Different activity and toxicity of immunotherapy in monozygotic twins diagnosed with early triple-negative breast cancer: a case report.

Breast cancer (BC) is the most common malignancy among women. Among 5%-10% of diagnoses are correlated with hereditary cancer syndromes, while the remaining cases are sporadic and linked to multiple factors. When a pathogenetic variant in one of the genes commonly associated with BC is detected, the patient is referred to a tailored surveillance program; otherwise, the standard follow-up guidelines are applied. We present a unique case of BC diagnosed in two monozygotic twins at the same age apparently unrelated to a hereditary syndrome known to date. Notably, despite comparable clinical-pathological features, the two neoplasms behaved differently to neoadjuvant chemo-immunotherapy, showing different outcomes and toxicities. Very little is known about the predictive mechanisms of response and toxicity to immunotherapy and this clinical case might be a starting point for further investigations.