Current Roles of PET/CT in Thymic Epithelial Tumours: Which Evidences and Which Prospects? A Pictorial Review

Practical value of fluorodeoxyglucose positron emission tomography in treatment strategies for thymic epithelial tumors: implications for more specific use in routine clinical practice

Many studies have demonstrated that 18-fluorine fluorodeoxyglucose positron emission tomography (FDG-PET) is useful for predicting the grade of malignancy of thymic epithelial tumors (TETs), and there is a close relationship between the maximum standardized uptake value (SUVmax) and tumor stage. However, more specific usage of FDG-PET for TETs has not been proposed, and the actual value of FDG-PET in routine clinical practice should be firmly clarified. In this review, following three cutoff values of SUVmax that may be helpful in determining treatment strategies in cases of anterior mediastinal masses, particularly presented as discrete and resectable lesions, are identified: (I) SUVmax of 7.5 as an indicator for pretreatment biopsy: differential diagnosis between TETs and mediastinal lymphoma (ML); (II) SUVmax of 4.2 as an indicator for a minimally invasive approach (MIA): differentiation of noninvasive TETs and invasive TETs; and (III) SUVmax of 5.9 as a reference value for the necessity of lymph node dissection (LND). There are still several challenges in using FDG-PET for routine clinical practice that need to be addressed, such as variations between instruments and institutions, leading to lower reproducibility. Harmonization methods should be applied to make clinical practice more uniform. Due to the rarity of these diseases, multi-institutional studies are warranted.

Clinical significance of positron emission tomography-computed tomography in the classification of thymic tumors

OBJECTIVES

This study aimed to explore the possibility of positron emission tomography/computed tomography (PET-CT) in identifying histological classification of thymic tumours.

METHODS

Patients diagnosed as thymic tumours and accepted PET-CT scans were included. Thymic tumours were classified into three subgroups: low-risk thymoma (A, AB and B1), high-risk thymoma (B2, B3) and thymic carcinoma (TC). Logistic regression analysis was performed to identify potential factors differentiating the classification of thymic tumours. The receiver operating characteristic curve was applied to assess the diagnosis efficiency and the cut-off value.

RESULTS

From 2015 to 2023, a total of 176 patients, including 75 cases of low-risk thymoma, 60 cases of high-risk thymoma and 41 cases of TC, were included. The logistic regression models suggested maximum standardized uptake value (SUVmax) as a potential factor differentiating the three subgroups. Moreover, the receiver operating characteristic curve identified the SUVmax in differentiating low-risk thymoma vs high-risk thymoma (area under the curve [AUC]: 0.845, 95% CI: 0.776-0.913, specificity: 0.907, sensitivity: 0.716), low-risk thymoma vs TC (AUC: 0.976, 95% CI: 0.953-0.999, specificity: 0.933, sensitivity: 0.951) and high-risk thymoma vs TC (AUC: 0.84, 95% CI: 0.761-0.92, specificity: 0.865, sensitivity: 0.703), respectively. SUVmax was also an independent factor identifying thymic tumours with or without lymph node metastasis. The cut-off of 10 in SUVmax could well identify lymph node metastasis with the positive predict value of 0.684 and negative predict value of 0.981.

CONCLUSIONS

SUVmax is a reliable factor in distinguishing different histological subgroups and identifying lymph node metastasis in thymic tumours.

Case report: A challenging case of severe Cushing's syndrome in the course of metastatic thymic neuroendocrine carcinoma with a synchronous adrenal tumor

Ectopic ACTH syndrome (EAS) remains one of the most demanding diagnostic and therapeutic challenges for endocrinologists. Thymic neuroendocrine tumors account for 5%-10% of all EAS cases. We report a unique case of a 31-year-old woman with severe EAS caused by primary metastatic combined large-cell neuroendocrine carcinoma and atypical carcinoid of the thymus. The patient presented with severe hypercortisolemia, which was successfully controlled with continuous etomidate infusion. Complex imaging initially failed to detect thymic lesion; however, it revealed a large, inhomogeneous, metabolically active left adrenal mass infiltrating the diaphragm, suspected of primary disease origin. The patient underwent unilateral adrenalectomy, which resulted in hypercortisolemia resolve. The pathology report showed an adenoma with adrenal infarction and necrosis. The thymic tumor was eventually revealed a few weeks later on follow-up imaging studies. Due to local invasion and rapid progression, only partial resection of the thymic tumor was possible, and the patient was started on radio- and chemotherapy.

