Desmoplastic melanoma in Sweden in 2009-2022: A population-based registry study demonstrating distinctive tumour characteristics, incidence and survival trends.

Background: Desmoplastic melanoma (DM) is a rare subtype, accounting for less than 5% of primary cutaneous invasive melanomas. DM often arises in chronically sun-exposed skin, in older individuals. While the incidence of cutaneous melanoma has increased globally, trends specific to DM are less documented and studies on survival outcomes for DM are inconsistent.

Objectives: To study patient and tumour characteristics, incidence trends and survival outcomes of desmoplastic melanoma in comparison with other melanoma subtypes using data from the population-based Swedish Melanoma Registry (SweMR).

Methods: Clinicopathological variables were obtained from SweMR, for patients diagnosed with invasive cutaneous melanomas diagnosed in Sweden from 2009 to 2022. Incidence rates of DM per 100,000 inhabitants were calculated. DM cases were identified and survival outcomes were compared across DM, superficial spreading melanoma (SSM), nodular melanoma (NM) and lentigo maligna melanoma (LMM). Survival was analysed using Kaplan-Meier and Cox regression methods.

Results: Among 48,945 invasive melanomas, 388 (0.8%) were identified as DM. The incidence ranged between 0.1 and 0.4 cases per 100,000, without a evident rise, beside a certain peak in 2019-2020 in the men. DM had compared to other melanoma subtypes the thickest primary tumours (p < 0.001). When adjusting for baseline prognostic factors, DM patients had an improved melanoma-specific survival (HR 0.36, 95% CI 0.26-0.49). No significant improvement in survival was observed for DM diagnosed in 2015-2022 compared to those diagnosed in 2009-2014 (adjusted HR 0.99, 95% CI 0.45-2.14), whereas other subtypes showed an improvement (adjusted HR 0.74, 95% CI 0.68-0.80).

Conclusion: In contrast to the rising incidence of other melanoma subtypes, the incidence of DM has remained more stable over recent years. Despite presenting with adverse prognostic features, survival from DM is superior when adjusting for these factors. However, the introduction of anti-PD-1 therapy has not significantly improved survival for DM patients.