Boosted abscopal effect from radiotherapy and pembrolizumab in anaplastic thyroid cancer: a mini-review and case report

Background: The abscopal effect, in which radiation induces a systemic anti-tumour immune response, has been demonstrated with radiotherapy. Immunotherapy boosts the abscopal effect by facilitating the immune response to radiation. Radiotherapy and programmed cell death protein 1/programmed cell death ligand 1 (PD-1/PD-L1) blockade has resulted in the boosted abscopal effect in solid cancers, but its role in anaplastic thyroid cancer (ATC) is unknown. In this mini-review, we describe the abscopal effect and summarise its proposed underlying mechanisms. We then present a potential case of boosted abscopal effect in ATC. Case Description: In our case presentation, we describe a 51-year-old female who presented with 3 weeks of rapidly enlarging thyroid mass. Examination revealed a 3-cm thyroid nodule which was Bethesda V on fine needle aspiration cytology (FNAC). Intraoperatively, there was a gross extrathyroidal extension into the cricoid cartilage. After total thyroidectomy, post-operative histopathology showed widely invasive follicular thyroid cancer with anaplastic transformation (>50%). Immunohistochemistry showed high PD-L1 expression [combined positive score (CPS) >70%]. Due to residual cricoid cartilage disease and several perihilar and lung metastases on positron emission tomography-computed tomography (PET-CT) scan, she underwent post-operative palliative radiotherapy and pembrolizumab. After two cycles of pembrolizumab, repeat PET-CT scan showed complete response (CR) of local and distant disease. She remained well for 32 months, before recent discovery of a right mandible bony metastasis planned for radiotherapy. Conclusions: This case demonstrates exceptional response to radiotherapy and anti-PD-1 immunotherapy in ATC, potentially illustrating the first known abscopal effect in ATC with this treatment.