Optic disc melanocytoma (ODM) is a benign tumor arising from proliferation of melanocytes in the optic disc. It originates from melanocytes of the lamina cribrosa and is considered a congenital, non-hereditary condition 1).
In 1933, Reese reported it as “melanoma of the optic disc,” and in 1962, Zimmerman and Garron named it “melanocytoma” as a benign lesion. It can occur anywhere in the uvea but is most common in the optic disc. The rate of malignant transformation is extremely low, about 1–2% 1).
In the past, due to its heavy pigmentation, it was often mistaken for malignant melanoma, and some cases underwent enucleation. Today, its benign nature is widely understood, and unnecessary surgical intervention has decreased 1).
Histopathologically, it consists of polygonal cells containing large melanosomes. Nucleoli are inconspicuous and the nuclear-to-cytoplasmic (N/C) ratio is low. It shows an immunohistochemical profile of Melan A positivity and S100 negativity.
The rate of malignant transformation is extremely low, about 1–2% 1). However, severe visual loss due to tumor necrosis or vascular occlusion can occur. Severe vision loss does not immediately indicate malignant transformation; details are discussed in the section “Pathophysiology and Detailed Mechanisms” (#6).
Most patients are asymptomatic, and the condition is discovered incidentally during fundus examination.
When complications occur, decreased vision and visual field defects become apparent (see Clinical Findings for details).
It occurs unilaterally and is observed as a dark brown to black pigmented mass on ophthalmoscopy.
Tumor location and extent:
Neuro-ophthalmologic findings:
Frequency of complications:
Papillary and retinal edema
Papilledema: Found in about 25% of cases. It is thought to be caused by tumor compression and circulatory disturbance.
Intraretinal edema: Found in about 16% of cases. The main mechanism is exudative changes associated with necrosis1).
Subretinal fluid: Found in about 14% of cases.
Vascular occlusion and hemorrhage
76% are asymptomatic and have no effect on vision. However, if complications such as necrosis or vascular occlusion occur, severe vision loss may result 1). RAPD is observed in 9–30%, and visual field abnormalities are present in 90%.
ODM is a congenital lesion, and no specific environmental factors or acquired causes are known.
Epidemiological features (at diagnosis):
Tumor growth and risk of malignant transformation:
The following table summarizes the risk factors.
| Risk factor | Details |
|---|---|
| Tumor thickness >1.5 mm | Major risk factor for growth 1) |
| Growth rate | Slow growth in 10–15% 1) |
| Malignant transformation rate | Approximately 1–2%1) |
The cumulative risk of slow growth is reported as 11% at 5 years, 32% at 10 years, and 38% at 20 years1). Tumor growth itself does not imply malignant transformation (see “Pathophysiology and Detailed Pathogenesis” section).
Diagnosis is based on typical findings on fundus examination. Various imaging studies are used to evaluate tumor characteristics and differentiate from malignant melanoma.
A characteristic finding is a dark brown pigmented mass attached to the optic disc. Regular color fundus photography is used to determine the presence of growth.
OCT and OCT-A
OCT: Nodular elevation and posterior shadowing are characteristic findings. Hyperreflective dots may be seen1). It is also useful for evaluating tumor-related edema; in one case report, central retinal thickness was 158 μm after one year1).
OCT-A: Visualizes capillaries on the tumor surface without contrast agent. It can assess the correspondence between avascular areas and visual field defects. This is a new noninvasive technique useful for differentiating malignant melanoma.
Ultrasound B-Scan
B-mode ultrasound: Dome-shaped morphology and high internal reflectivity are suggestive of benignity. In a case by Khadka et al., thickness was 1.68 mm and transverse diameter was 3.01 mm1).
Fluorescein angiography (FA): Diffuse hypofluorescence is characteristic. In this case, delayed arterial filling (32 seconds) is observed, useful for confirming vascular occlusion 1).
Fundus autofluorescence (FAF):
CT/MRI: In Khadka et al.’s case, CT revealed a hyperdense lesion measuring 0.1×0.3 cm. MRI was unable to identify the lesion 1).
The most important differential diagnosis is choroidal malignant melanoma.
| Feature | Melanocytoma | Malignant Melanoma |
|---|---|---|
| Color | Dark brown, homogeneous | Gray to yellowish-white, heterogeneous |
| Ultrasound internal reflectivity | High (suggestive of benignity) | Low |
| Fundus autofluorescence | SWAF hypofluorescence | Various |
It has high diagnostic accuracy with sensitivity of 84% and specificity of 98% 1). It may not be performed if the tumor is small or to avoid invasiveness.
Regular OCT images the cross-sectional structure of the retina, while OCT-A is a technique that captures blood flow information without using contrast agents and visualizes vascular structures. In optic disc melanocytoma, it allows evaluation of capillaries and avascular areas on the tumor surface, and is expected to be useful for non-invasive differentiation from malignant melanoma.
ODM is a benign tumor and basically does not require treatment. Regular follow-up is the standard management strategy.
Management options:
In the case reported by Khadka et al., the tumor was small and FNAB was avoided, and enucleation was refused by the patient, so observation was chosen 1). As the benign nature has become widely recognized, unnecessary enucleations have significantly decreased 1).
Recommended follow-up methods:
Optic disc melanocytoma is a benign tumor, and treatment is not required in the majority of cases. Annual regular observation with color fundus photography and OCT is recommended. However, because the rate of malignant transformation is about 1-2%, it is important to continue regular follow-up 1).
ODM is thought to originate from melanocytes of the lamina cribrosa 1).
Mechanisms of visual dysfunction:
An important clinical fact is that the degree of nerve fiber compression or vascular occlusion does not necessarily correlate with tumor size 1). Therefore, while some tumors may enlarge without causing serious complications, even small tumors can sometimes cause vascular occlusion.
Relationship between tumor growth and malignant transformation:
OCT-A is attracting attention as a novel technique for evaluating optic disc melanocytoma. It has been reported that the capillary network on the tumor surface can be visualized noninvasively, and the spatial correspondence between avascular areas and visual field defects can be assessed.
As a noninvasive alternative to conventional fluorescein angiography (FA), it is expected to contribute to improved diagnostic accuracy in differentiating from malignant melanoma.
Compared to conventional spectral-domain OCT, SS-OCT allows evaluation of deeper tissues, and case reports are accumulating. Detailed observation of the choroidal structure and lamina cribrosa beneath the tumor is expected to advance understanding of the tumor’s origin and pattern of progression.
Larger long-term cohort studies are needed to identify predictors of tumor growth and malignant transformation. Future challenges include identifying predictors other than tumor thickness >1.5 mm and analyzing the relationship between OCT-A patterns and visual field prognosis.
