Distinguishing thymic cysts from low-risk thymomas via [ <sup>18</sup>F]FDG PET/CT

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Abstract

Background

Thymic cysts are a rare benign disease that needs to be distinguished from low-risk thymoma. [ ¹⁸F]fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) is a non-invasive imaging technique used in the differential diagnosis of thymic epithelial tumours, but its usefulness for thymic cysts remains unclear. Our study evaluated the utility of visual findings and quantitative parameters of [ ¹⁸F]FDG PET/CT for differentiating between thymic cysts and low-risk thymomas.

Methods

Patients who underwent preoperative [ ¹⁸F]FDG PET/CT followed by thymectomy for a thymic mass were retrospectively analyzed. The visual [ ¹⁸F]FDG PET/CT findings evaluated were PET visual grade, PET central metabolic defect, and CT shape. The quantitative [ ¹⁸F]FDG PET/CT parameters evaluated were PET maximum standardized uptake value (SUVmax), CT diameter (cm), and CT attenuation in Hounsfield units (HU). Findings and parameters for differentiating thymic cysts from low-risk thymomas were assessed using Pearson’s chi-square test, the Mann-Whitney U-test, and receiver operating characteristics (ROC) curve analysis.

Results

Seventy patients (18 thymic cysts and 52 low-risk thymomas) were finally included. Visual findings of PET visual grade ( P < 0.001) and PET central metabolic defect ( P < 0.001) showed significant differences between thymic cysts and low-risk thymomas, but CT shape did not. Among the quantitative parameters, PET SUVmax ( P < 0.001), CT diameter ( P < 0.001), and CT HU ( P = 0.004) showed significant differences. In ROC analysis, PET SUVmax demonstrated the highest area under the curve (AUC) of 0.996 ( P < 0.001), with a cut-off of equal to or less than 2.1 having a sensitivity of 100.0% and specificity of 94.2%. The AUC of PET SUVmax was significantly larger than that of CT diameter ( P = 0.009) and CT HU ( P = 0.004).

Conclusions

Among the [ ¹⁸F]FDG PET/CT parameters examined, low FDG uptake (SUVmax ≤ 2.1, equal to or less than the mediastinum) is a strong diagnostic marker for a thymic cyst. PET visual grade and central metabolic defect are easily accessible findings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-024-01108-3.

Related collections

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Although the histologic classification of thymic epithelial tumors has been confusing and controversial, an agreement on the universal classification system for thymic epithelial tumors was achieved by the World Health Organization (WHO) in 1999. The authors previously reported that the WHO histologic classification system reflects invasiveness and immunologic function of thymic epithelial tumors. In this subsequent study, they examined the prognostic significance of this classification system. Clinical features as well as postoperative survival of patients with thymoma, but not thymic carcinoma, were examined with reference to WHO histologic classification based on an experience with 273 patients over a 44-year period. There were 18 type A tumors, 77 type AB tumors, 55 type B1 tumors, 97 type B2 tumors, and 26 type B3 tumors. In patients with type A, AB, B1, B2, and B3 tumors, the respective proportions of invasive tumor were 11.1%, 41.6%, 47.3%, 69.1%, and 84.6%; the respective proportions of tumors with involvement of the great vessels were 0%, 3.9%, 7.3%, 17.5%, and 19.2%; and the respective 20-year survival rates were 100%, 87%, 91%, 59%, and 36%. According to the Masaoka staging system, the 20-year survival rates were 89%, 91%, 49%, 0%, and 0% in patients with Stage I, II, III, IVa, and IVb disease, respectively. By multivariate analysis, the Masaoka staging system and the WHO histologic classification system were significant independent prognostic factors, whereas age, gender, association with myasthenia gravis, completeness of resection, or involvement of the great vessels were not significant independent prognostic factors. This study showed that histologic appearance reflects the oncologic behavior of thymoma when the WHO classification system is adopted. The WHO classification system may be helpful in clinical practice for the assessment and treatment of patients with thymoma. Copyright 2002 American Cancer Society. DOI 10.1002/cncr.10225

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Cystic masses of the mediastinum are well-marginated round lesions that contain fluid and are lined with epithelium. Major cystic masses include congenital benign cysts (ie, bronchogenic, esophageal duplication, neurenteric, pericardial, and thymic cysts), meningocele, mature cystic teratoma, and lymphangioma. Many tumors (eg, thymomas, Hodgkin disease, germ cell tumors, mediastinal carcinomas, metastases to lymph nodes, nerve root tumors) can undergo cystic degeneration-especially after radiation therapy or chemotherapy-and demonstrate mixed solid and cystic elements at computed tomography (CT) or magnetic resonance (MR) imaging. If degeneration is extensive, such tumors may be virtually indistinguishable from congenital cysts. A mediastinal abscess or pancreatic pseudocyst also appears as a fluid-containing mediastinal cystic mass. However, clinical history and manifestations, anatomic position, and certain details seen at CT or MR imaging allow correct diagnosis in many cases. Familiarity with the radiologic features of mediastinal cystic masses facilitates accurate diagnosis, differentiation from other cystlike lesions, and, thus, optimal patient treatment. Copyright RSNA, 2002

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Author and article information

Contributors

Yong-il Kim:

ORCID: http://orcid.org/0000-0002-4113-8351

kyi821209@naver.com

Journal

Journal ID (nlm-ta): EJNMMI Res

Journal ID (iso-abbrev): EJNMMI Res

Title: EJNMMI Research

Publisher: Springer Berlin Heidelberg (Berlin/Heidelberg )

ISSN (Electronic): 2191-219X

Publication date (Electronic): 3 May 2024

Publication date PMC-release: 3 May 2024

Publication date Collection: 2024

Volume: 14

Electronic Location Identifier: 45

Affiliations

[1 ]GRID grid.267370.7, ISNI 0000 0004 0533 4667, Department of Nuclear Medicine, Asan Medical Center, , University of Ulsan College of Medicine, ; Seoul, Republic of Korea

[2 ]GRID grid.289247.2, ISNI 0000 0001 2171 7818, Department of Nuclear Medicine, Kyung Hee University Hospital, School of Medicine, , Kyung Hee University, ; Seoul, Republic of Korea

[3 ]GRID grid.267370.7, ISNI 0000 0004 0533 4667, Department of Thoracic and Cardiovascular Surgery, Asan Medical Center, , University of Ulsan College of Medicine, ; Seoul, Republic of Korea

Author information

Yong-il Kim http://orcid.org/0000-0002-4113-8351

Article

Publisher ID: 1108

DOI: 10.1186/s13550-024-01108-3

PMC ID: 11068711

PubMed ID: 38702532

SO-VID: a74c2d0d-aeb8-4ebf-91c3-7b71f0a10e8a

License:

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

History

Date received : 13 March 2024

Date accepted : 26 April 2024

Funding

Funded by: FundRef http://dx.doi.org/10.13039/100008903, Ministry of Health and Welfare;

Award ID: HR18C0016

Award Recipient : Yong-il Kim Sangwon Han Jin-Sook Ryu

Custom metadata

ScienceOpen disciplines: Radiology & Imaging

Keywords: thymus neoplasms,fluorodexoyglucose f18,positron emission tomography computed tomography,sensitivity and specificity

Data availability:

ScienceOpen disciplines: Radiology & Imaging

Keywords: thymus neoplasms, fluorodexoyglucose f18, positron emission tomography computed tomography, sensitivity and specificity