Page 805 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 34 Miscellaneous Tumors 783

chromatin. The most characteristic feature is the presence of
numerous discrete clear cytoplasmic vacuoles, often referred to as
a “string of pearls.” In R phase, TVTs contain a higher number
VetBooks.ir of infiltrating lymphocytes. Other round cell tumors, including
lymphomas, mast cell tumors, plasma cell tumors, histiocytomas,
and amelanotic melanomas, are important differential diagnoses
but are generally not confused with TVT on cytopathology.
Histopathology of TVT reveals compact masses of round or
polyhedral cells with slightly granular, vacuolated, eosinophilic
cytoplasm. 187–189,192 The neoplastic cells are arranged in a diffuse
pattern and supported by a thin trabecula of fibrovascular tissue.
Regressing tumors are infiltrated by lymphocytes, plasma cells,
and macrophages. 189 For atypical TVTs, if there is doubt about
the diagnosis, specific molecular techniques can be used (e.g., in
situ PCR of the rearranged LINE–c-myc gene sequence). 222,223
The incidence of metastatic spread of TVT has been reported
as less than 15%. However, in most cases of TVT, tumor staging
is not performed. Therefore the actual metastatic rate might be
higher. Regional LNs should always be evaluated for metastasis by
palpation and cytopathology. A thorough physical examination is
A essential to rule out other possible sites of involvement (i.e., skin,
subcutis, nasal and oral cavities, eye, orbit). Diagnostic imaging is
usually not required except with invasive TVT of the nasal cavity,
orbit, or unusual locations. However, abdominal ultrasound may
be used to image regional LNs. Complete blood count (CBC),
serum biochemistry profile, and urinalysis do not reveal specific
changes. Dogs bearing a large tumor burden of TVT have been
associated with a paraneoplastic erythrocytosis that may require
temporary symptomatic therapy. 189

Therapy
TVTs will respond to many forms of therapy; however, chemo-
therapy is the most effective. Single-agent vincristine (0.5–0.7 mg/
2
m or 0.025 mg/kg intravenously [IV], once weekly for 3–6
treatments) results in a complete and durable response in 90%
to 95% of treated dogs. 186,190,196,197,200,232,238,241,244–248 Other
single-agent and combination multiagent protocols employing
B
cyclophosphamide, vinblastine, methotrexate, l-asparginase, and
• Fig. 34.6 Typical appearance of external genitalia affected by canine prednisolone have not demonstrated superiority to vincristine
transmissible venereal tumor. (A) Female: irregular, friable, bleeding cauli- alone. 244,245 Resistant cases can be treated with DOX (25–30 mg/
2
flowerlike mass on the vagina. (B) Male: multinodular, friable cauliflowerlike m IV, every 21 days for three treatments). 190,195 Theoretically,
mass at the base of the penis. (Reprinted with permission from Ostrander if/when immune checkpoint inhibitors become available in vet-
EA, Davis BW, Ostrander GK. Transmissible tumors: breaking the cancer erinary medicine, they may be effective against TVT because it is
paradigm. Trends Genet. 2016;32:1–15.) an immunogenic tumor with regression correlated with immune
cells and inflammation. The immune modulators might be used
to treat chemotherapy resistant TVT or to reduce the dose of che-
Diagnostic Techniques and Workup motherapy needed to achieve remission.
RT has demonstrated efficacy against TVT. In a study using
A presumptive diagnosis of TVT can be obtained based on history orthovoltage RT at total doses of 1000 to 3000 cGy, all 18 dogs
(including travel), signalment, clinical signs, and physical findings treated responded with a complete and durable response, with
in dogs with the classic presentation. Definitive diagnosis is based seven dogs requiring a single fraction of 1000 cGy and 11 dogs
on cytologic examination of cells obtained by swabs, FNAs or requiring two or three fractions. 249 Another study using megavolt-
imprints of the tumors or histologic examination of a biopsy from age radiation (Co60) reported all 15 dogs achieving complete and
the mass. TVT is described as a discrete (or round) cell tumor. durable responses with three fractions administered over 1 week,
TVT has a characteristic morphologic appearance on cytopathol- for an average total dose of 1500 cGy. 196 Therefore RT can be
ogy and is often diagnosed without the need for histopathology considered an effective treatment for TVT, particularly for lesions
(see Fig. 7.37). Exfoliative cytology demonstrates uniform dis- showing resistance to chemotherapy or located in sanctuary sites
crete round to polyhedral-shaped cells with moderately abundant from chemotherapy (i.e., brain, testicle, eye).
pale blue cytoplasm and an eccentrically located nucleus, with Surgery can be an effective treatment for small localized
occasional binucleation and mitotic figures. 187–189,192 Single or TVT; however, surgery has an overall recurrence rate of 30% to
multiple nucleoli are often observed, surrounded by clumped 75%. 248,250,251 Marginal surgical excision is not effective, and it