DP07 Clinicopathologic features and outcomes of amelanotic melanoma: a 20-year retrospective case series

Abstract Amelanotic melanoma (AM) is a subtype of melanoma that lacks pigmentation, and often mimics various benign and malignant pathology, leading to delays in referral and diagnosis. AM is diagnosed through clinical evaluation, biopsy and immunohistochemical staining. We conducted a retrospective study over a 20-year period to identify the relative prevalence of amelanotic melanoma in our population and the clinicopathological features. Cases of AM were identified through a free-text search of the pathology database from 2005 to 2024 for ‘amelanotic melanoma’. A separate search for ‘melanoma’ was completed to establish the total number of cases diagnosed. The case notes and pathology reports were reviewed. In total there were 153 cases of melanoma within the study period, of which four cases were amelanotic melanoma (2.6%). In total 32 cases were identified in which amelanotic melanoma was part of the clinical differential diagnosis, including all four cases of AM. AM subtypes were nodular amelanotic melanoma (NAM; n = 3) and subungual spindle cell AM (SSAM; n = 1). All four cases were in male patients with Fitzpatrick skin types I–II, and the age at diagnosis ranged from 33 to 90 years. The cases of NAM were located on the lower back, ear and sole of the foot. The SSAM was on the hallux. Time from referral to definitive excision ranged from 20 to 68 days. Breslow thickness ranged from 2.0 to 6.4 mm. Ulceration was present in two cases. Clinical stage at presentation was stage 2A (n = 2), 2B (n = 1) and 2C (n = 1). Immunohistochemically all four cases were positive for S100 and Sox-10, while melan-A was positive only in the cases of NAM. Disease-specific survival at 2 years was 75%, with one patient dying from brain metastasis. Comparing the data from this study with existing literature we found that the 2.6% relative prevalence of AM falls within the reported range of 1–8% of all cases of melanoma. All cases were patients with Fitzpatrick skin types I–II, which is a well-established risk factor for amelanotic melanoma in addition to old age. Despite 32 clinically suspected cases over 20 years, only four were pathologically confirmed, reflecting the heterogeneity of amelanotic melanoma and its more common clinical mimics such as pyogenic granuloma and nonmelanoma skin cancer. The time from referral to definitive excision was within the national target range, so in this series there was no significant delay to treatment due to misdiagnosis. To minimize diagnostic delays clinicians should maintain a high suspicion for AM in nonpigmented lesions with atypical features, particularly in Fitzpatrick skin types I–II and older patients with chronic photodamage.