Malignancy Risk Associated with the Thyroid Stimulating Hormone Receptor (TSHR) pI568T Variant in Bethesda III/IV Thyroid Nodule Cytology

In indeterminate thyroid nodules, TSHR mutations usually predict benign final pathology and when malignant, low risk disease. The TSHRpI568T variant is a rare and poorly described TSHR variant. A 53-year-old female presented with a 1.3 cm ultrasonographically ATA high risk thyroid nodule that had a Bethesda V (BV) cytology result. Afirma Xpression Atlas (XA) analysis revealed TSHRpI568T variant expression. Surgery revealed a 2cm classic papillary thyroid cancer (PTC) with tracheal invasion. This prompted investigation of the Afirma database for samples with TSHRpI568T variant expression and correlation of those with Afirma Gene Sequencing Classifier suspicious (GSC-S) results with final pathology.

Methods:

TSHRpI568T variants were evaluated in Afirma GSC- Benign (GSC-B) and GSC-S samples across BIII and IV cytology categories. For GSC-S samples, surgical pathology reports were requested under an IRB protocol and the rate of malignancy was calculated. Non-invasive follicular neoplasm with papillary like nuclear features (NIFTP) was considered malignant.

Results:

There were 427 samples from 388 patients harboring the TSHRpI568T variant, representing 0.33% of all Afirma GSC tests. Ninety percent (n=383) were BIII and 10% (n=44) were BIV. GSC-S was reported on 25/427 samples (6% overall): 17 from BIII cytology (4%) and 8 from BIV cytology (18%) (p < 0.01, two proportion z-test). Eight patients had no follow up data. Of the 17 remaining, 14 pathology reports were obtained. Three malignancies were found (all with BIII cytology), including a 1.2cm NIFTP, a 0.8cm PTC and a 2.0cm invasive follicular variant PTC. Thus, 3/14 (21%) were malignant. Assuming the GSC-B samples are benign, the malignancy rate based on TSHRpI568T variant detection alone is 3/427 (0.7%). Making the extreme assumption that all 11 GSC-S patients without follow up have malignancy, the malignancy rate would be 14/427 (3%). In contrast, of the 25 GSC-S samples, we know 11 are benign and 3 are malignant. If we assume the 11 unknown samples are all benign or malignant, the malignancy rate within GSC-S ranges from 12% (3/25) to 56% (14/25), respectively.

Discussion/Conclusion:

TSHRpI568T is a rare mutation in indeterminate thyroid cytology. If all GSC-B samples in our study are truly benign, the overall estimated risk of malignancy of the TSHRpI568T variant alone is at most 3%. However, within the GSC-S category, the risk estimate is 21% (12-56%). Therefore, nodules with the GSC-S molecular result have a significantly increased risk estimate of cancer, which is clinically meaningful and relevant to clinicians and patients when deciding whether to proceed with surgery. As shown by our index case, these rarely can present with aggressive features. Conversely, 94% of TSHRpI568T samples are identified as molecularly benign (GSC-B) where clinical observation is favored.