26.1.1 Melanocytic Choroidal Tumors

Choroidal Nevus

Choroidal nevus is a common benign melanocytic tumor with an incidence in the general population that ranges from 1 to 3% and is found mostly in the postpubertal age group. ⁷ More than 90% of choroidal nevi occur posterior to the equator and therefore are likely to be detected on routine ophthalmic examination. A choroidal nevus characteristically appears as a placoid or minimally elevated lesion (< 2 mm) ranging in diameter from 1.5 to 5 mm, with variable pigmentation: brown to slate gray to amelanotic. ⁷ The clinical picture is often altered by secondary changes such as surface drusen, RPE clumping, atrophy, fibrous metaplasia or detachment, subretinal neovascular membrane, and serous retinal detachment. ⁷ It is estimated that 1 in 8845 choroidal nevi undergoes malignant transformation into melanoma. ⁸ Despite the rarity, careful evaluation and follow-up of all choroidal nevi are advised. Factors predictive of nevus transformation into melanoma include thickness greater than 2 mm, the presence of subretinal fluid, orange pigment, juxtapapillary location, and symptoms of blurred vision or photopsia. ^{9 ,​ 10} In the setting of a suspicious nevus, OCT is useful in identifying some of these differentiating signs.

Although time-domain OCT (TD OCT) features of choroidal nevus are extensively documented, limited visualization of the deeper layers of the choroid and sclera secondary to light scattering from the RPE and choroid had limited the early descriptions to the retina and anterior choroid. ⁴ In an assessment of 120 eyes with choroidal nevus using OCT, Shields et al described secondary retinal alterations that included overlying retinal edema (15%), subretinal fluid (26%), retinal thinning (22%), drusen (41%), and RPE detachment (12%). ¹¹ The overlying retinal edema could be further classified as focal cystoid (3%), diffuse cystoid (8%), coalescent cystoid (3%), and noncystoid (1%), which may have value in deciding on treatment in patients who have visual loss and in prognosticating the outcome. By OCT, the overlying retina was of normal thickness (32%), thin (22%), or thick (45%) and photoreceptor loss or attenuation present in 51% of cases. The choroidal findings in nevi were limited to the anterior surface and included hyporeflectivity in 62%, isoreflectivity in 29%, and hyperreflectivity in 9%. Anterior choroidal reflectivity is affected by overlying RPE alterations and the amount of pigmentation. Nevi with dense pigment tend to show posterior shadowing, whereas those with minimal pigment allow some light transmission. The OCT findings rather reflect pigment within the mass and do not correlate to internal reflectivity and acoustic quality as elicited by ultrasonography (which implies density of cellularity). ¹¹

Compared with the clinical examination, OCT is more sensitive in detecting the secondary changes: retinal edema, subretinal fluid, retinal thinning, photoreceptor attenuation, and RPE detachment. ¹¹ Retinal edema, RPE alterations, photoreceptor loss, and RPE detachment are related to chronic retinal degeneration and suggest a stable, chronic choroidal nevus. ¹¹ In contrast, the presence of subretinal fluid and photoreceptor preservation suggests a relatively acute process with risk for growth into a melanoma or even a small melanoma. ¹² Serial OCT in a suspicious nevus can demonstrate subtle clinically undetectable changes that may help in making a treatment decision.

The more recent descriptions have used high-resolution spectral-domain (SD) EDI OCT, which allows detailed study of the internal architecture of the tumor and its precise measurement (Fig. 26.1 , Fig. 26.2). ^{5 ,​ 12} EDI OCT is sensitive in identifying drusen and photoreceptor loss. Subretinal fluid, a predictor of activity of a melanocytic choroidal lesion, can be detected by EDI OCT when it has been overlooked clinically and ultrasonographically. Visualization of explicit alterations specifically within the various layers of the retina allows characterization of subtle alterations such as RPE atrophy (43%), RPE loss (14%), RPE nodularity (8%), inner segment/outer segment (IS/OS) junction irregularity (37%), IS/OS junction loss (6%), external limiting membrane irregularity (18%), outer nuclear and outer plexiform layer irregularity (8%), and inner nuclear layer irregularity (6%). ⁵ Choroidal shadowing deep to the nevus (partial 59%, complete 35%) and choriocapillary thinning overlying the nevus (94%) can be reliably demonstrated on EDI OCT. Although EDI OCT provides much useful information, its inability to acquire images of good quality in about half the cases remains a barrier. Ideal candidates with choroidal nevus for EDI OCT imaging include younger (i.e., < 60 years) patients with nevus located in the posterior pole and < 3 mm tumor thickness. ⁵

