The solid variant of papillary thyroid carcinoma: a multi‐institutional retrospective study

Abstract

Aims The definition of papillary thyroid carcinoma, solid variant (PTC‐SV) varies from >50% to 100% of solid/trabecular/insular growth (STI). We aimed to identify prognostic factors and to establish an appropriate STI cutoff for PTC‐SV in this multi‐institutional study of 156 PTCs with STI. Results Nodal metastases were seen in 18% and were associated with a higher percentage of papillary and STI. When substratified by infiltration/encapsulation status, the STI percentage did not impact the risk of nodal metastasis. pN1 stage was seen in 51% of infiltrative tumours and 1% of encapsulated lesions. Overall, PTC with STI had an excellent prognosis. The 10‐year disease‐free survival (DFS) was 87% in the entire cohort, 94% in encapsulated lesions, and 76% in infiltrative tumours. The STI percentage did not impact DFS. Fifty‐four patients had noninvasive encapsulated lesions with 2–100% STI. None developed recurrence. Encapsulated lesions were enriched with RAS mutations (54%), whereas infiltrative lesions lacked RAS mutations (4%). The BRAF V600E mutation was an infrequent event, being seen in 11% of the entire cohort. Conclusion In PTC with STI, the determining factor for nodal metastasis and DFS is the encapsulation/infiltration status rather than the STI percentage. Encapsulated noninvasive tumours with STI follow an indolent course with a very low risk of nodal metastasis and recurrence. Overall, PTC with STI has an excellent prognosis, with a 10‐year disease‐specific survival (DSS) and DFS of 96% and 87%, respectively. Therefore, the classification of SV‐PTC as an aggressive PTC subtype may be reconsidered.

Molecular and clinical features of papillary thyroid cancer in adult patients with a non-classical phenotype

“…We presumed that these delicate papillae were formed in the following ways: (1) originating from a solid structure with delicate fibrovascular cores by cellular dehiscence (Figure S3); or (2) originating from packed microfollicles by follicular-septum cracking, which may be different from the formation of the common papillae present in CPTC (Figure S4). Meanwhile, considering that the solid growth pattern was predominant (> 50%) in our cases, a diagnosis of a solid variant of PTC (SVPTC) may also be established (50,51), which is most commonly seen in children after the Chernobyl nuclear accident (52,53) or in few adults without a history of radiation exposure (51), accounting for 1-3% of all PTCs (54). Different from the most common encapsulated type (65%) (54) in reported SVPTC, our general solid growth cases were almost non-encapsulated and invasive (87.5%, 7/8), and showed a lower proportion (0.76%, 8/ 1059) in Chinese, suggest it may be clinically rare.…”

“…Meanwhile, considering that the solid growth pattern was predominant (> 50%) in our cases, a diagnosis of a solid variant of PTC (SVPTC) may also be established (50,51), which is most commonly seen in children after the Chernobyl nuclear accident (52,53) or in few adults without a history of radiation exposure (51), accounting for 1-3% of all PTCs (54). Different from the most common encapsulated type (65%) (54) in reported SVPTC, our general solid growth cases were almost non-encapsulated and invasive (87.5%, 7/8), and showed a lower proportion (0.76%, 8/ 1059) in Chinese, suggest it may be clinically rare. Furthermore, extensive fibrosis was quite common in our series.…”

Clinical significance of papillary thyroid carcinoma with solid/trabecular growth

“…7 The PTC-ST subtype has been associated with a less favourable outcome, although the data are conflicting. [8][9][10][11][12][13][14] Several studies suggest that, compared with classic PTC, the PTC-ST subtype is associated with a higher frequency of nodal and distant metastasis, higher recurrence risk, and decreased disease-specific survival (DSS). 9,10,13 In contrast, other studies report that PTC-ST may not be as aggressive as previously thought.…”

The metamorphosis of papillary thyroid carcinoma

“…Drastic shifts in our understanding of papillary thyroid carcinoma (PTC) have occurred in the last decade due to advances in molecular diagnostics and long‐term clinical follow‐up of patients. The article “The solid variant of papillary thyroid carcinoma: a multi‐institutional retrospective study” by Xu, et al ., in the current issue of Histopathology , represents part of the burgeoning literature emphasizing the indolent behavior and urging the reconceptualization of various types of PTC 1 . The initial shift in this direction was the proposal by the Endocrine Pathology Society working group (Nikiforov, et al ., published online in April, 2016 and revised in June 2018 in JAMA Oncology) to reclassify select cases of noninvasive, encapsulated follicular variant of papillary thyroid carcinoma to noninvasive follicular thyroid neoplasm with papillary‐like nuclear features (NIFTP) if certain criteria were met, in light of their indolent nature (minimal to absent metastatic potential and recurrence) as well as their molecular genetic profile being more similar to follicular neoplasms ( RAS ‐like) rather than classic PTCs ( BRAF ‐like) 2,3 …”

“…The study by Xu, et al . in this month's Histopathology evaluates PTCs with solid, trabecular, or insular growth (STI) without other adverse high‐grade features (including necrosis and high mitotic activity), and they conclude that noninvasive tumors with STI (of any amount) demonstrate RAS ‐like genetics, have a very low risk of metastasis, and should be regarded as very low risk lesions 1 . While this group does not explicitly advocate expanding the definition of NIFTP to include lesions with greater than 30% STI, this article may pave the way for additional reevaluation of the NIFTP criteria.…”