AKT-1284

A 55-year-old woman visits her General Practitioner with a pigmented skin lesion on her back that has grown quickly over the past few months. She has a history of frequent sunbed use. The lesion measures 9.5 mm in diameter. She is referred to Dermatology, where a diagnosis of malignant melanoma is confirmed.
What characteristic of the lesion would indicate the worst prognosis for this disease?