Page 702 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

680 PART IV Specific Malignancies in the Small Animal Patient

Choroidal Melanomas
Choroidal melanomas are rare intraocular melanocytic tumors,
VetBooks.ir comprising only 4% to 7% of canine uveal melanomas, with
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no clear breed or sex predisposition. Middle-aged (6–7 years),
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medium- to large-breed dogs predominate. Generally, these
tumors are well-delineated, raised subretinal pigmented masses
with tapering margins, bulging centers, and a propensity for the
peripapillary region and optic nerve. 46,47 In some cases, the tumor
may remain virtually static for many years, whereas others exhibit
infiltration into the overlying retina, through the sclera, up the
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optic nerve, and into the orbit. Nuclear anaplasia is minimal and
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generally mitotic figures are absent. Despite these benign cyto-
logic features, metastasis has been described in one dog 21 months
after exenteration, and follow-up in most studies is incomplete.
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In general, however, these tumors appear to be benign in the
vast majority of dogs. Most dogs with tumors involving a lim-
ited portion of the choroid are asymptomatic, and the mass is • Fig. 32.6 Diffuse iris melanomas may first appear as multifocal to diffuse
noted incidentally on ophthalmoscopy. Larger tumors frequently pigmentary changes, as seen in the coalescing darker regions of the iris
in this cat.
present with chronic uveitis, secondary glaucoma, retinal detach-
ment, intraocular hemorrhage, or blindness. 46,47 Extension into
the orbit can occur, and documentation of this is important in Most ophthalmologists have noted unilateral or occasionally
planning for enucleation surgery. Ocular ultrasonography may bilateral slowly progressive iridal pigmentary changes (especially
demonstrate mass lesions if anterior segment changes or retinal in older orange cats) over many years to a decade or more that
detachment obscures an underlying mass. Therapy usually con- apparently do not lead to disease beyond the pigmentation,
sists of enucleation once progression has been documented or if although eventual removal of these eyes can show melanoma.
the eye is painful. Diode laser ablation may offer an alternative to It is possible in some cats that these initially benign-appearing
enucleation if the lesion is small and does not involve the optic accumulations of small, angular pigmented cells on the ante-
nerve. Optic nerve or scleral invasion may warrant a more cau- rior iridal surface undergo transformation to the larger, rounded
tious prognosis. cells typical of the potentially malignant diffuse iris melanoma.
Of concern to the clinician waiting to document progression
Feline Primary Intraocular Melanomas (Feline before advising treatment, however, is that malignant transfor-
Diffuse Iris Melanoma) mation is not readily observable clinically and that these cells,
once transformed, appear to be capable of quickly dropping off
Incidence and Risk Factors into the anterior chamber and entering the drainage apparatus
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Anterior uveal melanomas are the most common primary intra- and vasculature.
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ocular tumor in cats (Fig. 32.6). They account for 55% of all
neoplastic submissions to the COPLOW in cats. There appears to
be no breed or sex predisposition, and most cats are more than 9 History, Clinical Signs, Diagnostic Techniques, and Workup
years of age at the time of diagnosis, 49,50 although the prodromal Slowly progressive, diffuse iridal hyperpigmentation is the most
period for many of these tumors may be quite long. common clinical sign, although occasionally a pigmented iridal
nodule or amelanotic mass is seen. Secondary glaucoma will even-
Pathology and Natural Behavior tually occur, and the diffuse form may be mistaken for chronic
In the malignant form of uveal melanoma, the rate of metastasis anterior uveitis with iridal hyperpigmentation.
(frequently to the liver and lungs) has been reported to vary from The diagnosis of melanoma, generally made clinically, requires
55% to 66% or higher. 49–52 Iridal hyperpigmentation, however, demonstration of progression and iridal thickening or irregular-
frequently takes months to years to progress to the extent to which ity of the iris surface or pupil. The prognostic and diagnostic
the eye must be enucleated, and an additional 1 to 3 years after value of FNAs of the iridal surface or iridal biopsies is unclear
enucleation are required before metastatic disease may become evi- and worthy of further study. A recent report described multiple
dent. 49,50,52,53 No single morphologic feature is predictive of out- cases of darkly pigmented iridociliary cysts in cats that were erro-
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come, but in some studies metastasis has been linked to a greater neously diagnosed as iris melanomas. These cases presented
mitotic index, larger tumors, and extension through the iris into single to multiple cysts that failed to transilluminate and caused
the ciliary body stroma and involvement of the scleral venous distortion and discoloration of the iris, leading to enucleation of
plexus. 49,50 In a recent study, an increased risk for metastasis was the globe. The presence of bilateral masses protruding through
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found if there was extrascleral extension, necrosis within the neo- the pupil may help distinguish iridociliary cysts from melano-
plasm, a mitotic index of more than seven mitoses in 10 high-power mas and should prompt the clinician to pursue additional diag-
(×400) fields, choroidal invasion, increased labeling for E-cadherin nostic imaging, such as high-resolution ultrasound.
(a transmembrane protein responsible for cell adhesion) or melan-
A (an antigen expressed by melanocytes) label intensity. PNL2 Therapy
label homogeneity was associated with a decreased rate of metasta- The treatment of feline uveal melanomas is controversial. Ideally,
sis. PNL2 is an antibody directed against an unidentified antigen enucleation would be delayed until just before malignant transfor-
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expressed by normal and neoplastic melanocytes. mation, invasion into other ocular structures, secondary glaucoma,