Re-evaluation and operative indications after induction therapy for thymic epithelial tumors

Thymic epithelial tumors (TETs), encompassing thymoma and thymic carcinoma, represent a rare and heterogeneous group of thoracic malignancies with varying prognoses and treatment strategies. Surgical resection is the cornerstone of therapy for localized stages, but the management of locally advanced or unresectable TETs often involves induction therapy, including chemotherapy and/or radiation therapy, as a neoadjuvant approach aimed at downstaging the tumor to facilitate subsequent resection. This review synthesizes current knowledge on the re-evaluation process and operative indications following induction therapy for TETs, highlighting the pivotal role of accurate assessment in guiding surgical decisions and optimizing patient outcomes. Induction therapy's efficacy is contingent upon precise re-evaluation methods to accurately gauge treatment response and assess resectability post-therapy. This review discusses the various modalities employed in re-evaluation, including computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography-CT (PET-CT), and the significance of tumor markers, underlining their strengths and limitations. The adoption of modified RECIST criteria for TETs by the International Thymic Malignancy Interest Group (ITMIG) underscores the necessity for standardized assessment guidelines to ensure consistency and reliability across studies and clinical practices. Furthermore, we explore the implications of induction therapy on surgical decision-making, emphasizing the criteria for determining the suitability of patients for surgical intervention post-therapy. The review addresses the challenges and future perspectives associated with the re-evaluation process, including the potential for advanced imaging techniques and the integration of molecular and genetic markers to enhance the precision of treatment response assessment. In conclusion, the re-evaluation of TETs post-induction therapy is a complex but critical component of the multidisciplinary management approach for these patients. Standardizing re-evaluation methodologies and incorporating novel diagnostic tools could significantly improve the prognostication and treatment stratification, ultimately enhancing the therapeutic outcomes for patients with advanced TETs.

Differentiating thymic epithelial tumors from mediastinal lymphomas: preoperative nomograms based on PET/CT radiomic features to minimize unnecessary anterior mediastinal surgery

PURPOSE

Clinical feasibility nomograms were developed to facilitate the differentiation between thymic epithelial tumors (TETs) and mediastinal lymphomas (MLs), aiming to minimize the occurrence of non-therapeutic thymectomy.

METHODS

A total of 255 patients diagnosed with TETs or MLs underwent pre-treatment 18F-FDG PET/CT. Comprehensive clinical and imaging data were collected, including age, gender, lactate dehydrogenase (LDH) level, pathological results, presence of myasthenia gravis symptoms, B symptoms, mass size, location, morphology, margins, density, and metabolic parameters derived from PET/CT. Radiomic features were extracted from the region of interest (ROI) of the primary lesion. Feature selection techniques were employed to identify the most discriminative subset of features. Machine learning methods were utilized to build candidate models, which were subsequently evaluated based on their area under the curve (AUC). Finally, nomograms were constructed using the optimal model to provide a clinical tool for improved diagnostic accuracy. The performance of the radiomic models was evaluated by their calibration, discrimination, and clinical utility.

RESULTS

Several independent risk factors were identified for distinguishing TETs from MLs, including average standardized uptake value (SUVavg), LDH, age, mass size, and radiomic score (rad-score). Significance was observed in differentiating the two types of tumors based on these factors. The best clinical efficacy was demonstrated by the combined model, with an impressive AUC of 0.954. Decision curve analysis and calibration curves indicated that the combined model was clinically advantageous for discriminating TETs from MLs. Besides, the results of external validation showed a sensitivity of 0.8 and a specificity of 0.78.

