Main points

• The peritumoral computed tomography (CT) features of thymic epithelial tumors (TET) provide additional information for predicting transcapsular invasion.

• The intratumoral and peritumoral CT features of TETs exhibit good reproducibility.

• In patients with TETs, the CT features of shape, capsule integrity, and peritumoral vascularity help differentiate between those with transcapsular invasion and those without.

Thymic epithelial tumors (TETs), originating in thymic epithelial cells, are rare tumors of the anterior (prevascular) mediastinum, with a reported annual incidence ranging from 1.3 to 3.2 per million globally.^1,2 Histologically, various staging systems have been proposed for TETs, including the tumor, nodes, and metastases (TNM) classification of malignant tumors and non-TNM based staging systems.³ The Masaoka–Koga staging system has been extensively used to classify and stage TETs, including thymomas and thymic carcinomas, based on the extent of tumor invasion.^4,5 In the Masaoka–Koga staging system, the distinction between stages I and II is based on whether the tumor exhibits transcapsular invasion.⁶ Transcapsular invasion is a key factor associated with disease recurrence and patient survival.^7,8 In Masaoka–Koga stages I and II, the 10-year survival rates are 90% and 70% and the average recurrence rates 3% and 11%, respectively.⁷ Additionally, prognostic difference can influence the treatment plan for patients with TET.

Enhanced chest computed tomography (CT) is currently the preferred imaging examination for the initial assessment of patients with TET. Although several studies have suggested an association between the Masaoka–Koga staging system and TET CT features, such as tumor size, shape, and internal characteristics, distinguishing between Masaoka–Koga stages I and II remains challenging when using a preoperative chest CT.^{9,10,11,12,13} Although pleural effusion, enlarged lymph nodes, and pleural or pulmonary nodules have been assessed in patients with TETs, CT analysis has limited value in the prediction of the Masaoka–Koga stage.^{10,11,12,13,14} However, to our knowledge, no studies have analyzed the association between the peritumoral CT features of TETs and the Masaoka–Koga staging system.

Distinguishing between Masaoka–Koga stages I and II preoperatively is crucial for determining an appropriate surgical plan of either complete thymectomy or thymomectomy.^15,16,17 The aim of this study was to retrospectively investigate the potential factors relating to tumoral and peritumoral CT features that influence transcapsular invasion using preoperative chest CT.

Methods

The institutional review board of the participating hospitals approved this retrospective study, and the requirement for patient consent was waived (KNUH 2021-03-004).

Study population and computed tomography acquisition

We retrospectively reviewed 132 preoperative chest CT scans of patients diagnosed with TETs between January 2012 and December 2020 from Kyungpook National University Hospital and Kyungpook National University Chilgok Hospital, Daegu, South Korea. Patients who only underwent chest CT at other hospitals were analyzed using the CT images acquired from these hospitals. All patients underwent chest CT scans within 2 weeks prior to surgery. In total, 16 patients were excluded from our study for the following reasons: radiation therapy before surgery (3/16, 18.8%), inappropriate pathology results (1/16, 6.3%), definite signs of seeding or the invasion of adjacent structures such as large vessels (4/16, 25%), surgical biopsy without total tumor resection (2/16, 12.5%), non-enhanced CT scans (4/16, 25%), and poor CT imaging quality (2/16, 12.5%). In total, 116 chest CT images from 116 patients were included in this study, some of which originated from our hospital (n = 57) and the remainder from other hospitals (n = 59). According to the Masaoka–Koga stage, there were 37 (31.9%) patients at stage I, 37 (31.9%) at stage IIa, 27 (23.3%) at stage IIb, 13 (11.2%) at stage III, and 2 (1.7%) at stage IV. All CT scans were performed using multiple multidetector CT scanners, including 16-, 64-, 128-, 192-, and 320-slice scanners. The CT scans were obtained in the supine position at full inspiration using contrast media. Axial and coronal images were reconstructed with a sharp or standard reconstruction kernel. In all patients, the CT examination used slice thicknesses of 1.0–5.0 mm, a slice gap of 1.0–5.0 mm, kVp values in the range of 90–140, CT dose index–volume values in the range of 2.251–62.24, and dose-length product values in the range of 148.1–914. An automatic tube current was used for all patients.

