Page 37 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

16 PART I The Biology and Pathogenesis of Cancer

carcinogens in the same setting makes discerning the “smoking been conducted in an attempt to discern links between magnetic
gun” a difficult task. Nonetheless, the study of animals as sentinels fields and a variety of human cancers, ranging from hematopoietic
malignancies to breast cancer. The extremely-low-frequency mag-
of environmental health hazards has been recommended and pro-
VetBooks.ir vides supportive evidence for carcinogenic risk assessment across netic fields (<60 Hz) in question are ubiquitous in today’s society
The results of a hospital-based case-control
172,189–192
and are generated by household appliances, industrial machinery,
species.
study conducted in Naples, Italy, and nearby cities with known and electrical power lines. Because pets share our environment
high levels of illegal waste dumping suggest that living in these and have similar exposure to magnetic fields, a similar risk of can-
sites of waste emission increases the risk of cancer development cer development has been presumed for companion animals. In a
in dogs, but not cats. This may relate to reduced exposure of cats 1995 study the risk for the development of lymphoma was found
to environmental carcinogens, because they are more often exclu- to be highest in dogs from households with the highest measured
sively indoor pets. 159 exposure to magnetic fields. 208 The risk was related both to the
duration and the intensity of exposure and was highest for dogs
Physical Factors that spent more than 25% of the day outdoors. The next year, at
the request of Congress, a report was published by the National
Sunlight Research Council (NRC) that reviewed more than 500 studies
on the subject of cancer risk and exposure to electromagnetic
The relationship between sunlight exposure or ultraviolet (UV) fields. 209 The report concluded that, although a weak association
irradiation and the subsequent development of skin cancer is one has been shown between the development of childhood leukemia
of the better known examples of physical carcinogenesis. Rec- and exposure to electromagnetic fields, no clear evidence exists to
ognized for its role in human SCC induction, sunlight also has suggest that exposure to electromagnetic fields is a true threat to
been implicated as a cause of SCC in domestic animals and live- human health. To the authors’ knowledge, no reports on a possible
stock. 193–196 In particular, light skin pigmentation and chronic link between magnetic fields and cancer in companion animals
sun exposure are associated with the development of facial, aural, have been published since the 1995 report, although the magnetic
and nasal planum SCC in white or partially white cats and may field debate continues in the human literature. The NRC report
play a similar role in some cutaneous SCC lesions in dogs. 197–199 suggested that other factors, including air quality and proximity
The portion of the UV spectrum most likely to be responsible for to high traffic density, may be more likely environmental causes of
nonmelanotic skin lesions in people and animals is UV B (UV- cancer than low-frequency magnetic fields.
B), which is in the range of 280 to 320 nm. Cumulative long-
term exposure to UV-B may induce skin tumors directly through Radiation
genetic mutations, including mutations in p53, and indirectly
by impairing the response of the immune system to tumor anti- The first report of cancer development after therapeutic irradia-
gens. 200–202 Pets are at greatest risk of exposure to UV-B during tion in a dog dates back more than 30 years, when orthovoltage
the midday hours and should be protected from this exposure, radiation was considered state of the art. 210 At that time the term
especially if they are a lightly pigmented breed. malignant transformation was used to describe the development of
epithelial malignancies at the site of prior irradiation for acantho-
Trauma/Chronic Inflammation matous epulides in four dogs. After a review of more recent cases
involving megavoltage irradiation, the author of the original report
Tumor-promoting inflammation recently was deemed an enabling has suggested that the concept of malignant transformation should
characteristic, because it allows cancer the ability to acquire core be discarded, because the occurrence of second tumors was not
and emerging hallmarks necessary to continue uncontrolled likely due to a true transformation of epulides into carcinomas. 211
propagation. 203 In four dogs with chronic pigmentary keratitis, Rather, the relatively high rate of carcinomas at previously irradi-
neoplastic lesions of the cornea were reported, including three ated sites for epulides is a result of less effective forms of irradiation
SCCs and one squamous papilloma. 204 Although the underlying or misclassification of the tumor type. Radiation carcinogenesis
etiology of the keratitis could not be confirmed, the neoplastic is considered the cause of secondary tumors arising in radiation
transformation likely was related to chronic inflammation. Ear- fields. In human oncology most tumors occurring in heavily irra-
lier reports have linked feline eye tumors to ocular trauma that diated treatment fields are mesenchymal, rather than epithelial, in
induces secondary uveitis and lens rupture (see Chapter 32). 205 origin. 212–214 Several reports of sarcomas occurring in sites of prior
Unlike the corneal tumors reported in dogs with pigmentary irradiation also can be found in the veterinary literature. 215–217 A
keratitis, the ocular lesions in cats were intraocular sarcomas. retrospective review of 57 dogs undergoing definitive megavolt-
Despite the varied histology, the underlying etiology in all cases age radiation therapy with 205 cobalt photons for acanthomatous
was thought to be related to inflammatory changes. Chronic epulides 211 described the development of a second tumor (one sar-
inflammation also has been suggested as a cause of intraocular coma and one OSA) in two dogs, occurring 5.2 and 8.7 years after
sarcoma in the domestic rabbit. 206 Another companion animal the initial treatment, respectively. The overall incidence of second
malignancy thought to be associated with inflammation is injec- tumors (3.5%) was lower than in previous reports. The report sug-
tion site sarcoma in the cat. This tumor type and its etiology are gests that the risk of second tumors at sites of radiation therapy
discussed in detail in Chapter 22. is primarily of clinical concern for young dogs expected to enjoy
long-term survival. Secondary tumors have been reported in at
Magnetic Fields least 36 people who have undergone stereotactic radiosurgery; thus
the risk for a secondary neoplasm is low but not zero. 218 As this
Nearly 40 years ago a potential link was proposed between radiation technique becomes more commonplace in veterinary
chronic low-dose exposure to magnetic fields and the develop- medicine, the possibility of second tumors may need to be consid-
ment of childhood cancer. 207 Since then multiple studies have ered in companion animals undergoing stereotactic radiosurgery.