OP62 – An analysis of the American Joint Committee on Cancer 8th edition revised T staging system for papillary thyroid carcinoma and exclusion of microscopic extrathyroidal extension

      Tran, Bryan1; Roshan, David2; Abraham, Earl1; Graham, Susannah1; Lehane, Christopher1; Wykes, James1; Campbell, Peter1; Ebrahimi, Ardalan1 1 Head and Neck Unit, Department of Surgery, Liverpool Hospital, Sydney, Australia 2 South Western Sydney Clinical School, UNSW, Sydney, Australia   Background/Purpose: Increasing evidence that microscopic extrathyroidal extension (m-ETE) is not an independent prognostic factor in papillary thyroid carcinoma (PTC) prompted its removal from the 8th edition AJCC T staging (pT8) [1-4]. Tumours staged as T3 in the 7th edition (pT7) based on m-ETE are hence down-staged according to size. Objectives: We aimed to compare the prognostic performance of pT8 and pT7, particularly in patients ?55 years old without distant metastases, in whom changes to T classification impact TNM stage. Method: A retrospective analysis of disease-free survival (DFS) in 577 patients with PTC comparing pT8 versus pT7 using the Akaike information criterion (AIC), Harrel’s C-index, proportion of explained variation (r2) and visual inspection for stratification into distinct prognostic groups on Kaplan-Meier curves. Results: There were 105 T3 patients in pT7, with 74 based on m-ETE. Thirty (28.6%) were down-staged to T1a, 27 (25.7%) to T1b and 17 (16.2%) to T2. In patients ?55 years old without distant metastases, pT8 was inferior to pT7 on the basis of higher AIC, lower C-index (0.68 versus 0.75) and lower r2 (0.34 versus 0.41). In pT8 stage T1a-T3a disease, m-ETE was associated with reduced DFS (HR 2.8, p=0.002) irrespective of tumour size. Discussion and Conclusion In our patient population, the prognostic performance of pT8 was inferior to pT7, particularly in patients ?55 years old without distant metastases.  Microscopic ETE appears to be an important factor in T staging and consideration should be given to reintroducing this to the AJCC staging if these results are validated.   References:
    1. Ito, Y., et al., Prognostic significance of extrathyroid extension of papillary thyroid carcinoma: massive but not minimal extension affects the relapse-free survival. World journal of surgery, 2006. 30(5): p. 780-786.
    2. Mete, O., L. Rotstein, and S.L. Asa, Controversies in thyroid pathology: thyroid capsule invasion and extrathyroidal extension. Annals of surgical oncology, 2010. 17(2): p. 386-391.
    3. Nixon, I.J., et al., The impact of microscopic extrathyroid extension on outcome in patients with clinical T1 and T2 well-differentiated thyroid cancer. Surgery, 2011. 150(6): p. 1242-1249.
    4. Wang, L.Y., et al., Operative management of locally advanced, differentiated thyroid cancer. Surgery, 2016. 160(3): p. 738-746.


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