Residency · Residency · Ophthalmology
Choroidal Nevus vs. Small Melanoma: The TFSOM-DIM Mnemonic
Introduction
Choroidal nevi are among the most common intraocular findings, present in approximately 5-7% of the adult population. The critical clinical challenge lies in distinguishing a benign nevus from an early small choroidal melanoma. The TFSOM-DIM mnemonic ("To Find Small Ocular Melanoma, Doing IMaging") codifies the risk factors associated with growth and malignant transformation, guiding surveillance intensity and treatment decisions.
Choroidal Nevus: Baseline Features
Clinical Characteristics
Flat or minimally elevated (< 2 mm thickness) pigmented lesion. Typically < 5 mm in basal diameter. Well-defined or slightly feathered margins. Slate-grey or brown color; may have overlying drusen. Usually posterior to the equator. Overlying RPE changes: drusen, fibrous metaplasia, RPE atrophy.
No subretinal fluid, no orange pigment, no symptoms.
Prevalence and Natural History
Estimated growth rate to melanoma: < 1% over 5 years for lesions with no risk factors. Risk increases dramatically with the number of TFSOM-DIM risk factors present. Annual fundus photography and OCT monitoring recommended at baseline.
The TFSOM-DIM Mnemonic
"To Find Small Ocular Melanoma, Doing IMaging"
Each letter represents a clinical risk factor that increases the likelihood of malignant transformation:
T - Thickness > 2 mm
Measured by A-scan or B-scan ultrasonography. Lesions > 2 mm are more likely to be melanomas. The single strongest individual predictor of growth. Hazard ratio for growth: approximately 4.0.
F - Fluid (Subretinal)
Detected on clinical exam and confirmed by OCT. Represents breakdown of the RPE barrier and tumor metabolic activity. May be subtle (subfoveal fluid causing metamorphopsia) Hazard ratio for growth: approximately 2.7.
S - Symptoms
Visual changes: blurred vision, metamorphopsia, visual field defect, photopsia. Indicates tumor involvement of the visual axis or secondary effects (fluid, hemorrhage) Symptomatic lesions warrant closer follow-up regardless of other features.
O - Orange Pigment (Lipofuscin)
Accumulation of lipofuscin granules over the tumor surface. Best detected with fundus autofluorescence (FAF): hyperautofluorescent signal. Indicates RPE disturbance by metabolically active tumor. Hazard ratio for growth: approximately 2.1.
M - Margin within 3 mm of the Optic Disc
Peripapillary location associated with increased growth risk. Possibly due to greater vascularity and different microenvironment. Also complicates treatment (radiation optic neuropathy risk with plaque)
D - Drusen Absent
Overlying drusen suggest chronicity and are associated with benign behavior. Absence of drusen suggests a more active, potentially malignant lesion. A marker of longstanding, stable lesions when present.
I - Internal Reflectivity (Ultrasonographic) Low
Measured on A-scan ultrasonography. Low internal reflectivity is characteristic of melanoma. Nevi typically show medium-to-high internal reflectivity. Acoustic hollowing on B-scan further supports melanoma.
M - Melanoma Hollow on Ultrasound
B-scan finding of acoustic hollowing within the lesion. Indicates homogeneous tumor tissue characteristic of melanoma. Combined with low A-scan reflectivity, highly suggestive of malignancy.
Risk Stratification
Risk Based on Number of Factors
0 factors: growth risk approximately 3% at 5 years (observe) 1 factor: growth risk approximately 38% at 5 years. 2 factors: growth risk approximately 50% at 5 years. 3 or more factors: growth risk > 50% at 5 years (consider treatment)
| TFSOM-DIM Factor | Finding | Detection Method | Hazard Ratio |
|---|---|---|---|
| T - Thickness | > 2 mm | B-scan ultrasonography | ~4.0 |
| F - Fluid | Subretinal fluid | OCT | ~2.7 |
| S - Symptoms | Visual changes | History | -- |
| O - Orange pigment | Lipofuscin | Fundus autofluorescence | ~2.1 |
| M - Margin | Within 3 mm of disc | Clinical exam / photography | -- |
| D - Drusen absent | No overlying drusen | Clinical exam / OCT | -- |
| I - Internal reflectivity | Low on A-scan | Ultrasonography | -- |
| M - Melanoma hollow | Acoustic hollowing | B-scan ultrasonography | -- |
| Number of Factors | 5-Year Growth Risk | Recommended Management |
|---|---|---|
| 0 | ~3% | Observe; serial imaging every 6-12 months |
| 1 | ~38% | Monitor every 6-12 months |
| 2 | ~50% | Closer monitoring every 3-6 months; discuss treatment |
| 3+ | >50% | Strong consideration for treatment |
Practical Application
Lesions with 0-1 factors: monitor with serial imaging every 6-12 months. Lesions with 2 factors: closer monitoring every 3-6 months; discuss treatment. Lesions with 3+ factors: strong consideration for treatment as a small melanoma.
Multimodal Imaging Protocol
Fundus Photography
Baseline wide-field color and red-free photography. Serial comparison images to detect subtle growth (as little as 0.5 mm)
Fundus Autofluorescence
Detects orange pigment (lipofuscin) as hyperautofluorescent signal. More sensitive than clinical examination for lipofuscin detection. Hyperautofluorescence also seen over active melanomas.
Optical Coherence Tomography
Detects subclinical subretinal fluid not visible on clinical exam. Characterizes RPE changes: drusen (benign) vs. shaggy photoreceptors (malignant) Enhanced depth imaging (EDI-OCT) can measure tumor thickness for small lesions.
Ultrasonography
A-scan: internal reflectivity assessment; thickness measurement. B-scan: shape, acoustic properties, hollowness. Essential for any lesion > 1 mm in elevation.
When to Treat
Documented growth on serial imaging (most definitive indicator) High-risk features by TFSOM-DIM (3+ factors) even without documented growth. Patient anxiety and preference (shared decision-making) Small melanomas can be treated with plaque brachytherapy with excellent local control (>95%) TTT may be used as adjunct for very small lesions.
Patient Communication
Most choroidal nevi are benign and will never transform. Regular monitoring is effective for detecting early change. Use the mnemonic to explain risk factors in a structured, understandable manner. Document discussion of risk factors and plan in the medical record. Empower patients to report new visual symptoms between visits.
Key Clinical Pearls
The TFSOM-DIM mnemonic provides a systematic framework: Thickness > 2 mm, Fluid, Symptoms, Orange pigment, Margin near disc, Drusen absent, Internal reflectivity low, Melanoma hollow. The risk of growth increases with each additional risk factor; 3 or more factors should prompt strong consideration for treatment. Fundus autofluorescence is more sensitive than clinical examination for detecting orange pigment (lipofuscin). Overlying drusen are a reassuring finding and suggest chronicity of a benign nevus.
References
- Shields CL, Dalvin LA, Ancona-Lezama D, et al. Choroidal nevus transformation into melanoma: analysis of 2514 consecutive cases. Arch Ophthalmol. 2019;137(3):283-289.
- Shields CL, Furuta M, Berman EL, et al. Choroidal nevus transformation into melanoma per millimeter increment in size using multimodal imaging in 2355 cases: the TFSOM-DIM study. Arch Ophthalmol. 2009;127(8):981-988.
- Say EA, Shah SU, Ferenczy S, Shields CL. Optical coherence tomography of retinal and choroidal tumors. J Ophthalmol. 2011;2011:385058.
- Shields CL, Shields JA. Clinical features of small choroidal melanoma. Curr Opin Ophthalmol. 2002;13(3):135-141.