[18F]FDG positron emission tomography/computed tomography and multidetector computed tomography roles in thymic lesion treatment planning

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.

Value of 18F-FDG PET/computed tomography in predicting the simplified WHO grade of malignancy in thymic epithelial tumors

OBJECTIVE

To investigate the value of 18F-fluorodeoxyglucose (F-FDG) PET/computed tomography (CT) in predicting the simplified WHO grade of malignancy in thymic epithelial tumors.

MATERIALS AND METHODS

We retrospectively reviewed 81 patients with pathologically proven thymic epithelial tumors who underwent F-FDG PET/CT before surgical resection. Maximum standardized uptake value (SUVmax) and SUVmax/tumor size were measured on the primary lesion. A receiver operating characteristics (ROC) curve were performed for assessing the ability of F-FDG PET/CT as a predictor of the simplified WHO classification.

RESULTS

There were 43 male patients (53.1%) and 38 female patients (46.9%), and the mean age was 55.6 ± 11.9 years. The mean tumor size was 53.2 ± 21.4 mm. There were 24 low-risk thymomas (29.6%) (A, AB, and B1), 29 high-risk thymomas (35.8%) (B2 and B3), and 28 thymic carcinomas (34.6%). The SUVmax and SUVmax/tumor size were found to be predictive factors that were useful to distinguish thymomas and thymic carcinomas, and area under the ROC curve were 0.820 and 0.691, respectively (P < 0.05), and the cutoff value for discriminating thymomas and thymic carcinomas was 5.34.

CONCLUSION

In conclusion, a significant relationship was observed between SUVmax, SUVmax/tumor size and histological WHO classification of thymic epithelial tumors. F-FDG PET/CT may be useful for predicting the grade of malignancy in thymic epithelial.

The role of positron emission tomography/computed tomography in the evaluation of anterior mediastinal masses and differentiating between the histological subtypes of thymic epithelial neoplasms

Background

This study aims to investigate the role of positron emission tomography/computed tomography in differentiating between benign and malignant anterior mediastinal masses and between the histological subtypes of thymic epithelial neoplasms.

Methods

This retrospective study included a total of 57 patients (30 males, 27 females; mean age 48.9 years; range, 14 to 78 years) who underwent an fluorodeoxyglucose positron emission tomography/ computed tomography with an indication of an anterior mediastinal mass between May 2010 and November 2018. The maximum and mean standardized uptake values, metabolic tumor volume, total lesion glycolysis, and mean Hounsfield units of the lesions were determined. Thymic epithelial neoplasms were classified as low-risk thymomas (A, AB, B1), high-risk thymomas (B2, B3), and thymic carcinomas. All lesions were evaluated together and divided into two groups as benign and malignant, and positron emission tomography/computed tomography parameters of the two groups were compared.

Results

Histopathological examination identified 29 thymic epithelial neoplasms (13 low-risk, 14 high-risk, two thymic carcinomas), 13 benign lesions, and 15 malignant lesions. No significant correlation was identified between the positron emission tomography/computed tomography parameters and histological subtype of thymic epithelial neoplasms. There was no significant difference in the maximum and mean standardized uptake values, metabolic tumor volume, total lesion glycolysis, and mean Hounsfield units of the lesions between low-risk and high-risk thymic carcinomas. The maximum and mean standardized uptake values, metabolic tumor volume, and mean Hounsfield units of the lesions were significantly higher in the malignant group than in the benign group.

Conclusion

Our study results show that positron emission tomography/ computed tomography is useful in the differentiation of benign and malignant anterior mediastinal masses, although it fails to differentiate between the histological subtypes of thymic epithelial neoplasms.

Value of 18F-FDG PET/CT for Predicting the World Health Organization Malignant Grade of Thymic Epithelial Tumors: Focused in Volume-Dependent Parameters

PURPOSE

We investigated whether preoperative parameters of 18F-FDG PET/CT were correlated with the World Health Organization (WHO) classification and/or Masaoka staging of thymic epithelial tumors.

PATIENTS AND METHODS

We reviewed 61 patients retrospectively who were diagnosed with thymic epithelial tumors after surgical resection and PET/CT. A volume of interest was drawn on the primary lesion, using an SUV cutoff of 2.5, and metabolic indices such as SUVmax, metabolic tumor volume (MTV), and total lesion glycolysis (TLG) were measured.

