Dermatology: Practical and Conceptual Image Letter | Dermatol Pract Concept. 2023;13(4):e2023227 1 Abscopal Effect After Localized Radiotherapy in a Patient With Sezary Syndrome Under Treatment with Mogamulizumab Vieri Grandi1, Isabella Ciardetti1, Andrea Gemignani1, Eleonora Gherardi1, Tommaso Grassi2, Nicola Pimpinelli1 1 Department of Health Sciences, Section of Dermatology, University of Florence, P. Palagi Hospital, Florence, Italy 2 Public Health, Hygiene and Preventive Medicine Unit, P. Palagi Hospital, Florence, Italy Citation: Grandi V, Ciardetti I, Gemignani A, Gherardi E, Grassi T, Pimpinelli N. Abscopal Effect After Localized Radiotherapy in a Patient with Sezary Syndrome Under Treatment with Mogamulizumab. Dermatol Pract Concept. 2023;13(4):e2023227. DOI: https://doi .org/10.5826/dpc.1304a227 Accepted: April 5, 2023; Published: October 2023 Copyright: ©2023 Grandi et al. This is an open-access article distributed under the terms of the Creative Commons Attribution- NonCommercial License (BY-NC-4.0), https://creativecommons.org/licenses/by-nc/4.0/, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original authors and source are credited. Funding: None. Competing Interests: None. Authorship: All authors have contributed significantly to this publication. Corresponding Author: Grandi Vieri, Department of Health Sciences, Section of Dermatology, University of Florence, P. Palagi Hospital, Viale Michelangelo 41, 50141, Florence, Italy. Email: Vieri.grandi@unifi.it Case Presentation A 50-year-old lady with Sezary Syndrome (stage IVA1), di- agnosed 2 years before, had been treated previously with extracorporeal photophoresis + Interferon-alfa, metho- trexate, and gemcitabine. After 12 months, we introduced mogamulizumab i.v. After 6 cycles, she achieved a partial response in the skin, with only a large thick plaque residual on the back (modified Severity Assessment Tool, mSWAT =  8), and a complete response in the blood. After two months, her disease progressed with bulky nodal involve- ment in the right cervical basin (IVA2), treated with pallia- tive surgery. Subsequently, she developed a rapidly growing tender mass on the occiput. We decided to treat it with elec- tron radiotherapy (15 Gy in 5 fractions), obtaining rapid local control. After a few weeks from the scalp irradiation, the large plaque on the back disappeared, leaving a hyper- pigmented area. Teaching Point The abscopal effect refers to an uncommon activation of the immune system secondary to radiotherapy that may lead to clinical responses in areas far from the irradiation site  [1]. Mogamulizumab is a defucosylated, humanized IgG1 kappa immunoglobulin selective against CC Receptor type 4 (CCR4). It binds directly to CCR4 expressed on tumor cells, yet it is also known to exert indirect immune- mediated antitumor effects via inhibition of CCR4 on Tregs [2]. Rapid lysis of tumor cells induced by radiotherapy, alongside the positive immune activation induced by mogamulizumab, might explain the impressive response obtained in a large, refractory skin plaque in our patient. We believe that mog- amulizumab combined with radiotherapy is an interesting approach in patients with cutaneous T-cell lymphoma, yet further studies are needed in order to better explain the re- ported abscopal effect. 2 Image Letter | Dermatol Pract Concept. 2023;13(4):e2023227 2. Kim YH, Bagot M, Pinter-Brown L, et al. Mogamulizumab ver- sus vorinostat in previously treated cutaneous T-cell lymphoma (MAVORIC): an international, open-label, randomised, con- trolled phase 3 trial. Lancet Oncol 2018;19:1192–204.Image Letter References 1. Yilmaz MT, Elmali A, Yazici G. Abscopal Effect, From Myth to Reality: From Radiation Oncologists’ Perspective. Cureus 2019;11:e3860. Figure 1. Clinical photos of a 50-year-old lady affected by Sezary Syndrome with mogamulizumab-refractory skin lesions. A) Rapidly grow- ing, painful tumor mass on the scalp. B) Same lesion, 45 days after electron-beam radiotherapy (15 Gy in 5 fractions). C) Large infiltrated, pruritic plaque on the back. D) Asymptomatic, post- inflammatory hyper- and hypopigmented macules as result of the complete spontaneous resolution of the lesion