Background: Mycosis fungoides (MF) is the most common variant of cutaneous T-cell lymphoma, a heterogeneous group of non-Hodgkin lymphomas that primarily affect the skin. Narrowband UVB (NB-UVB) phototherapy is a first-line treatment for early-stage MF across Fitzpatrick skin types (FSTs). However, existing NB-UVB guidelines are largely derived from studies with limited skin type diversity. In a 2023 pilot study, our group demonstrated that MF patients with higher FSTs experience slower improvement on NB-UVB. Here, we validate and expand these findings using a multicenter cohort.

Methods: We conducted a retrospective chart review of MF patients at Columbia, Johns Hopkins, and Emory Universities who underwent NB-UVB monotherapy between January 2010 and June 2025. Patients were included if they had demonstrable disease within 90 days of NB-UVB initiation and at least two documented disease severity assessments (measured as % total body surface area TBSA involved). Patients receiving concurrent systemic therapy and radiation were excluded. Patients were stratified by FST (I-III vs IV-VI).

Timepoints were categorized as baseline, mid-year, year-end, and end of follow-up. The primary outcome was the change in TBSA involvement at each timepoint. Statistical analyses included Chi-square and Fisher’s exact tests for categorical variables, and Wilcoxon rank-sum and Welch t tests for continuous variables; α = 0.05.

Results: A total of 114 patients (63 FST I-III, 51 FST IV-VI) met inclusion criteria. Baseline demographic and disease characteristics differed between groups: patients with FST IV-VI were younger (mean age 49 vs 60 years, p < 0.001) and more likely to present with hypopigmented MF (43% vs 4.8%, p < 0.001). Patients with FST I-III experienced significantly greater percent reductions in TBSA than those with FST IV-VI, with reductions of 73% versus 35% at mid-year (p = 0.009), 78% versus 26% at year-end (p = 0.009), and 78% versus 47% at end of follow-up (p = 0.006). Absolute TBSA reductions mirrored these trends, with greater improvement observed in FST I-III at year-end (−14 vs −10, p = 0.037).

Conclusion: Across three academic centers, patients with FST I-III demonstrated significantly greater clinical improvement on NB-UVB than patients with FST IV-VI at mid-year, year-end, and end of follow-up. Potential contributors include disease presentation differences, variability in dosing regimens, and post-inflammatory hyperpigmentation mimicking persistent disease in patients with skin of color, leading to perception of lesser improvement. Larger retrospective and prospective studies are needed to elucidate these disparities and inform treatment strategies to promote equitable outcomes.