Page 803 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 34 Miscellaneous Tumors 781

in elderly patients (60 years or older) and a gender or race predilec- Ireland, Japan, China, the Far East, the Middle East, and parts of
tion has not been identified. 182,183 A clinicopathologic classification Africa. 186,190 In enzootic areas, where breeding is poorly controlled
and there are high numbers of free-roaming sexually active dogs,
adopted by the World Health Organization (WHO) correlates well
VetBooks.ir with biologic behavior of thymomas. In the WHO system, cells are TVT is the most common canine tumor. 186,190,191,193 In North
America, the prevalence of TVT is correlated with increased rain-
classified as spindle (predominant in the medulla), oval and epi-
thelioid (predominant in the cortex), or dendritic. The tumors are fall and mean annual temperature. 193 Occasional cases occur in
then further divided into medullary, mixed, predominantly corti- regions otherwise free of TVT after travel to endemic areas as a
cal and cortical thymomas, and well-differentiated and high-grade result of tourism. 194 Pets traveling abroad can be exposed to TVT
thymic carcinoma. Medullary and mixed thymomas are considered and carry it back to nonendemic areas; therefore veterinarians may
benign tumors and, even in the face of capsular invasion, will not act as the first line of defense against the introduction of TVT as
recur. Predominantly cortical and cortical thymomas display inter- an emerging disease in nonendemic areas.
mediate aggressiveness and have a low risk of relapse independent Because TVT is primarily spread by coitus, free-roaming, sexu-
of their invasiveness. Well-differentiated and high-grade thymomas ally intact mature dogs are at greatest risk. 186 Dogs of any breed,
are highly invasive and associated with a high frequency of relapse age, or sex are susceptible. 186,190,189 No heritable breed-related pre-
and death. The staging system created by Masaoka uses both sur- disposition has been found. 186,189 In endemic areas, although dogs
gical and histologic signs of invasiveness to describe five different older than 1 year of age are at high risk, TVT is most common in
stages that correlate well with prognosis 182,183 : dogs 2 to 5 years of age. 186 The physical exertions associated with
coitus in the dog with extensive abrasions and bleeding make both
Stage I: Tumor is grossly encapsulated and no capsular invasion is sexes susceptible to injury to the genital mucosa, which facilitates
noted microscopically. the exfoliation and implantation of tumor cells. 186,189 Transmis-
Stage II: Gross invasion occurs to surrounding fatty tissue or me- sion can occur efficiently in either direction between the male dog
diastinal pleura. Microscopic invasion of the capsule is noted. and the bitch. The most common sites of involvement are the
Stage III: Gross invasion into neighboring organs (pericardium, external genitalia, but other sites that can be affected through lick-
great vessels, lungs) ing or sniffing include the nasal and oral cavities, subcutaneous
Stage IVa: Pleural or pericardial dissemination tissues, and the eyes. 186–192,195–200
Stage IVb: Lymphatic or hematogenous metastasis TVT is a transmissible allograft spread directly from dog to
dog across major histocompatibility complex (MHC) barriers,
MG is the most common paraneoplastic syndrome associated through transplantation of viable tumor cells. 201 TVT and Tas-
with thymomas, occurring in 30% to 50% of patients. Red cell apla- manian devil facial tumor disease (DFTD) are the only known
sia and hypogammaglobulinemia occur in 5% to 10% of patients. naturally occurring clonally transmissible cancers that behave
Surgery is the standard-of-care in people with resectable tumors like an infectious parasitic neoplastic tissue graft. 202,203 Similar
and complete surgical resection is the best predictor for longterm to host–parasite interactions, the successful transmission of TVT
survival in people with thymomas. RT is most commonly indi- requires a confluence of multiple tumor and host traits includ-
cated for people with extensive or recurrent disease. A variety of ing environment and behavior to facilitate transfer of tumor cells
chemotherapy drugs have been used to treat inoperable thymomas between hosts, tumor tissue that promotes shedding of large num-
or in cases in which gross residual disease is present after surgery. ber of cancer cells, tumor cell plasticity to survive transmission
Cisplatin, ifosfamide, and prednisone are considered the most and grow in the new host, and permissible host tissue involving
effective agents. In addition, neoadjuvant chemotherapy has been angiogenesis. 204,205
shown to influence longterm survival for thymomas in patients These cancers have evolved into a unique niche by overcoming
with Masaoka stages II and IVa. 182,184,185 the limitations of existing within the single host that gave rise to
the tumors by gaining the ability to spread between individuals
and thus survive long after the original hosts have died.
SECTION C: CANINE TRANSMISSIBLE
VENEREAL TUMOR Pathology and Natural Behavior
TVT was initially recognized in 1876 and was used for the first
J. PAUL WOODS successful experimental transmission of a tumor. 188,189 A number
of characteristics of TVT suggest that the tumor originated in
Incidence and Risk Factors inbred wolves or dogs about 10,000 to 15,000 years ago, around
the time that the dog was domesticated. Subsequently, the tumor
Canine transmissible venereal tumor (TVT), also known as spread worldwide. 206 TVT has evolved into a transmissible para-
transmissible venereal sarcoma and Sticker’s sarcoma, infectious site representing the oldest known colony of cloned somatic mam-
sarcoma, venereal granuloma, canine condyloma, transmissible malian cells in continuous propagation.
sarcoma, and transmissible lymphosarcoma, 186,187–189 is a natu- Tumor growth generally appears on the external genitalia or
rally occurring, horizontally transmitted infectious histiocytic nasal or oral mucosa within 2 to 6 months of mating and can
tumor of dogs usually spread by coitus, but it may also be spread either grow slowly and unpredictably for years or grow invasively
by licking, biting, and sniffing tumor-affected areas. 186,187,190–192 and eventually become malignant and metastasize. 186–190 Extra-
It has been observed occasionally in other canids, such as foxes, genital lesions can occur both alone (in isolation) and in associa-
coyotes, and jackals. 186,190 tion with genital lesions; however, it has been suggested that in
Although TVT has a worldwide distribution, its prevalence is most cases neoplastic foci can be detected on the genitalia.
highest in tropical and subtropical areas, particularly in the south- TVT usually remains localized, but metastasis occurs in up to
ern United States, Central and South America, southeast Europe, 5% to 17% of cases to regional lymph nodes (LNs), subcutaneous