2699 - Effect of Timing of Total Skin Electron Therapy on Clinical Outcomes in Patients with Mycosis Fungoides

Purpose/Objective(s): Patients with mycosis fungoides (MF) have numerous treatment options, often resulting in prolonged and complex therapeutic courses. The optimal timing for initiation of total skin electron therapy (TSET) remains uncertain. We evaluated whether outcomes differ between patients who received TSET after prior therapies and those who received TSET as an initial treatment, excluding topical corticosteroids.

Materials/Methods: We queried our institutional CTCL database to identify patients with MF treated between 2013 and 2025. Patients received TSET either upfront or after prior therapies (delayed). TSET was delivered to 36 Gy in 24 fractions. We analyzed clinicopathologic features, prior treatments, time from diagnosis to TSET, and time to next treatment (TTNT). Continuous variables were summarized as medians (IQR) and compared using the Mann–Whitney test; categorical variables were reported as frequencies and proportions.

Results: Seventy patients were identified; 61 (87%) received delayed TSET and 9 (13%) upfront TSET. Median age at diagnosis was 61 vs 51 years, respectively (p=0.15). In the delayed group, 77% had T1–2 and 23% T3–4 disease, compared with 56% and 44% in the upfront group (p=0.23). Median time from diagnosis to TSET was 29 months (IQR 14–109) in the delayed group vs 2 months (IQR 1–9) in upfront (p<0.001). Delayed patients had a median of 4 prior therapies (range 1–15), including narrowband UVB phototherapy (74%), systemic therapy (72%), local radiation (41%), and topicals (39%). Progression after TSET occurred in 75% of delayed patients and all upfront patients. Among the delayed TSET patients who progressed, 19% received 1 prior line, 22% received 2 lines, and 59% received =3 lines of treatment. Median TTNT after TSET was similar between groups, 3 months (IQR 1–8) in delayed group and 3 months (IQR 2-5) in upfront group (p=0.59). Within the delayed group, TTNT did not differ by number of prior therapies with a median of 3 months across patients treated with 1, 2, or =3 prior lines. No meaningful correlation was observed between time from diagnosis to TSET and TTNT overall (?=0.05, p=0.54). When stratified by treatment group, a moderate but non-significant positive correlation was observed in upfront group (?=0.42, p=0.14), while no correlation was observed in delayed group (?=0.01, p=0.90).

Conclusion: In this single-institution cohort of patients with MF, most patients received TSET after multiple prior therapies. TTNT following TSET was similar between upfront and delayed treatment groups and was not influenced by number of prior therapies or timing of TSET. These findings suggest that delayed use of TSET does not adversely impact short-term disease control. Multi-institutional pooled analyses are warranted to validate these observations.