Choroidal Melanoma

Choroidal melanoma is the most common primary intraocular malignant tumor. It is relatively rare in pigmented races and has known association with oculocutaneous melanocytosis. Choroidal melanomas manifest a range of growth patterns. Clinical appearances often change with the increasing size of the tumor. A small choroidal melanoma appears as a placoid, dome-shaped, or nodular well-circumscribed choroidal mass. As it grows, it may break through the Bruch membrane, assume a collar-button shape, and infiltrate the retina. A rare variant is a diffuse choroidal melanoma where the tumor thickness is < 20% of its overall basal diameter. Degree of pigmentation varies from characteristic deep brown to rare pale white. Secondary changes associated with choroidal melanoma include RPE mottling and localized detachment, orange pigment, subretinal neovascular membrane, overlying retinal phototoreceptor degeneration, retinoschisis, exudative retinal detachment, vitreous hemorrhage, vitreous pigment and tumor seeding, and scleral invasion and extraocular extension. ¹³ A good indirect ophthalmoscopy and ultrasound B-scan often aided by FFA and ICG are generally adequate to diagnose a typical melanoma. ¹³

Although TD OCT is not helpful in imaging the tumor itself, it can elucidate secondary retinal changes. Detection of overlying subretinal fluid by OCT could be important in confirming the suspicion of melanoma in eyes with borderline tumors. ¹² Muscat et al studied 20 untreated choroidal melanomas and detected subretinal fluid using TD OCT in all cases. ¹⁴ Espinoza et al showed that localized serous retinal detachment associated with retina of normal thickness was highly associated with documented tumor growth and future treatment. ¹² In contrast, a chronic OCT pattern with a thin retina, intraretinal cysts, and RPE thickening was associated with a long-standing dormant lesion. ¹² Sayanagi, using three-dimensional SD OCT, found subretinal fluid, retinal edema, and subretinal deposits in choroidal melanoma. ¹⁵ Singh and colleagues used SD OCT to describe dispersed subretinal deposits corresponding to orange pigment over a small choroidal melanoma that was missed with TD OCT. ¹⁶

Choroidal melanomas showed a highly reflective band in the anterior choroid by EDI OCT with lack of visibility of either the choroidal vessels or inner sclera. ¹⁷ Shields used EDI OCT to compare the features of small choroidal melanomas (≤ 3 mm thick on ultrasonography) with similarly sized choroidal nevi. ⁶ Of 37 eyes with small choroidal melanoma imaged using EDI OCT, the mean tumor thickness was 55% less by EDI OCT compared with ultrasonography. ⁶ Choroidal features included optical shadowing in and overlying choriocapillary thinning in all. Outer retinal features included shaggy photoreceptors in 49%, an absence of photoreceptors in 24%, IS/OS junction in 65%, external limiting membrane in 43%, outer nuclear layer in 6%, and outer plexiform layer in 11% (Fig. 26.3 , Fig. 26.4). ⁶ Inner retinal features were irregularity of the inner nuclear layer, inner plexiform layer, and ganglion cell layer in 8% each, nerve fiber layer in 5%. Secondary changes were subretinal fluid in 92%, subretinal lipofuscin in 95%, and intraretinal edema in 16%. Using EDI OCT, a comparison with similar-sized choroidal nevus revealed that small choroidal melanoma showed increased tumor thickness, subretinal fluid, subretinal lipofuscin, RPE atrophy, and retinal irregularities, including shaggy photoreceptors. ⁶ The difficulty of imaging the overlying retina for large melanomas and the inability to characterize the tumor itself beyond its anterior surface limit the routine use of OCT in the diagnosis of melanoma. ³

