Page 685 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 31 Tumors of the Nervous System 663

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A B C D E

F G H I J

• Fig. 31.4 Serial imaging of brain tumors treated with different approaches. (A, B) Grade III oligodendro-
glioma in the right frontal lobe associated with perilesional vasogenic edema. Compared with the pretreat-
ment (A) image there is improvement in the edema and a decrease in the size of the mass after 3 days
of dexamethasone therapy (B). Sequential CT (C, D) and magnetic resonance imaging (E) examinations
of an incidental meningioma managed with annual watchful monitoring. There is minimal enlargement of
the mass over the depicted 3-year period. Feline parasagittal meningioma with calvarial hyperostosis (F)
treated via bilateral radical rostrotentorial craniectomy and reconstructive titanium mesh cranioplasty (G,
arrow; H). Pre- (I) and immediate postresection (J) computed tomography reconstructions of a dog with a
multilobular osteochondroma compressing the cerebrum (I, inset).

of case reports or small case series examining specific drugs. The and evaluate brain tumor-specific drug protocols, such as high-
most commonly used chemotherapeutics for brain tumors are throughput drug library screening and the identification of bio-
the alkylating agents lomustine (CCNU), carmustine (BCNU), markers that predict chemoresponsiveness.
and temozolomide (TMZ), or the antimetabolite hydroxyurea, all
of which penetrate the blood–brain barrier (BBB). 79–81 A small Surgery
number of case reports have reported objective tumor responses Advantages associated with surgical resection of brain tumors
or survival benefits associated with chemotherapy. 82,83 In general, include the rapid reduction or elimination of the tumor burden,
however, chemotherapy appears to have very limited objective the secondary beneficial effect this has on reducing intracranial
efficacy for the treatment of brain tumors, especially when used pressure, and definitive histopathologic diagnosis of the tumor.
as a monotherapy in the setting of gross disease. One retrospec- Several variables complicate critical comparisons of studies eval-
tive study in 71 dogs with presumptively diagnosed brain tumors uating the efficacy surgical treatment of specific tumor types
reported that CCNU-treated dogs (MST 93 days) experienced no including the experience of the surgeon, availability or standard-
survival benefit compared with dogs receiving palliative therapy ized protocols for usage of a growing number of operating room
79
(MST 60 days). No difference in survival was reported between technologies, and a nearly universal lack of inclusion of quantifi-
groups of dogs with predominantly presumptively diagnosed glio- able surrogates of surgical success, such as the extent of resection
mas treated with stereotactic volume modulated arc radiotherapy (EOR) in PBT studies. Given that attainment of wide excision
(VMAT) with (MST 420 days) or without (MST 383 days) margins of brain tumors is generally not possible, objective evalu-
81
TMZ. At the dose intensities reported in the literature, toxici- ation of EOR is currently dependent on MRI-based assessments,
ties associated with CCNU, TMZ, and hydroxyurea appear to which are associated with practical limitations, including expense
be uncommon and rarely life-threatening in companion animals and difficulties distinguishing residual tumor from surgically
with brain tumors. 79,81 induced reactive changes in the acute postoperative period. 85,86
In vitro studies have demonstrated that therapeutic responses Cytoreductive surgery is currently the preferred primary mode
to chemotherapeutic agents (such as bleomycin, carboplatin, of therapy for feline supratentorial meningiomas. 3,29,87,88 Feline
CCNU, irinotecan, and TMZ), as well as mechanisms of chemo- supratentoral meningiomas are often located over the cerebral
resistance observed in canine glioma cell lines, parallel those seen convexities, visibly well demarcated, and are not usually infiltra-
70
in human glioma cell lines. Thus it is reasonable to expect that tive into the underlying brain parenchyma. These characteristics
the inclusion of adjuvant chemotherapy in multimodality com- make them amenable to gross total resection (see Figs. 31.4F, G)
panion animal brain tumor treatment protocols could result in with low procedural mortality, even in instances of large tumor
34
modest clinical benefits, as is generally seen in humans. Metro- burdens, multiple tumor foci, or recurrent tumor. Reconstruc-
nomic chemotherapy with chlorambucil in conjunction with sur- tion of extensive cranial surgical defects necessary to remove
gical resection is currently being evaluated in an early phase trial large tumors can be performed using MRI-compatible materi-
84
in dogs with gliomas. The creation of animal patient-derived als, including fascial or calvarial autografts, or polymethylmeth-
89
tumor cell lines will be paramount to efforts necessary to develop acrylate or titanium mesh cranioplasties (see Figs. 31.4G, H).