Christine Do Cao

Introduction Anaplastic thyroid carcinoma (ATC) is the most aggressive form of thyroid cancer with a bleak prognosis. Favorable outcomes are rare but help decipher molecular pathophysiology, investigate prognosis factors, and discover new therapeutic targets. Case presentation Two patients were diagnosed with locally advanced nonresectable ATC, one with metastatic extension. Each patient received chemotherapy and radiotherapy, allowing thyroid surgical resection. In both cases, the pathological examination was consistent with complete response with no viable tumor cells. After follow-ups of 48 and 70 months, both patients remain disease-free. Molecular explorations on thyroid biopsies revealed microsatellite instability (MSI) and alterations on mismatch repair–gene complex, also PTEN and ATM variants in both cases. Both also presented with non-classical immune infiltrate composed of equal parts T CD4 + lymphocytes and macrophages. Conclusion We report two cases of patients cured from advanced ATC and for the first time provide genetic and immunological explorations in this setting. It seems with these two cases that MSI-ATCs may indicate a better prognosis. Our study hypothesizes different responsible mechanisms including increased sensitivity to chemoradiotherapy and/or immune tumor infiltrate modulation.