Clinical variables and computed tomography image analysis

The following clinical data of each patient at admission were collected from the medical records: age, sex, and presence of myasthenia gravis. Two radiologists (J.P. and B.P., with 8- and 5-years’ experience in thoracic radiology, respectively) independently performed a retrospective assessment of TET chest CT features based on enhanced images, including tumor size, shape, capsule integrity, internal necrosis, presence of calcification, pleural effusion, pericardial effusion, and vascularity grade, without knowledge of the clinical and pathological results. We also evaluated non-contrast CT images to evaluate the presence of calcification and the heterogeneous enhancement of TETs.

In case of disagreement, final decisions on the TET features on chest CTs were reached by consensus. Tumor size was defined as the longest diameter among the axial and coronal images of the chest CT. Shape was classified as round, oval, lobulated, or irregular, with or without capsule integrity. Capsule integrity was defined as almost complete (smooth or shallow lobulation) and partial complete (deep lobulation or spiculate protuberance).^18,19 Vascularity grade was defined as the extent of peritumoral vascular structures in the anterior mediastinum and was evaluated at the largest distribution of the vascular structure among the axial and coronal images in the mediastinal setting [width, 440 Hounsfield units (HU); level, 45 HU] of the enhanced chest CT based on the use of CT images at magnifications in the range of ×3–4. It was evaluated subjectively using a 3-point grading scale according to the following criteria: 0, no visible peritumoral vascular structure; 1, non-measurable slit-like peritumoral vascular structure; 2, measurable peritumoral hypertrophied vascular structure, with a size >1 mm (Figure 1). Representative cases of TETs are presented in Figure 2.

Pathological analysis

All pathological specimens were formalin fixed, paraffin embedded, and stained with hematoxylin and eosin. All histopathological specimens were classified by two pathologists with more than 10-years’ experience in line with the fourth edition of the World Health Organization (WHO) classification and Masaoka–Koga staging system.

Statistical Analysis

Categorical variables were expressed as frequency and percentages (%) and were compared between the two groups using the chi-square or Fisher’s exact tests. Continuous variables were tested for normality using the Shapiro–Wilk test. Normally distributed data were presented as mean ± standard deviation and analyzed using the Student’s t-test; however, non-normally distributed data were expressed as median and interquartile range (IQR) and analyzed using the Mann–Whitney U test. The odds ratio (OR) and confidence interval (CI) from the multivariable logistic analysis were used to identify factors that were independently associated with transcapsular invasion in patients with TETs. Variables with P < 0.20 in the univariable analysis were included as input variables for the multivariable logistic regression analysis using the backward stepwise method. Goodness of fit for the logistic regression model was assessed using the Hosmer–Lemeshow and Omnibus tests. We also constructed a nomogram to predict the transcapsular invasion probability of a TET. Interobserver agreements for TETs on chest CT were assessed using Cohen’s or weighted kappa coefficients. For all statistical analyses, the level of significance was set as P < 0.05. All statistical analyses were performed using SPSS statistical software (IBM, Armonk, NY, USA) and the R software package (version 4.0.3, The R Foundation for Statistical Computing).

Results

Clinical variables

The demographics of patients with TETs are listed in Table 1. A total of 116 patients, 52 (44.8%) men and 64 (55.2%) women and a mean age of 54.4 ± 12.3 years, were enrolled in the study. The patients were divided into a group without transcapsular invasion (n = 37) and a group with transcapsular invasion (n = 79) based on their pathology results. According to the WHO pathological classification, there were 5 (13.5%) patients with type A, 8 (21.6%) with type AB, 10 (27.0%) with type B1, 11 (29.7%) with type B2, 1 (2.7%) with type B3, and 2 (5.4%) with thymic carcinoma in the group without transcapsular invasion. There were 5 (6.3%) patients with type A, 9 (11.4%) with type AB, 19 (24.1%) with type B1, 19 (24.1%) with type B2, 9 (11.4%) with type B3, and 18 (22.8%) with thymic carcinoma in the group with transcapsular invasion. There were no statistically significant differences in age, sex, or the presence of myasthenia gravis between the two groups (P > 0.05 for all).