RESULTS

There were 24 male patients (38.7%), and the mean (SD) age was 50.23 (12.54) years. The mean (SD) tumor size was 6.11 (3.41) cm. There were 22 low-risk thymomas (36.9%) (A, AB, B1), 32 high-risk thymomas (51.6%), and 7 thymic carcinomas (11.5%). The Masaoka stage was I in 15 (24.6%), II in 30 (49.2%), III in 11 (18.0%), and IV in 5 patients (8.2%). Mean (SD) SUVmax was 3.43 (1.01) in low-risk thymomas, 4.42 (1.70) in high-risk thymomas, and 8.23 (2.61) in thymic carcinoma; the differences were significant (P < 0.001). Mean (SD) MTV and TLG were 90.74 (114.56) and 229.36 (300.56) in low-risk thymomas, 80.82 (112.49) and 233.93 (340.91) in high-risk thymomas, and 90.63 (90.74) and 390.94 (437.62), respectively, in thymic carcinomas. MTV and TLG showed no correlation with the WHO classification. On receiver operating characteristic curve analysis, the cutoff value for discriminating thymomas and thymic carcinomas was 5.05. SUVmax and TLG were correlated with Masaoka stage.

CONCLUSIONS

Although volume-dependent parameters were not correlated with the WHO classification, a significant relationship was observed between SUVmax and WHO classification and Masaoka stage.

Is (18)F-FDG PET useful in predicting the WHO grade of malignancy in thymic epithelial tumors? A meta-analysis

AIM

To perform a systematic review and meta-analysis of published data on the role of fluorine-18-fluorodeoxyglucose positron emission tomography ((18)F-FDG PET) in predicting the WHO grade of malignancy in thymic epithelial tumors (TETs).

METHODS

A comprehensive literature search of studies published up to March 2014 was performed. Data on maximum standardized uptake value (SUVmax) in patients with low-risk thymomas (A, AB, B1), high-risk thymomas (B2, B3) and thymic carcinomas (C) according to the WHO classification were collected when reported by the retrieved articles. The comparison of mean SUVmax between low-risk thymomas, high-risk thymomas and thymic carcinomas was expressed as weighted mean difference (WMD) and a pooled WMD was calculated including 95% confidence interval (95%CI).

RESULTS

Eleven studies were selected for the meta-analysis. The pooled WMD of SUVmax between high-risk and low-risk thymomas was 1.2 (95%CI: 0.4-2.0). The pooled WMD of SUVmax between thymic carcinomas and low-risk thymomas was 4.8 (95%CI: 3.4-6.1). Finally, the pooled WMD of SUVmax between thymic carcinomas and high-risk thymomas was 3.5 (95%CI: 2.7-4.3).

CONCLUSIONS

(18)F-FDG PET may predict the WHO grade of malignancy in TETs. In particular, we demonstrated a statistically significant difference of SUVmax between the different TETs (low-grade thymomas, high-grade thymomas and thymic carcinomas).

Distinguishing benign thymic lesions from early-stage thymic malignancies on computed tomography

INTRODUCTION

The increasing use of computed tomography (CT) has led to frequent identification of asymptomatic lesions in the anterior mediastinum. The purpose of this study is to identify CT features that distinguish benign thymic lesions from early-stage malignant thymic neoplasms.

METHODS

We retrospectively reviewed preoperative CT imaging for 66 patients, who had undergone thymectomy for benign thymic lesions or early-stage malignant thymic neoplasms. All variables with a p value of less than 0.2 on univariate logistic regression analysis were evaluated by multivariate analysis. Stepwise selection was performed, and variables with a p value less than 0.05 were retained in the final model.

RESULTS

Thirty-eight malignant (58%) and 28 benign thymic lesions (42%) were included. Patients with benign thymic tumors were significantly younger (median age, 49.5 years) than patients with malignant tumors (60.0 years; p = 0.007). Malignant tumors were larger in short-axis dimension (p = 0.028) and more frequently in a nonmidline location in the anterior mediastinum (p = 0.029). Intralesional fat was seen exclusively in benign masses (p = 0.002). Seven benign tumors (25%) and one malignant tumor (2.6%) had a triangular thymic shape (p = 0.023). In multivariate analysis, lower age, smaller short-axis dimension, and lack of infiltration of the mediastinal fat were significant independent predictors of benign pathologic results.