CONCLUSION

Preoperatively, the differentiation of TETs from MLs can be facilitated by the utilization of the combined clinical information and radiomics model. This approach holds promise in minimizing the occurrence of unnecessary anterior mediastinal surgeries.

Incidental Detection of a Type B3 Thymoma on 68Ga-FAPI-04 PET/CT Imaging

ABSTRACT

18F-FDG PET/CT performed for staging of colon cancer in a 60-year-old woman revealed a thymic neoplasm with increased radiopharmaceutical uptake in the anterior mediastinum. 68Ga-FAPI PET/CT showed low FAPI uptake in the anterior mediastinal mass. The patient was diagnosed with type B3 thymoma by histopathologic evaluation. In this case, thymoma demonstrated lower uptake with FAPI than FDG. Larger studies are required for the differentiation of malignant and benign mediastinal masses with FAPI.

Is PET/CT Able to Predict Histology in Thymic Epithelial Tumours? A Narrative Review

BACKGROUND

The usefulness of ¹⁸FDG PET/CT scan in the evaluation of thymic epithelial tumours (TETs) has been reported by several authors, but data are still limited and its application in clinical practice is far from being defined.

METHODS

We performed a narrative review of pertinent literature in order to clarify the role of ¹⁸FDG PET/CT in the prediction of TET histology and to discuss clinical implications and future perspectives.

RESULTS

There is only little evidence that ¹⁸FDG PET/CT scan may distinguish thymic hyperplasia from thymic epithelial tumours. On the other hand, it seems to discriminate well thymomas from carcinomas and, even more, to predict the grade of malignancy (WHO classes). To this end, SUVmax and other PET variables (i.e., the ratio between SUVmax and tumour dimensions) have been adopted, with good results. Finally, however promising, the future of PET/CT and theranostics in TETs is far from being defined; more robust analysis of imaging texture on thymic neoplasms, as well as new exploratory studies with "stromal PET tracers," are ongoing.

CONCLUSIONS

PET may play a role in predicting histology in TETs and help physicians in the management of these insidious malignancies.

Multiparameter diagnostic model based on 18F-FDG PET and clinical characteristics can differentiate thymic epithelial tumors from thymic lymphomas

OBJECTIVE

To evaluate the diagnostic performance of combined multiparametric ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸FDG PET) with clinical characteristics in differentiating thymic epithelial tumors (TETs) from thymic lymphomas.

PATIENTS AND METHODS

A total of 173 patients with 80 TETs and 93 thymic lymphomas who underwent ¹⁸F-FDG PET/CT before treatment were enrolled in this retrospective study. All patients were confirmed by pathology, and baseline characteristics and clinical data were also collected. The semi-parameters of ¹⁸F-FDG PET/CT, including lesion size, SUVmax (maximum standard uptake value), SUVmean (mean standard uptake value), TLG (total lesion glycolysis), MTV (metabolic tumor volume) and SUVR (tumor-to-normal liver standard uptake value ratio) were evaluated. The differential diagnostic efficacy was evaluated using the receiver operating characteristic (ROC) curve. Integrated discriminatory improvement (IDI) and net reclassification improvement (NRI), and Delong test were used to evaluate the improvement in diagnostic efficacy. The clinical efficacy was evaluated by decision curve analysis (DCA).

RESULTS

Age, clinical symptoms, and metabolic parameters differed significantly between patients with TETs and thymic lymphomas. The ROC curve analysis of SUVR showed the highest differentiating diagnostic value (sensitivity = 0.763; specificity = 0.888; area under the curve [AUC] = 0.881). The combined diagnostics model of age, clinical symptoms and SUVR resulted in the highest AUC of 0.964 (sensitivity = 0.882, specificity = 0.963). Compared with SUVR, the diagnostic efficiency of the model was improved significantly. The DCA also confirmed the clinical efficacy of the model.

CONCLUSIONS

The multiparameter diagnosis model based on ¹⁸F-FDG PET and clinical characteristics had excellent value in the differential diagnosis of TETs and thymic lymphomas.