Not always a miracle - The case of the missing metastasis.

Spontaneous regression of malignant neoplasms is extremely rare, but renal cell carcinomas (RCC) are most often associated with this phenomenon. We report a case of a patient with personal history of RCC, who underwent nephrectomy and no other oncological treatment. One year after nephrectomy, a lung metastasis was detected and kept under follow-up for 3 years. Its size increased over time until a needle biopsy was performed, and its metastatic nature confirmed. Wedge resection of the lung nodule was performed, and no neoplastic cells were found, suggesting its spontaneous regression after biopsy. Different theories have been proposed to explain this phenomenon and, in most cases, the mechanism seems to involve the activation of the immune system. This case supports the importance of reducing tumor burden and the impact of the disturbance of the tumor microenvironment caused by instrumentation, in improving immune system activation and its essential role in neoplasm regression.