# 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.

![Comparison of a benign choroidal nevus (left) and a suspicious lesion with orange pigment and subretinal fluid (right)](images/nevus-vs-melanoma-comparison.jpg)

## 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.

![Multimodal imaging of a suspicious choroidal lesion: color photo, FAF, OCT, and B-scan](images/tfsom-dim-multimodal-imaging.jpg)

## 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.

![Fundus autofluorescence showing hyperautofluorescent lipofuscin over a suspicious choroidal lesion](images/faf-orange-pigment-nevus.jpg)

## 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

1. 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.
2. 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.
3. Say EA, Shah SU, Ferenczy S, Shields CL. Optical coherence tomography of retinal and choroidal tumors. *J Ophthalmol*. 2011;2011:385058.
4. Shields CL, Shields JA. Clinical features of small choroidal melanoma. *Curr Opin Ophthalmol*. 2002;13(3):135-141.