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Melanocytic Tumors

Alison H. Skalet, MD, PhD; David Huang, MD, PhD; Yali Jia, PhD; and Yan Li, PhD

Advances in optical coherence tomography (OCT) technology in the last decade have allowed OCT to gain a role in clinical diagnosis and management of ocular surface and intraocular tumors. High-speed OCT is now used routinely in intraocular tumor care to assist in diagnosis and to monitor the natural history as well as treatment response of ocular tumors. The use of OCT angiography (OCTA) for ocular tumor imaging remains experimental, but shows clinical promise for use in diagnosis of melanocytic tumors and monitoring treatment response.

MELANOCYTIC TUMORS

Melanocytic tumors of the eye include benign and malignant tumors of the conjunctiva, iris, ciliary body, and choroid. Melanocytic tumors may also arise in the orbit. Here we will focus on intraocular melanocytic tumors.

Melanocytic tumors of the iris, ciliary body, and choroid include benign nevi, uveal melanoma, and indeterminate lesions. Distinguishing between benign tumors and small melanomas can be difficult, and expert clinicians most often rely on clinical diagnosis based on ophthalmoscopic and imaging characteristics of the lesions rather than cytologic diagnosis because of technical challenges and surgical risk associated with biopsy of these thin tumors. Diagnostic imaging using echography, and more recently, structural high-speed OCT for choroidal lesions, is also important in differentiating melanomas from simulating lesions. Fluorescein angiography may also be helpful in select cases. Tumors judged to be benign or indeterminate based on clinical criteria are typically followed longitudinally for growth or change consistent with malignant transformation.

Uveal melanomas are the most common primary intraocular malignancy in adults, with an incidence of approximately 5 cases per million population.¹ Posterior uveal melanomas, including ciliary body and choroidal melanomas, have a high metastatic potential, with up to 50% of patients developing metastatic disease, for which there is currently no curative treatment. Iris melanomas have a fairly low rate of metastatic disease, in comparison, and therefore may, in some cases, be observed out of concern for vision complications associated with treatment.²

Treatment of uveal melanomas depends on the size and location of the tumor and vision potential for the eye, but the majority of patients are now offered eye-sparing treatment, most commonly radiation therapy. The local tumor control rate is very high for uveal melanomas treated with radiation. Nonetheless, treated melanomas must be monitored long term for clinical regression to ensure there is no tumor recurrence. Successful treatment is often associated with decreased tumor thickness and decreased vascularity.

The advent of spectral domain (SD) OCT, and subsequent development of enhanced-depth-imaging (EDI) OCT techniques, allowed clinicians to begin to image small choroidal tumors, including choroidal melanocytic tumors.3 More recently, swept source (SS) OCT has been used to image choroidal melanocytic tumors with further improvement of image penetration and range at the level of the choroid.⁴ The improved depth of penetration of SS-OCT is related to the use of longer wavelengths (1050 nm), which has reduced signal loss due to scattering and absorption in these turbid and pigmented lesions.

EDI-OCT features of choroidal nevi and small melanomas have been described.^3,5,6 Melanocytic tumors of both types are associated with choroidal shadowing and choriocapillaris thinning. There are some features that appear to distinguish tumors with malignant features, presumably melanomas, from their benign counterparts, including intraretinal edema, loss of photoreceptors, loss of the inner segment-outer segment junction and external limiting membrane, irregularity of the inner plexiform and ganglion cell layer, and elongated photoreceptors.⁶ Subclinical subretinal fluid, which may be an early sign of malignant transformation, can also be readily identified and monitored longitudinally by EDI-OCT. The thickness of lesions can be documented and followed by OCT in tumors too thin to measure accurately by echography.

SS-OCT at a longer wavelength (1050 nm) provided further improvement in image penetration over EDI-OCT. Intralesional vessels were visible in all of the 30 choroidal nevi evaluated with SS-OCT by Francis and colleagues.⁴ In addition, distended bordering vessels were present in 73% of tumors and were associated with the presence of subretinal fluid. However, SS-OCT is still less likely to visualize the nevus-scleral interface in pigmented choroidal nevi compared to amelanotic nevi, indicating that penetration of highly pigmented tumors remains a challenge even at 1050 nm wavelength.

Because of the ease of image acquisition, safety, and low cost of OCT imaging, EDI-OCT has now gained a role as an adjunctive imaging technique helpful in distinguishing small melanomas from choroidal nevi and for following indeterminate lesions for concerning change such as growth, increased subretinal fluid, and concerning retinal changes. It is likely that SS-OCT will supplant EDI-OCT when it becomes more readily available in the clinical setting. OCTA, which can be performed using the same high-speed SD-OCT or SS-OCT scanners in coordination with structural images, promises to further expand the role of OCT in the diagnosis and management of these small posterior tumors. OCTA may also provide a novel method for imaging iris and ocular surface tumor vasculature in the future.

OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY

Solid tumors undergo initial avascular and subsequent vascular phases of growth. Increased vascularity is a hallmark of malignant transformation.^7,8 This occurs through a process of angiogenesis in which neovascularization creates a network of disorganized, highly permeable intratumoral vessels that allow the tumor to grow beyond a size at which diffusion alone can support nutritional needs.^8,9 This access to highly permeable capillaries is also likely a prerequisite for metastatic spread of cancer. Increased intratumoral microvessel density is associated with various measures of increased tumor aggressiveness, and is a morphologic measure of angiogenesis.¹⁰ OCTA has the potential to provide a novel method for examining intratumoral microvessel density, and could play a role in cancer care in the future.

OCTA does not require injected contrast, which makes it safer and less expensive than traditional ophthalmic angiography techniques, and can provide very high-resolution images of the vasculature of the conjunctiva, iris, retina, and choroid. There is considerable interest in whether OCTA could be used to assist in diagnosis and management of melanocytic tumors of the eye. The technique may allow for earlier identification of malignant transformation among nevi being observed for growth, reducing risk for development of metastatic disease among patients being managed expectantly. It also may find an application in monitoring tumor regression after radiotherapy, as tumor regression is characterized by decreased intratumoral vascularity.

Current research is attempting to use OCTA techniques to image intraocular tumors, including melanocytic tumors. OCTA contrasts vasculature from static tissue by assessing the change in the OCT signal caused by flowing red blood cells in blood vessels. Hence, attenuation of OCT signal from the pigmentation of densely pigmented lesions will also affect OCTA imaging depth. Early work shows promise for imaging iris and choroidal tumors, and ongoing work to develop a system able to penetrate the more pigmented lesions is ongoing.

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