Germline Testing in Breast Cancer: A Single-Center Analysis Comparing Strengths and Challenges of Different Approaches.

Background/objectives: Compared to single gene testing (SGT), multigene panel testing (MGPT) improves pathogenic variants (PVs) detection. However, MGPT yields complex results, including secondary findings, heterozygous PVs in recessive genes, low-penetrance PVs, and variants of uncertain significance. We reported our mono-institutional experience of germline testing in breast cancer (BC), comparing SGT and MGPT.

Methods: We retrospectively analyzed clinical and molecular data from 1084 BC patients: 308 underwent SGT (/) and 776 MGPT (for 28 cancer-related genes). We compared these approaches regarding the genetic classification of the findings (positive, uncertain, uninformative) and their impact on clinical management (primary findings (PFs); complex and inconclusive results). Additionally, we described clinical features supporting one approach over the other and focused on copy number variation (CNV) frequency in non- genes.

Results: We found ≥1 PV in 165 patients (165/1084 = 15.2%), including 91 in / (91/1084 = 8.4%), with 42 identified by SGT (42/308 = 13.6%) and 49 by MGPT (49/776 = 6.3%). MGPT detected PVs in non- genes in 74 patients (74/776 = 9.5%), including 40 PFs. Overall, MGPT identified 89 PFs (89/776 = 11.5%). We observed complex results in 21 patients (21/308 = 6.8%) with SGT and in 300 (300/776 = 38.7%) with MGPT. Compared to MGPT, SGT detected a similar percentage of PFs (13.6% vs. 11.5%) but a significantly reduced percentage of complex results (6.8% vs. 38.7%) ( < 0.001). Triple-negative BCs prevailed in carriers, while ER-positive BCs were more prevalent in / carriers. Concerning non- genes, MGPT detected CNVs in , representing 20% of PVs in this gene.

Conclusions: Although MGPT increases hereditary BC detection, its complexity requires clear guidelines for optimal clinical management and strategies for merging the benefits of SGT and MGPT.