After radiotherapy, OCT has been used to monitor treatment response and complications of choroidal melanoma. Horgan et al performed pre- and post-plaque radiotherapy OCT and found that the mean time to onset of radiation maculopathy was 12 months. ¹⁸ They reported macular edema in 17% by OCT at 6 months, in 40% at 12 months, and in 61% at 24 months, much higher than by clinical evaluation: 1% at 6 months, 12% at 12 months, and 29% at 24 months. ¹⁸ Further, they were able to classify macular edema into extrafoveolar noncystoid (grade 1), extrafoveolar cystoid (grade 2), foveolar noncystoid (grade 3), mild-to-moderate foveolar cystoid (grade 4), and severe foveolar cystoid (grade 5). ¹⁸ This qualitative classification correlated with quantification of central foveolar thickness. OCT can therefore be used to diagnose and monitor radiation macular edema.

26.1.2 Choroidal Vascular Tumors: Circumscribed and Diffuse Choroidal Hemangioma

Circumscribed choroidal hemangioma is a common benign vascular tumor that manifests in middle-aged individuals with sudden-onset blurred vision or metamorphopsia. ^{19 ,​ 20} It is generally unifocal and unilateral, with no systemic association. Ophthalmoscopically, circumscribed choroidal hemangioma appears as a subtle reddish orange mass posterior to the equator. It is usually dome shaped, ranging in size from 3 to 10 mm in diameter and 2 to 4 mm thick. ^{19 ,​ 20} Whereas FFA features might not be diagnostic of circumscribed choroidal hemangioma, ICG angiography is characteristic: bright early filling and a characteristic late washout.

The diffuse choroidal hemangioma involves the entire choroid but with maximal thickness in the posterior pole. It is associated with facial hemangioma as part of the Sturge-Weber syndrome. Secondary changes in both circumscribed and diffuse choroidal hemangioma include subretinal fluid, overlying RPE atrophy, hyperplasia or fibrous metaplasia, retinal cystoid degeneration, and intraretinal macrocysts. ^{19 ,​ 20} On ultrasonography, choroidal hemangiomas have a high internal reflectivity and acoustic solidity matching that of the surrounding choroid, which helps to differentiate choroidal hemangiomas from other choroidal tumors, such as melanoma. ²⁰

On TD OCT, choroidal hemangioma shows poor resolution. The tumor appears to be hyperreflective at its anterior surface but with little deeper detail. On EDI OCT, circumscribed choroidal hemangiomas appear to have a medium-to-low reflective band with a homogeneous signal and intrinsic spaces (Fig. 26.5). ¹⁷ EDI OCT allows quantifying choroidal thickness in the posterior pole, even in diffuse choroidal hemangioma. It can detect choroidal thickness in eyes with subclinical choroidal abnormalities in patients with Sturge-Weber syndrome. ²¹

Optical coherence tomography helps image the overlying retina and elucidate the reason for visual loss, such as subretinal fluid, intraretinal edema, chronic photoreceptor loss, and tilt of the fovea (Fig. 26.6). OCT findings in circumscribed choroidal hemangioma include subretinal fluid (19%), retinal edema (42%), retinal schisis (12%), macular edema (24%), and localized photoreceptor loss (35%). ^{3 ,​ 22} OCT findings in diffuse choroidal hemangioma are subretinal fluid (28%), retinal edema (14%), and photoreceptor loss (43%). ^{3 ,​ 22} Newly active choroidal hemangioma shows subretinal fluid and preserved photoreceptor layer with minimal intraretinal edema. ² Chronically leaking choroidal hemangioma displays photoreceptor attenuation and overlying intraretinal edema and bullous retinoschisis. ² Together with the evaluation of the IS/OS photoreceptor line and the integrity of the RPE layer, this information may be useful when considering treatment options and predicting the potential outcome. ²³ Preservation of outer retinal thickness and integrity of the IS/OS photoreceptor line are important prognostic factors for visual recovery after treatment. ²⁴ When the tumor causes visual loss, treatment is advised. Options for treatment include laser photocoagulation, transpupillary thermotherapy, plaque brachytherapy, and photodynamic therapy. OCT is an important tool in depicting resolution of subretinal fluid and foveal edema after treatment. ² Recurrent subretinal fluid after therapy can be detected before the vision deteriorates and before this fluid becomes apparent clinically. ^{25 ,​ 26}