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

[18F]fluorodeoxyglucose positron emission tomography/computed tomography characteristics of primary mediastinal germ cell tumors

Primary mediastinal germ cell tumor (MGCT) is an uncommon tumor. Although it has histology similar to that of gonadal germ cell tumor (GCT), the prognosis for MGCT is generally worse than that for gonadal GCT. We performed visual assessment and quantitative analysis of [18F]fluorodeoxyglucose positron emission tomography/computed tomography ([18F]FDG PET/CT) for MGCTs. A total of 35 MGCT patients (age = 33.1 ± 16.8 years, F:M = 16:19) who underwent preoperative PET/CT were retrospectively reviewed. The pathologic diagnosis of MGCTs identified 24 mature teratomas, 4 seminomas, 5 yolk sac tumors, and 2 mixed germ cell tumors. Visual assessment was performed by categorizing the uptake intensity, distribution, and contour of primary MGCTs. Quantitative parameters including the maximum standardized uptake value (SUVmax), tumor-to-background ratio (TBR), metabolic tumor volume (MTV), total lesion glycolysis (TLG), and maximum diameter were compared between benign and malignant MGCTs. On visual assessment, the uptake intensity was the only significant parameter for differentiating between benign and malignant MGCTs (p = 0.040). In quantitative analysis, the SUVmax (p < 0.001), TBR (p < 0.001), MTV (p = 0.033), and TLG (p < 0.001) showed significantly higher values for malignant MGCTs compared with benign MGCTs. In receiver operating characteristic (ROC) curve analysis of these quantitative parameters, the SUVmax had the highest area under the curve (AUC) (AUC = 0.947, p < 0.001). Furthermore, the SUVmax could differentiate between seminomas and nonseminomatous germ cell tumors (p = 0.042) and reflect serum alpha fetoprotein (AFP) levels (p = 0.012). The visual uptake intensity and SUVmax on [18F]FDG PET/CT showed discriminative ability for benign and malignant MGCTs. Moreover, the SUVmax may associate with AFP levels.

Using CT to evaluate mediastinal great vein invasion by thymic epithelial tumors: measurement of the interface between the tumor and neighboring structures

OBJECTIVES

For thymic epithelial tumors, simple contact with adjacent structures does not necessarily mean invasion. The purpose of our study was to develop a simple noninvasive technique for evaluating organ invasion using routine pretreatment computed tomography (CT).

METHODS

This retrospective study analyzed the pathological reports on 95 mediastinal resections performed between January 2003 and June 2020. Using CT images, the length of the interface between the primary tumor and neighboring structures (arch distance; Adist) and maximum tumor diameter (Dmax) was measured, after which Adist/Dmax (A/D) ratios were calculated. Receiver operating characteristic (ROC) curves were used to analyze the Adist and A/D ratios.

RESULTS

An Adist cut-off of 37.5 mm best distinguished between invaded and non-invaded mediastinal great veins based on ROC curves. When Adist > 37.5 mm was used for diagnosis of invasion of the brachiocephalic vein (BCV) or superior vena cava (SVC), the sensitivity, specificity, positive predictive value, negative predictive value, accuracy, and area under the ROC curve for diagnosis of invasion were 61.9%, 92.5%, 81.25%, 82.2%, 81.97%, and 0.76429, respectively. Moreover, there were significant differences between BCV/SVC Adist > 37.5 mm and ≤ 37.5 mm for 10-year relapse-free survival and 10-year overall survival (p < 0.01).

CONCLUSIONS

When diagnosing invasion of the mediastinal great veins based on Adist > 37.5 mm, we achieved a higher performance level than the conventional criteria such as irregular interface with an absence of the fat layer. Measurement of Adist is a simple noninvasive technique for evaluating invasion using CT. Key Points • Simple contact between the primary tumor and adjacent structures on CT does not indicate direct invasion. • Using CT images, the length of the interface between the primary tumor and neighboring structures (arch distance; Adist) is a simple noninvasive technique for evaluating invasion. • Adist > 37.5 mm can be a supportive tool to identify invaded mediastinal great veins and surgical indications for T3 and T4 invasion by thymic epithelial tumors.

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

Simple Summary

Thymic epithelial tumours are uncommon malignancies. Histologically, they may be distinguished in different subtypes and different relapse risk classes. Surgery, sometimes after induction therapy, stays the best treatment option, and long-term results depend on the disease stage and completeness of resection. In this context, ¹⁸F FDG PET CT scan has been reported to play different roles in the care strategy of thymic epithelial tumours. In the present review, we analyse current evidences, the use of this imaging tool and future application prospects.

Abstract

Background: The use of ¹⁸F FDG PET/CT scan in thymic epithelial tumours (TET) has been reported in the last two decades, but its application in different clinical settings has not been clearly defined. Methods: We performed a pictorial review of pertinent literature to describe different roles and applications of this imaging tool to manage TET patients. Finally, we summarized future prospects and potential innovative applications of PET in these neoplasms. Results: ¹⁸FFDG PET/CT scan may be of help to distinguish thymic hyperplasia from thymic epithelial tumours but evidences are almost weak. On the contrary, this imaging tool seems to be very performant to predict the grade of malignancy, to a lesser extent pathological response after induction therapy, Masaoka Koga stage of disease and long-term prognosis. Several other radiotracers have some application in TETs but results are limited and almost controversial. Finally, the future of PET/CT and theranostics in TETs is still to be defined but more detailed analysis of metabolic data (such as texture analysis applied on thymic neoplasms), along with promising preclinical and clinical results from new “stromal PET tracers”, leave us an increasingly optimistic outlook. Conclusions: PET plays different roles in the management of thymic epithelial tumours, and its applications may be of help for physicians in different clinical settings.

