Introduction:
A well documented transformation from a choroidal nevus to melanoma is uncommon.

Aim of the research:
To report a series of choroidal melanocytic lesions diagnosed at the Ocular Oncology Service, Department of Ophthalmology, Poznan University of Medical Studies, Poland, presenting initially low-risk features for malignancy, which transformed into choroidal melanomas.

Material and methods:
Retrospective data analysis of patients with choroidal nevi without major features for growth (thickness over 2 mm in ultrasound (US), subretinal fluid (SRF), orange pigment). Lesions were evaluated at all visits using multimodal imaging (colour and autofluorescence fundus photography, OCT, and US measurements) until the melanoma diagnosis. We measured alterations in thickness, growth rate (GR) and tumour doubling time (TDT) and across the consecutive visits.

Results:
We identified 11 lesions with a median thickness of 0.95 mm at presentation (range: 0.3–1.9 mm). The median observation time until transformation was 47 months (range: 17–99 months). Ten (91%) lesions showed lipofuscin accumulation, 8 (73%) grew in thickness over 2 mm in US, 8 (73%) were diagnosed with SRF and 6 (54%) extended in their longitudinal diameter. The median overall GR was 0.67 mm/year (range: 0.0–3.4 mm/year) and the median overall TDT was 884 days (range: 167–3719 days). On follow-up, majority of lesions presented heterogenous GR and TDT, and at the time of fastest growth, median GR was 1.88 mm/year (range: 0.0–5.6 mm/year) and median TDT was 201 days (range: 83–3044 days).

Conclusions:
Choroidal nevus may transform into choroidal melanoma after years of stability and therefore should undergo a regular follow-up. Careful monitoring of GR and TDT may aid the diagnosis of malignant transformation.