CONCLUSION

Intralesional fat, midline location, and triangular thymic shape are more frequently found in benign thymic lesions. Lack of infiltration of the mediastinal fat, younger patient age, and smaller size are independent predictors of benign thymic lesions. These features may help characterize thymic masses as benign and avert potentially unnecessary invasive diagnostic procedures.

Evaluation of thymic tumors with 18F-FDG PET-CT: a pictorial review

Thymic tumors represent a broad spectrum of neoplastic disorders and pose considerable diagnostic difficulties. A non-invasive imaging study to determine the nature of thymic lesions can have significant impact on management of such tumors. 18F-flurorodeoxyglucose (18F-FDG) positron emission tomography-computed tomography (PET-CT) has shown promising results in characterization of thymic tumors. The objective of this article is to provide an illustrative tutorial highlighting the clinical utility of 18F-FDG PET-CT imaging in patients with thymic tumors. We have pictorially depicted the 18F-FDG PET-CT salient imaging characteristics of various thymic tumors, both epithelial and non-epithelial. Also discussed is the dynamic physiology of thymus gland which is to be kept in mind when evaluating thymic pathology on 18F-FDG PET-CT, as it can lead to interpretative pitfalls.

Differentiation of normal thymus from anterior mediastinal lymphoma and lymphoma recurrence at pediatric PET/CT

PURPOSE

To evaluate the role of positron emission tomography (PET)/computed tomography (CT) in the differentiation of normal thymus from mediastinal lymphoma and lymphoma recurrence in pediatric patients.

MATERIALS AND METHODS

The study was approved by the institutional review board, and informed consent was waived. The study was HIPAA compliant. Two hundred eighty-two fluorine 18 fluorodeoxyglucose PET/CT studies in 75 pediatric oncology patients were reviewed retrospectively. Patients were divided into four groups: anterior mediastinal lymphoma (group A, n=16), anterior mediastinal lymphoma with subsequent recurrence (group B, n=5), lymphoma outside the mediastinum (group C, n=16), and other malignant tumors outside the thymus (group D, n=38). Analyses included measurements of the maximum anteroposterior and transverse dimensions of the anterior mediastinal mass or thymus on axial CT images and measurements of maximum standardized uptake values of anterior mediastinal mass, thymus (SUVt), and bone marrow at the level of the fifth lumbar vertebra (SUVb) on PET images. Quantitative parameters were compared by using an analysis of variance test.

RESULTS

Mean prechemotherapy SUVt was 4.82 for group A, 8.45 for group B, 2.00 for group C, and 2.09 for group D. Mean postchemotherapy SUVt for group B was 4.74. Thymic rebound (mean SUVt, 2.89) was seen in 44% of patients at a mean interval of 10 months from the end of chemotherapy. The differences between prechemotherapy SUVt of mediastinal lymphoma and normal thymus and postchemotherapy SUVt of lymphoma recurrence and thymic rebound were highly significant (P<.001).

CONCLUSION

SUVt is a sensitive predictor for differentiation of normal thymus or thymic rebound from mediastinal lymphoma. SUVt of 3.4 or higher is a strong predictor of mediastinal lymphoma.

Molecular imaging of neuroendocrine tumors

Neuroendocrine tumors (NET) are a heterogeneous group of tumors that arise from neuroendocrine cells. These tumors may arise from various organs, including lung, thymus, thyroid, stomach, duodenum, small bowel, large bowel, appendix, pancreas, adrenal, and skin. Most are well differentiated and have the ability to produce biogenic amines and various hormones. NET usually occur sporadically but they also be associated with various familial syndromes. For the vast majority of NET, surgical resection is the treatment of choice whenever feasible. Localization of NET prior to surgery and for staging and follow-up relies on both anatomic and functional imaging modalities. In fact, the unique secretory characteristics of these tumors lend themselves to imaging by molecular imaging modalities, which can target specific metabolic pathways or receptors. Neuroendocrine cells have a variety of such target receptors and pathways for which radiopharmaceuticals have been developed, including [(123)I/(131)I]-metaiodobenzylguanidine (MIBG), [(111)In]pentetreotide, [(68)Ga] somatostatin analogs, [(18)F] fluorodeoxyglucose (FDG), [(11)C/(18)F] dihydroxyphenylalanine (DOPA), [(11)C] 5-hydroxytryptophan (5-HTP) (99m)Tc pentavalent dimercaptosuccinic acid ([(99m)Tc] (V) DMSA, and [(18)F] fluorodopamine (FDA). Here, we review the molecular imaging approaches for NET using various radiopharmaceuticals.