Introduction: Desmoplastic melanoma (DM) is an uncommon melanoma subtype that can be challenging for clinicians to diagnose. High rates of amelanosis and lack of conformity to established diagnostic criteria cause it to be often confused for scar tissue or other benign dermatoses1. Behaviour of pure desmoplastic melanoma (defined as >80% invasive tumour being associated with stromal fibrosis) is distinct from other melanoma subtypes, it tends to recur locally despite adequate clearance margins with a penchant for neurotropism over lymphatogenous metastasis2, 3 and low rates of BRAF mutations4, 5. There is a paucity of Australian data with respect to typical anatomical subsite and patient characteristics for DM.
Method: A prospectively collected cohort of patients reviewed at a state-wide tertiary referral service for melanoma between 1994 and 2016 had all melanoma pathology reviewed by expert dermatopathologists. Patient characteristics were collected by clinician examination and review of each patient.
Main findings: Amongst 4270 total cases of invasive cutaneous melanoma, 135 (2%) cases of DM were identified with a mean age of 71 years. Two thirds of DM occurred in males (67%). Median Breslow thickness was 4.0mm with 18% ulcerated. The mean mitotic rate was 2.9/mm2; 9% of DM were associated with a pre-existing naevus, 62% were diagnosed in the head and neck region (with 39% located on the scalp, 44% on the face, 13% on the neck and 4% on ears), 16% on the arms and 10% on the upper back. 41% were clinically amelanotic. Perineural invasion was present in 50% of DM with lymphovascular invasion detected in only 6%. 79% were diagnosed as the first melanoma for the patient with reasonably even distribution across early Fitzpatrick Skin phototypes. Over half of patients had at least one non-melanoma skin cancer (56%) and 70% recalling at least one episode of a severe sunburn. The majority of patients had less than 50 total naevi (65%) with 83% having no clinically dysplastic naevi. Median Breslow thickness of DM in sun-exposed subsites was 5.0mm compared to 2.8mm in non-sun-exposed sites (Wilcoxon rank-sum P<0.01).
Principal conclusions and implications for the field: We describe an Australian cohort of patients diagnosed with desmoplastic melanoma. The findings add to published international studies for DM, particularly with reference to novel data on patient characteristics and anatomical location to identify sites/patients at risk.