Primary indolent cutaneous B cell lymphomas (PCBCL) are rare: although data on outcomes and treatment are limited, traditionally they have been treated with radiation doses in excess of 24 Gy. Recently, some trials that patients with primary cutaneous indolent lymphoma managed with very low dose (4 Gy) RT (LDRT) have shown that high response rates and durable remission can be achieved; unfortunately, given the retrospective nature of these studies, the role of LDRT in indolent PMCL remains undefined. The objective of this retrospective multicentric trial is to investigate the efficacy of low-dose involved-field radiation therapy in patients with primary indolent cutaneous B-cell Lymphoma.

Primary cutaneous B-cell lymphomas (PCBCL) represent approximately 20% of all primary cutaneous lymphoma and are defined by a restricted disease spread to the skin without evidence of extracutaneous involvement at diagnosis. They encompass three main types: primary cutaneous marginal zone lymphoma (PCMZL), primary cutaneous follicle center lymphoma (PCFCL) and primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL, LT), of which the first two entities are indolent. Radiotherapy (RT) is part of the therapeutic mosaic for PCBCL and recommended by both national and international guidelines for a localized disease pattern, conferring high local control rates with a conservative approach . Curative doses cover a range of 18-54 Gy and complete remission is common regardless of which treatment is used. There is accumulating evidence for a possible de-escalation of RT dose while maintaining high response rates and thereby opening a role for low-dose RT (LDRT), which has been successfully shown for other indolent non-Hodgkin lymphoma. However, the rarity of indolent PCBCL and retrospective nature of studies make a definitive dose recommendation troublesome.

Inclusion Criteria:

• Age greater than or equal to 18 years
• Histological diagnosis of indolent B-cell lymphoma (follicular, marginal, NOS)
• Primary cutaneous location
• Early stage (I-II)
• First diagnosis or recurrence
• Low-dose radiotherapy treatment (4 Gy/2 sessions)

Exclusion Criteria:

• Aggressive histology lymphomas
• Advanced stages
• Extracutaneous localisation
• Radiotherapy dose administered > 4 Gy
• Patients without histological typing
• Patients without follow-up

The study involves adult patients with early-stage (I-II) primary cutaneous indolent B-cell lymphoma (PCBCL) treated with low-dose radiotherapy (4 Gy in two sessions). Patients with a confirmed histological diagnosis of indolent B-cell lymphoma (follicular, marginal, NOS) and adequate follow-up are included. Patients with aggressive lymphomas, advanced stages, extracutaneous localisations or radiotherapy doses greater than 4 Gy are excluded.