Computed tomography image features

The TETs in the group with transcapsular invasion were more likely to have lobular or irregular shapes (65/79, 82.3%) than those in the group without (18/37, 48.6%) (P < 0.001). A significant difference was observed for capsule integrity (P < 0.001). The percentage of TETs with almost complete capsule integrity was higher in the group without transcapsular invasion (27/37, 73%) than in the group with transcapsular invasion (27/79, 34.2%). A significant difference was also observed for vascularity grade (P < 0.001). The percentage of TETs with vascularity grade 2 was higher in the group with transcapsular invasion (61/79, 77.2%) than in the group without (12/37, 32.4%). The median size of TETs was 4.3 (IQR = 2.8; range, 1.9–9.5) in the group without transcapsular invasion and 4.8 (IQR = 2.6; range, 1.6–12.2) in the group with transcapsular invasion (P = 0.438). Other CT image features, such as internal necrosis, heterogeneous enhancement, presence of calcification in the TET, pleural effusion, and pericardial effusion, were not significantly different between the two groups (P > 0.05 for all).

Interobserver agreement

Table 2 presents the interobserver agreement for the TET CT image evaluation of both observers. The shape classification yielded almost perfect agreement, with a Cohen’s kappa value of 0.840 (95% CI: 0.761–0.893; P < 0.001). Capsule integrity yielded moderate agreement, with a Cohen’s kappa value of 0.526 (95% CI: 0.292–0.682; P < 0.001). The vascularity grade indicated substantial agreement, with a weighted kappa value of 0.752 (95% CI: 0.662–0.841; P < 0.001). Other variables (calcification, internal necrosis, and heterogeneous enhancement) also yielded almost perfect agreement (Cohen’s kappa value > 0.9 for all; P < 0.001).

Univariable and multivariable analyses for transcapsular invasion

Table 3 lists the results of the univariable and multivariable analyses used to assess the association between transcapsular invasion and the clinical and radiological features on the preoperative chest CT in patients with TETs. The result of the Hosmer–Lemeshow test was P = 0.681 and that of the Omnibus test was P < 0.001, indicating that the logistic regression model had a good fit. In the univariable analysis, lobular or irregular shape (OR = 4.90; 95% CI: 2.09–11.90; P < 0.001), partial complete capsule integrity (OR = 5.20; 95% CI: 2.26–12.79; P < 0.001), and vascularity grade 2 (OR = 11.18; 95% CI: 3.44–41.35; P < 0.001) were significantly associated with transcapsular invasion. The variables of sex, shape, capsule integrity, heterogenous enhancement, and vascularity grade were included in the multivariable analysis using backward stepwise selection.

The multivariable analyses indicated that shape, capsule integrity, and vascularity grade were consistently associated with transcapsular invasion. A lobular or irregular shape had a 4.19 times higher risk (95% CI: 1.53–12.09; P = 0.006) than a round or oval shape of transcapsular invasion. In addition, partial complete capsule integrity had a 5.03 times higher risk (95% CI: 1.85–15.13; P = 0.002) than almost complete capsule integrity. Vascularity grade 2 had a 10.09 times higher risk (95% CI: 2.59–45.48; P = 0.001) than vascularity grade 0. Based on the multivariable logistic regression analysis, a graphical representation of a nomogram was constructed to predict transcapsular invasion in patients with TETs (Figure 3).

Discussion

The main results of our study revealed that shape classification, capsule integrity, and vascularity grade were independently associated with transcapsular invasion in patients with TETs. The interobserver agreements for shape classification, capsule integrity, and vascularity grade suggest that TET CT features have good reproducibility and help predict transcapsular invasion.

