Spontaneous Complete Regression of Breast Cancer: Two Case Report.

Spontaneous regression (SR) is a tumor's partial or complete disappearance without any treatment. In the literature, it has been documented that SR is uncommon in breast cancer (BC) and other types of cancer. Multiple mechanisms are believed to contribute to the development of SR. However, its mechanism still needs to be clearly demonstrated. Although two SR patients were presented in our study, the evidence needed to be more sufficient to determine the mechanism. However, due to Programmed Death-Ligand 1 (PD-L1) negativity in both patients, the hypothesis in the literature that PD-L1 has strong antitumoral activity was not supported. In addition, it was determined that the patient in case 2 was the first Cerb B2 positive case reported in the literature and had the earliest SR period. Due to this, it has been disclosed that the SR mechanism of BC will be concluded within 21 days at the earliest. This situation suggests that breast surgeons, in particular, should conduct a thorough physical examination and, if necessary, re-radiological examination before surgery on patients for whom surgery is decided after diagnosis. Being careful in this regard may increase the number of SR in BC cases and allow molecular investigations on living tissue samples to reveal the underlying mechanism.