumors of any kind after fracture fixa- Prostatic epithelial tumors All
tion, compared with 1635 dogs that underwent fracture fixa- (including transitional cell)
tion without subsequent tumor development. 220 The authors
concluded that the use of metallic implants was not a risk fac- Urinary bladder transitional All
tor for bone tumor development. A second case-control study, cell carcinoma
using the Veterinary Medical Databases (VMDB), included more Lymphoma All, golden retrievers (<1 yr of
than 19,000 dogs with fracture fixation; this study also found no age at castration)
increased risk of a case reappearing in the VMDB a minimum Tumors with Decreased Risk After Castration
of 2 years later with a diagnosis of OSA. 222 This was compared
with dogs undergoing open joint reduction. The second report Testicular All
was a bi-institutional cohort study that evaluated more than 2400 Tumors with Increased Risk After Spay
dogs undergoing tibial plateau leveling osteotomies (TPLO), Cardiac tumors (including All
with more than 1 year of follow-up. 223 The authors concluded hemangiosarcoma)
that the incidence of OSA at TPLO sites is rare and did not find
an increased association between infection or fracture and tumor Osteosarcoma All, purebred dogs, rottweilers
(<1 yr of age at spay)
development. The median time to occurrence of OSA after TPLO
was approximately 4.5 years, longer than tumors occurring at sites Splenic hemangiosarcoma All, Vizslas, golden retrievers
distant to TPLO. The investigators postulate that the pathogenesis (>1 yr of age at spay)
for implant-associated tumors was different from that of sponta- Mast cell tumors All, Vizslas, golden retrievers
neously developing tumors. 223
Newer reports have tried to establish a cause and effect rela- Lymphoma All
tionship between implants and secondary tumors 224,225 ; however, Tumors with Decreased Risk After Spay
the small number of cases and questionable controls prevent these Ovarian tumors All
studies from providing a definitive link. Other types of implants
and foreign materials related to surgery sporadically are impli- Uterine tumors All
cated in carcinogenesis in human and veterinary case reports. Canine mammary tumors (spay All
Published examples include dogs developing sarcomas associated before third estrus)
with pacemakers, and multiple dogs with tumor formation caused
by retained surgical swabs or gauze. 226–230 Although a true cause Feline mammary tumors (spay All
and effect relationship has yet to be determined, neoplasia at the before third estrus)
site of a previous surgery should be a differential in animals with Reproduced with permission. Smith AN. The role of neutering in cancer development. Vet Clin
new pain or lameness. North Am Small Anim Pract. 2014;44:965–975.

Asbestos

Asbestos exposure is a known risk factor for the development of some cancers, recent breed-specific reports have shown a poten-
mesothelioma in people. 231 In fact, an estimated 60% to 88% tial association between neutering and cancer development (Table
of all cases of human mesothelioma are attributable to asbestos 1.1). 235 In the next section, the authors discuss organ-specific rela-
exposure. 231 A similar association has been found for dogs whose tionships between gonad status, potential hormone-related patho-
owners have an asbestos-related occupation or hobby. 232 This asso- genesis, and tumor development. However, although spaying and
ciation was further supported by a study in which significantly neutering may be associated with an increased risk of cancer in
more asbestos bodies were found in dogs with mesothelioma than some breeds, if we extrapolate that information to the entire pet
in control dogs. 233 Pericardial mesothelioma was reported in five population, the risk does not outweigh the issue of overpopulation
golden retrievers with histories of chronic idiopathic hemorrhagic and multiple, life-threatening diseases.
pericardial effusion, suggesting that other factors, including breed
predispositions and chronic inflammation unrelated to asbestos Canine Mammary Cancer
exposure, may be involved in the etiology of mesothelioma affect-
ing the pericardium. 234 Canine mammary cancer is a well-established model of hormonal
carcinogenesis in domestic animals (see Chapter 28). The most
Hormonal Factors common neoplasm of female intact dogs, 235 mammary tumors
affect approximately 260 in 100,000 dogs in the United States
The debate about spaying and neutering pets has increased over each year. 194,236 Dogs spayed before their first estrus cycle have a
the past decade. Although neutering may be protective against greatly reduced risk of developing breast cancer; the risk rises to