Based on the results, we assumed that vascularity grade reflects the degree of vascular proliferation in the anterior mediastinum. The vascular structures around the TETs were also confirmed in the pathological findings. Angiogenesis is indispensable for a tumor’s growth.²⁰ Tomita et al.²¹ demonstrated a significant correlation between tumor angiogenesis and invasiveness in patients with TETs. However, there are no studies on the growth of blood vessels around the TETs. The clinical implications of peritumoral vascularity on malignant tumors have been reported. In breast cancer, adjacent vessel signs, defined as the presence of peritumoral vessels that lead to a tumor, were significantly associated with the malignancy of a breast lesion and tumoral neovascularization; this finding was suggested as a possible explanation of the phenomenon. Additionally, adjacent vessel signs were significantly associated with malignancy and were observed more frequently in invasive cancers than in preinvasive cancers.^22,23 Zhang et al.²⁴ demonstrated that peritumoral vascularity was significantly associated with more aggressive subtypes of renal cell carcinoma. Therefore, the authors believe that peritumoral vascularity can represent the neovascularization of the tumor and may be associated with the invasiveness of TETs.

A few studies have reported interobserver agreements for shape, calcification, cystic or necrotic changes, enhancement patterns, enhancement degree, and the margin in TETs.^25,26 These findings are consistent with the results of the present study with the exception of shape classification. Although Yamazaki et al.²⁶ found that the interobserver agreement for shape classification demonstrated moderate agreement with a kappa value of 0.55, our interobserver agreement for shape classification yielded an almost perfect agreement, with a kappa value of 0.840. These differences are related to whether lobulated and irregular TET shapes were categorized in the same group when shape groups were classified. In the present study, classifying lobulated and irregular TET shapes in the same group was also associated with transcapsular invasion and yielded improved reproducibility.

Although additional validation is needed, the constructed nomogram can help predict transcapsular invasion in patients with TETs using preoperative chest CT. Complete thymectomy, which includes the complete removal of the thymus and surrounding mediastinal fat, is a standard method for treating TETs,^15,17 especially in patients at stage II or higher and those with concomitant myasthenia gravis. Thymomectomy, defined as the resection of the thymoma without perithymic tissue, is performed routinely on patients with TETs at Masaoka–Koga stage I. Thymomectomy is a less invasive procedure and is associated with a shorter operative time and hospitalization period.^27,28 Therefore, it is crucial to distinguish Masaoka–Koga stage I (no transcapsular invasion) from Masaoka–Koga stage II (or higher) in patients with TETs before surgery.

Our study has several limitations. First, this study was a retrospective analysis using clinical data from two institutions. Consequently, there may be an unintended selection bias. Second, we applied vascularity grade, which is a subjective, not quantitative, variable, in the prediction of the transcapsular invasion of TETs. We believe that the impact was minimal because the vascularity grades identified by the two experienced thoracic radiologists were in agreement. Third, we analyzed the CT features of TETs produced using various CT scanners with different CT protocols and contrast medium techniques, which could affect the CT features. However, we considered that the impact was minimal because the patients with poor CT image quality were excluded. We also assumed that the visual analysis of the TET CT images was minimally affected by the chest CT protocol, but additional research is needed on the qualitative analyses of TETs using CT images performed with standardized protocols. Finally, although vascularity grade reflects the extent of the peritumoral vascular structure, there is a lack of evidence to explain the association between peritumoral vascularity and tumor invasiveness. Therefore, an additional study is needed to clarify this association.

In conclusion, lobular or irregular shapes with partial complete capsule integrity and vascularity grade 2 exhibited good reproducibility outcomes and were independent predictors of transcapsular invasion on chest CT in patients with TETs. Therefore, the TET CT features of shape classification, capsule integrity, and vascularity grade can help identify transcapsular invasion in patients with TETs.

Conflict of interest disclosure

The authors declared no conflicts of interest.