Preoperative Radiation Therapy-Induced Molecular and Immune Modulation in Early-Stage Breast Cancer: Results From the YOUNGSTER trial.

Purpose: The impact of preoperative radiation therapy (RT) on early-stage breast cancer (BC) is underexplored but may significantly improve outcomes and offer new therapeutic strategies. The YOUNGSTER study aimed to characterize the molecular changes induced by preoperative RT across BC subtypes: luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)-enriched, and basal-like.

Methods And Materials: This exploratory study enrolled 20 patients with early-stage BC who were eligible for breast-conserving surgery and had not received prior treatment. Biological changes were assessed between baseline, 3 to 5 days post-RT, and surgical samples. A preoperative RT boost (5 × 2.67 Gy/fraction) was administered, followed by a core needle biopsy. Patients then proceeded to either surgery 4 weeks (2-8 weeks) after RT or neoadjuvant therapy. Gene expression was analyzed using a 192-gene panel, alongside immunohistochemistry for Ki67 and CD68, tumor-infiltrating lymphocytes quantification, and γH2AX staining for DNA damage assessment.

Results: At baseline, PAM50 subtype distribution was luminal A (35%), luminal B (25%), HER2-enriched (20%), and basal-like (20%). Early post-RT samples showed significant downregulation of proliferation genes, PAM50 proliferation signature, and Ki67 immunohistochemistry staining, increased DNA damage, and macrophage marker upregulation. In primary surgery samples (2-8 weeks post-RT) (n = 13), adaptive immune markers showed significant upregulation, along with a 14-gene immunoglobulin signature increase.

Conclusions: Preoperative RT induces early and late biological changes in BC, with initial effects on proliferation reduction and DNA damage within days, followed by adaptive immune activation within weeks. RT may serve as an effective primer for immunotherapy, especially in higher-risk subtypes, supporting the potential for combinatorial approaches in BC management.