Page 802 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

780 PART IV Specific Malignancies in the Small Animal Patient

The cytologic diagnosis of thymoma is confirmed with the Two retrospective studies have evaluated RT for the treatment
presence of neoplastic epithelial cells. These are often accompa- of thymomas. Seventeen dogs and seven cats with thymoma were
treated with RT alone or as adjunctive therapy. Twenty cases
nied by mast cells and variable numbers of small mature lympho-
VetBooks.ir cytes. 141,166,167 Unfortunately, nondiagnostic samples are common were available for followup with a 75% response rate (11 partial
responses and 4 complete responses). The MSTs for dogs and cats
because of a small percentage of neoplastic epithelial cells resulting
in the presence of only small mature lymphocytes or because of were 248 days and 720 days, respectively. In this study, the total
the presence of cysts within the mass. In one study, neoplastic radiation dose (15–54 Gy) and treatment interval (from daily to
epithelial cells were cytologically evident in only 61% of cases. once weekly) varied markedly and may have affected the response
3
The diagnosis is further complicated because of the fact that both rate and duration of responses. To additionally confound the
lymphoma and thymoma may be composed mainly of small lym- effects of RT, only five dogs received RT alone. The remaining
phocytes. In three studies, a presumptive diagnosis of thymoma patients were treated with adjuvant RT after surgery, prednisone,
was made in approximately 20%, 40%, and 77% of mediastinal and/or chemotherapy. 176 The second study evaluated eight dogs
masses after FNA cytology. A unique feature of thymoma, Hassal’s treated hypofractionated RT alone (48–49 Gy total dose, once
corpuscles, are cytoplasmic structures present in thymocytes that weekly, for 6–7 weeks). The overall response rate was 50% and the
can be used to aid on cytologic diagnosis. Unfortunately, these 1-year survival rate was 75%. 177
corpuscles are not usually visualized in Wright’s Giemsa prepara- The role of chemotherapy in the management of thymomas
tions in comparison to hematoxylin–eosin used for formalin-fixed has not been defined in cats and dogs. In a recent report, a cat
samples. 139,146,167,168 with thymoma achieved a partial response after treatment with
Flow cytometry may be used to aid in the specific diagnosis of DOX, vincristine, and l-asparaginase. 178 Progressive disease was
mediastinal tumors. Using flow cytometry, thymic lymphocytes can observed in 9 dogs treated with a variety of chemotherapy agents
be differentiated from peripheral lymphocytes by their simultane- (carboplatin, DOX, vincristine and cyclophosphamide, etc). 138
ous expression of CD4 and CD8. In one study, all cases of thymoma Chemotherapy and RT can result in a reduction in the size of
included 10% or more of lymphocytes coexpressing CD4 and thymomas, but this effect may be the result of reduction in the
CD8, whereas six of seven lymphomas contained fewer than 2% nonneoplastic lymphocyte population in the thymus rather than a
of CD4 CD8 lymphocytes. The one case of lymphoma expressing true anticancer effect. 139–141,146,178,179
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more than 2% of CD4 CD8 lymphocytes was readily differen-
tiated from thymomas by flow cytometric scatterplot analysis. 169 Prognosis
CT is recommended to determine the extent of disease,
whether the thymoma is invasive or noninvasive, and to help The prognosis is good for dogs and cats with noninvasive thymomas
the surgeon evaluate the feasibility of resection. The definition treated with surgery. Perioperative mortality rates range from 20%
of surgical resectability will depend on the experience and abil- to 27% in dogs and 11% to 22% in cats, 139,179 but no independent
ity of the surgeon. Vascular invasion, although more challeng- risk factors were identified for perioperative mortality. 180 Prognos-
ing, does not necessarily preclude surgery as a treatment option. tic factors in dogs include preoperative MG 139–142,147 and a low
Tumor thrombi have been successfully removed from within the percentage of intratumoral lymphocytes. 139 The presence of MG,
CVC, 138 and the CVC has been reconstructed with a jugular vein which was earlier thought to influence survival, did not do so in
autograft after excision of an invasive thymoma in a dog. 170 If the the most recent and largest retrospective study in dogs. The authors
tumor is deemed inoperable, CT will still be crucial for planning hypothesized that the lack of difference in survival despite these seri-
RT. Furthermore, CT-guided biopsies can be obtained during the ous conditions likely reflects an improvement in both perioperative
process. Despite these advantages, nonangiographic CT has been and postoperative care that has occurred in recent years. 138
shown to have limitations, including significantly underestimat- The prognostic effect of percentage of lymphocytes in the
ing vascular invasion when compared with surgical exploration. tumor was evaluated; results showed that a high percentage of
These limitations can be potentially overcome by the use of CT lymphocytes was associated with longer STs. Age, invasiveness of
angiography. 171–174 the tumor, and mitotic index had no effect on prognosis. 139 One
veterinary study evaluated the Masaoka–Koga staging system for
Therapy thymomas in dogs. They showed that dogs classified as lower Mas-
aoga–Koga stage (I or II) had significantly longer STs than dogs
A variety of different modalities have been described for the treat- with stages II or higher. 138 In cats, cystic thymomas are commonly
ment of thymomas in dogs and cats, including surgery, RT, che- reported and they have been associated with a better prognosis,
motherapy, and multimodality treatments. Unfortunately, there although no other possible prognostic factors, such as surgical
are no available studies comparing the survival times (STs) of ani- resectability, were critically evaluated. 139,180,181
mals treated by these different modalities. In addition, in many In conclusion, long-term STs should be expected for dogs
studies, animals were treated with a combination of different and cats with thymomas that can be completely resected. Tumor
methods. 139–142,146,150,157,173 In a retrospective study of 11 dogs recurrence may occur after excision and a second surgery can be
and 9 cats with invasive and noninvasive thymomas treated with successfully performed. Vascular invasion may increase surgical
surgery alone, the MST was 790 days and 1825 days for dogs complexity, but not necessarily deny surgery as an option. 138 RT
and cats, respectively. One- and 3-year survival rates were 64% appears to offer acceptable control rates for those tumors that are
and 42% and 89% and 74% for dogs and cats, respectively. 139 In unresectable or recur.
another study, the MST for dogs treated surgically was 635 days,
which was significantly better than the MST for dogs not treated Comparative Aspects
surgically (76 days). 138 The successful resection of noninvasive
thymomas in dogs by video-assisted thoracoscopy has also been Thymic neoplasms constitute 30% of anterior mediastinal masses in
reported. 175 adults and fewer than 15% in children. The majority are diagnosed