Conference Coverage

Revised thyroid Bethesda System resets malignant risks


 

EXPERT ANALYSIS FROM WCTC 2017

– Under the newly revised Bethesda System for Reporting Thyroid Cytology, slated for official release in October 2017, the six cytology-based diagnostic categories for thyroid lesions stay exactly the same as in the 10-year-old first edition, but some associated malignancy risks have changed.

Important changes include molecular testing to further assess malignancy risk in thyroid nodules and the introduction of lobectomy as a treatment option, “which really wasn’t an option 10 years ago,” in the first iteration of the Bethesda System (New York: Springer US, 2010), its coauthor Edmund S. Cibas, MD, said at the World Congress on Thyroid Cancer.

Dr. Edmund S. Cibas professor of pathology at Harvard Medical School and director of cytopathology at Brigham and Women’s Hospital in Boston Mitchel L. Zoler/Frontline Medical News

Dr. Edmund S. Cibas

He singled out reframing the malignancy risks for some of the six cytology categories as a top message of the revision, and he attributed these changes to two main factors: routine molecular testing, and creation of a new diagnostic category, the “noninvasive follicular thyroid neoplasm with papillary-like nuclear features” (NIFTP).

An Endocrine Pathology Society working group created the NIFTP designation in 2016 to describe an encapsulated follicular variant of papillary thyroid carcinoma that is characterized by lack of invasion, a follicular growth pattern, and nuclear features of papillary thyroid carcinoma with a very low risk of an adverse outcome (JAMA Oncology. 2016 Aug;2[8]:1023-9) (Cancer Cytopathol. 2016 Sep;124[9]:616-20).

NIFTP is not an overt malignancy. The revised Bethesda System “limits malignancy to cases with features of classic malignant papillary thyroid carcinoma,” explained Dr. Cibas, professor of pathology at Harvard Medical School and director of cytopathology at Brigham and Women’s Hospital, both in Boston.

Because the Bethesda System categories link to specific management recommendations, the new edition orients patients toward more conservative management decisions, specifically lobectomies instead of total thyroidectomies, he said in an interview.

The International Cytology Congress held a symposium during its meeting in Yokohama, Japan, in 2016, which resulted in the second edition of the Bethesda System (ACTA Cytol. 2016 Sep-Oct; 60[5]:399-405).

The changes in risk of malignancy occurred primarily in two categories, either “atypia of undetermined significance” (AUS) or “follicular lesions of undetermined significance” (FLUS). The risk of malignancy jumped from 5%-15% in the Bethesda System first edition up to 10%-30% in the revision. A smaller bump-up hit the category of “follicular neoplasm” or “suspicious for follicular neoplasm,” in which the risk of malignancy increased from 20%-30% in the first edition to 25%-40% in the revision. And, in the suspicion of malignancy category, the risk of malignancy actually lowered modestly, easing from 60%-75% in the first edition to 50%-75% in the revision.

Dr. Cibas highlighted the AUS/FLUS category with further notable features. The limit on laboratories reporting this category increased to 10% of total reports, up from 7% in the first edition. Management changed from the single options of a repeat fine-needle aspiration specimen to either that or molecular testing. Also, “the first edition was not clear that AUS and FLUS are synonyms. That will be a lot clearer” in the second edition, Dr. Cibas promised. The revision “will encourage labs that currently use [the terms] AUS and FLUS to mean two different things to make a choice between them.”

Another quirk of the AUS and FLUS category is that the risk of malignancy estimates are based on what Dr. Cibas called “flawed” data from only the selected subset of AUS or FLUS patients who have their nodule resected. “The reality is that most of the nodules are not resected” from patients with AUS or FLUS, so conclusions about the risk of malignancy come from a subset with considerable selection bias.

The definition of “follicular neoplasm” or “suspicious for follicular neoplasm” category also added “mild nuclear changes,” which can include increased nuclear size, contour irregularity, or chromatin clearing. The “suspicious for malignancy” category made a modest tweak to the risk of malignancy. Plus, “some of these patients will now undergo lobectomy rather than total thyroidectomy, which has been usual management.

The “suspicious for malignant” and “malignant” categories had little change aside from wider use of lobectomy, now feasible for any patient except those with metastatic disease, Dr. Cibas said.

On Twitter @mitchelzoler

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