Evaluation of PD-L1 antigen expression using immunohistochemistry technique in medullary thyroid carcinoma samples.

Background: Markers related to the mechanism of tumoral cell escape from the immune system have received more attention. The PD-L1 molecule encoded by the "CD274" gene binds to T lymphocytes and can inhibit these cells. Therefore, increasing the expression of this marker on inflammatory or tumor cells can indicate tumor progression invasiveness and long-term consequences. The present study aimed to determine the expression of the PD-L1 marker in thyroid medullary tumors and to evaluate its role in predicting long-term outcomes after cancer.

Methods: This retrospective longitudinal study was performed on pathology samples of patients with medullary thyroid carcinoma referred to the Cancer Institute of Imam Khomeini Hospital from 2015 to 2020. Slides related to medullary thyroid tumors were examined. A tissue microarray was used to evaluate the immunohistochemistry of PD-L1. Patients were followed up to assess the occurrence of recurrence. Out of 207 patients evaluated in the present study, histopathological information of 144 patients was available.

Results: The expression rate of PD-L1 in our community was 14.6% in lymphocyte cells, 35.4% in tumor cells, and 12.5% in both cells. The presence of metastasis at the time of diagnosis was reported in 35 cases (72.9%), and the occurrence of tumor recurrence was reported in 38 cases (79.2%). There was no relationship between the expression of this marker and the sex and age of patients. In addition, PD-L1 expression was unrelated to the two main characteristics of this cancer, namely tumor size and its focality. The presentation of tumor PD (L1) (but not lymphocytic) was a prognostic marker for synchronous metastasis at cancer diagnosis but could not predict tumor recurrence.

Conclusion: PD-L1 tumor marker expression is predictable in 14.6% of lymphocyte cells, 35.4% of tumor cells, and 12.5% in the selected Iranian population with medullary thyroid cancer. The expression of this marker is not related to the morphological characteristics of the tumor, such as tumor size or focality.