- Jasim S, Golding A, Bimston D, et al. Cytologic and molecular assessment of isthmus thyroid nodules and carcinomas. Thyroid. Epub 2024 Nov 11; doi: 10.1089/thy.2024.0254. PMID: 39527399.
- Background:
- Isthmus thyroid nodules:
- While less frequent than lobar nodules may carry a higher risk of thyroid cancer and be associated with more aggressive histopathologic features than nodules in either thyroid lobe
- The underlying reasons for these could be the anatomical location, thin thyroid tissue, and unique lymphatic drainage
- However, a recent study demonstrated potentially different molecular signatures:
- With higher ERK and lower thyroid differentiation scores (TDSs) in isthmus as compared to lobar papillary thyroid carcinoma (PTC)
- Whereas most thyroid nodules are cytologically benign:
- Approximately 20% to 30% have a Bethesda III or IV cytology:
- Considered indeterminate thyroid nodules (ITNs):
- Demonstrating a risk of malignancy ranging from 13% to 34%
- Considered indeterminate thyroid nodules (ITNs):
- Approximately 20% to 30% have a Bethesda III or IV cytology:
- Historically, surgical intervention was the mainstay for these ITNs:
- Though most are histopathological benign:
- Hence, molecular testing has emerged to guide the management of ITNs, of which the Afirma genomic sequencing classifier (GSC) uses exome-enriched RNA sequencing:
- Providing data on gene and exon expression, mitochondrial expression, loss of heterozygosity, and detection of expressed gene variants and fusions to provide diagnostic information for Bethesda III /IV, and potentially prognostic information for Bethesda V and GSC-suspicious nodules
- Hence, molecular testing has emerged to guide the management of ITNs, of which the Afirma genomic sequencing classifier (GSC) uses exome-enriched RNA sequencing:
- Though most are histopathological benign:
- This study demonstrates the cytologic and molecular differences between nodules and PTC based on isthmus or lobar location
- Isthmus thyroid nodules:
- Methods:
- This observational study analyzed two cohorts in a complementary manner:
- Afirma thyroid nodule database (n = 177,227) with genome wide differential expression analysis to decipher transcriptomic differences between isthmus and lobar nodules
- Histopathology reports (n = 583) of classic PTC (n = 389) and infiltrative follicular subtype of PTC (IF-PTC) (n = 194) from Afirma discovery cohorts and thyroid cancer patients treated at an integrative endocrine surgery community care practice for molecular differences between isthmic and lobar cancers
- For molecular difference assessment, 54 gene expression signatures were evaluated, including activity scores of 50 hallmarks of cancer pathways, BRAF-like to RAS-like molecular score (BRS), ERK signaling, TDS, and follicular to mesenchymal transition score
- For pathway enrichment analysis, transcriptome-wide differential expression analysis using 26,268 genes was conducted to identify genes upregulated and downregulated in isthmus nodules by using the Wilcoxon rank-sum statistical test. Deconvolution of bulk gene expression data was used to estimate levels of infiltrating immune cells
- This observational study analyzed two cohorts in a complementary manner:
- Results:
- There were 8527 (4.8%) isthmus nodules identified in the Afirma database
- Isthmus nodules were twice as likely to be Bethesda V / VI (8.2% vs. 4.3%, P<0.0001) and had twice the frequency of BRAF V600E (21% vs. 10.6%, P<0.0001), an increased frequency of ALK/NTRK/RET fusions (4.6% vs. 2.5%, P<0.0001) and SPOP variants (1.5% vs. 0.8%, P<0.0001):
- While demonstrating a lower frequency of NRAS mutations (7.8% vs. 13.2%, P<0.0001), PAX8/PPAR-gamma fusions (1.1% vs. 2.3%, P<0.0001, and RET variants (0.06% vs. 0.53%, P = 0.001) than lobar nodules
- Isthmus nodules also demonstrated higher BRS, ERK activity, and FMT scores (P<0.0001 for all) but lower inflammation immune activity scores on transcriptome and genomewide analysis
- Complementarily, isthmus nodules showed downregulation of immune response regulation genes on pathway enrichment analysis (OR, 0.74; P<0.001) relative to lobar nodules
- As compared to lobar IF-PTCs (n = 181), those from the isthmus (n = 13) demonstrated higher BRS, ERK activity, and FMT scores (P<0.01 for all)
- Analysis of clinical outcomes from 454 samples did not show differences in the frequency of vascular or capsular invasion, extrathyroidal extension, positive surgical margins, or lymph node metastases by cancer location
- Conclusions:
- Isthmus nodules were more likely to be malignant and to have increased rates of higher-risk molecular alterations compared to lobar nodules
- IF-PTC from the isthmus was molecularly different than lobar IF-PTC
- More investigation is needed to validate these findings before impacting the management of isthmus thyroid nodules
- COMMENTARY:
- The location of a nodule in the thyroid may provide a clue to the likelihood of malignancy and to the possibility of an aggressive type, but the current data supporting this are limited
- Using the Afirma molecular test on validation and clinical data sets, this study found isthmus thyroid nodules to be more likely to have Bethesda V/VI cytology and higher rates of BRAF-like molecular signatures and other molecular markers associated with aggressive behavior than lobar nodules
- When restricted to cancerous nodules, isthmus IF-PTC showed higher BRAF-like, ERK, and FMT expression scores consistent with aggressive molecular profiles than lobar IF-PTC
- This study builds on prior work using The Cancer Genome Atlas demonstrating molecular differences between isthmus and lobar PTC, confirming previous literature demonstrating higher risk and histopathologic aggressiveness of PTC in isthmus nodules
- The authors acknowledge study limitations, including its retrospective observational nature, making it prone to selection bias, low number of cancerous isthmus nodules, and lack of longterm outcome data, limiting their ability to potentially demonstrate histopathologic and clinical differences as compared to lobar nodules
- While the molecular signatures associated with aggressive PTC behavior were statistically higher in isthmus nodules, their clinical significance needs further investigation
- In summary, this study provides insights into different malignancy and aggressiveness risk of isthmus thyroid nodules based on molecular signatures
- The findings should guide future investigations into the underlying reasons for these molecular differences and prospective analyses across different cohorts to improve the management of thyroid nodules
Rodrigo Arrangoiz MS, MD, FACS, FSSO 