94P Biological effects of a pre-operative radiotherapy boost in early-stage breast cancer: Primary results of the YOUNGSTER unicentric single-arm clinical trial

Preoperative radiotherapy (RT) impact on early-stage breast cancer (BC) remains underexplored, holding promise for improved outcomes and innovative treatments. The YOUNGSTER trial aimed to analyze molecular changes induced by preoperative RT across BC subtypes: Luminal A, Luminal B, HER2-enriched, and Basal-like. From May 2021 to September 2022, YOUNGSTER enrolled 20 early-stage BC patients eligible for breast-conserving surgery and no prior treatment. The RT boost (5 x 2.67 Gy/fx) was administered preoperatively, followed by core-needle biopsy 3-5 days later. Gene expression was analyzed using a 192-gene panel, including BC and immune-related genes. Additional assessments included Ki67 and CD68 immunohistochemistry, tumor-infiltrating lymphocytes (TILs) via hematoxylin/eosin staining, and RT-induced DNA damage with γH2AX staining. Thirteen surgical samples from primary surgery patients were also analyzed. At baseline, PAM50 subtypes were distributed as follows: Luminal A (35%), Luminal B (25%), HER2-enriched (20%), and Basal-like (20%). Paired samples (baseline vs post-RT) revealed a significant downregulation in proliferation genes (e.g. MKI67) and the PAM50 proliferation signature. Ki67 IHC staining decreased (p=0.010). Intriguingly, RT upregulated genes such as MDM2, CD86, CD68, and S100A9, consistent across subtypes. Post-RT, a significant increase in DNA damage (p=0.004) occurred. Surgical samples showed a rebound in proliferation genes and MDM2, while sustained macrophage marker and CD86 expression persisted. Notably, immune-related genes (CXCL8, CD4, CD8A, PDCD1, CD79a, IGKC, IGL, IGLV3-25, IGJ) exhibited significant upregulation. Early changes induced by RT involve reduced proliferation, increased DNA damage, and broad immunomodulation across subtypes. RT upregulation of immune-related genes, sustained macrophage infiltration, and adaptive immune response activation suggest a broader impact than previously recognized.