18F-FDG PET/CT Evaluation of Thymomas: a Pictorial Review

The World Health Organization classification divides thymomas according to morphology, epithelial component, and cell atypia. They are grouped into 3 large subgroups: low-risk thymomas (types A, AB, and B1), high-risk thymomas (types B2 and B3), and thymic carcinomas. Tumor subtype represents an independent prognostic factor, which determines therapeutic decision. All thymomas show some degree of ¹⁸F-FDG uptake, which tends to increase with the grade of malignancy; this is related to glucose transporter 1 (GLUT1) expression. This review collects all types of thymomas with illustrative images and provides a guide to get familiar with histological characteristics of the lesions and have them in mind because, even imaging findings can overlap among subtypes, certain characteristics can be combined to make an accurate diagnosis based on ¹⁸F-FDG PET-CT findings.

Advancement in Diagnostic Imaging of Thymic Tumors

Simple Summary

Diagnostic imaging is pivotal for the diagnosis and staging of thymic tumors. It is important to distinguish thymoma and other tumor histotypes amenable to surgery from lymphoma. Furthermore, in cases of thymoma, it is necessary to differentiate between early and advanced disease before surgery since patients with locally advanced tumors require neoadjuvant chemotherapy for improving survival. This review aims to provide to radiologists a full spectrum of findings of thymic neoplasms using traditional and innovative imaging modalities.

Abstract

Thymic tumors are rare neoplasms even if they are the most common primary neoplasm of the anterior mediastinum. In the era of advanced imaging modalities, such as functional MRI, dual-energy CT, perfusion CT and radiomics, it is possible to improve characterization of thymic epithelial tumors and other mediastinal tumors, assessment of tumor invasion into adjacent structures and detection of secondary lymph nodes and metastases. This review aims to illustrate the actual state of the art in diagnostic imaging of thymic lesions, describing imaging findings of thymoma and differential diagnosis.

Glycoconjugated Metal Complexes as Cancer Diagnostic and Therapeutic Agents

The possibility of selectively delivering metal complexes to a defined cohort of cells on the basis of their metabolic features is a highly challenging goal, which may be extremely useful for a series of purposes, including diagnosis and therapy of pathological states, such as cancer. Tumor cells display augmented requests for carbohydrates and, in particular, for glucose in order to sustain their high proliferation rate, which causes an increased glycolytic process (Warburg effect). Since several metal complexes display diagnostic and/or therapeutic properties, their conjugation to carbohydrate portions often induce their preferential accumulation in cancer cells, similarly to what is observed with fluorodeoxyglucose (FDG). In this review we have considered the latest developments of glycoconjugates containing metal complexes in their structures. These compounds are classified as diagnostic or therapeutic agents and are further systematically discussed on the basis of the metal atom they contain. Several diagnostic techniques are possible with these probes, since, depending on the metal species included in their structures, they may be employed in nuclear medicine (PET, SPECT), magnetic resonance imaging, luminescence and phosphorescence. At the same time, the lack of selective cytotoxicity displayed by several metal-based chemotherapeutic agents, may also be solved by the conjugation of these agents to carbohydrate portions. Overall, data so far available reveal the great potential of this chemical class in the early detection and in the cure of severe neoplastic diseases, which still needs to be fully explored in the clinic.

Thymic Granuloma Mimicking Malignancy on 18F-FDG PET/CT

We present a case with an unexpected nodule in the thymus showing increased F-FDG uptake on PET/CT. The nodule was surgically removed. Histopathologic examination revealed granuloma in the thymus. This case indicates physicians and radiologists must be aware that although thymic granuloma is rare, it should be included in the differential diagnosis of the thymic lesions with F-FDG accumulation along with neoplastic and nonneoplastic conditions.

Thymic carcinoma initially presented with geographic destruction of scapula in a child

As the conventional histopathologic examination of thymic carcinoma (TC) is nonspecific, immunohistochemical studies along with correlative radiographic investigations are needed for its correct diagnosis. TC commonly occurs in the late 5th to early 6th decades of life but is extremely rare in childhood. It may be incidentally detected from chest radiographs taken as routine or for other reasons. However, most patients present with symptoms such as chest pain, cough, shortness of breath, dysphagia and hoarseness, which are directly attributable to the mediastinal mass. Although TC frequently invades the neighboring organs, pleura and pericardium and metastasizes to the lymph nodes, liver and lung at the time of the first diagnosis, initial or late metastasis to the bone has been seldom reported in adults. Indeed, the English literature revealed no earlier report on initial bony metastasis in a child to date. We report a case of TC in a 12-year-old boy who initially presented with scapular osteolysis masquerading as a primary bone tumor to emphasize the usefulness of combined imaging for staging and histologic studies, particularly for such an unexpected case.

Imaging Evaluation of Mediastinal Masses in Children and Adults: Practical Diagnostic Approach Based on A New Classification System

A compartmental approach to the diagnosis of the mediastinal masses in children and adults has been widely used to facilitate the diagnosis and planning of diagnostic interventions and surgical treatment for many years. Recently, a new computed tomography-based mediastinal division scheme, approved by the International Thymic Malignancy Interest Group, has received considerable attention as a potential new standard. In this review article, this new computed tomography-based mediastinal division scheme is described and illustrated. In addition, currently used imaging modalities and techniques, practical imaging algorithm of evaluating mediastinal masses, and characteristic imaging findings of various mediastinal masses that occur in children and adults are discussed. Such up-to-date knowledge has the potential to facilitate better understanding of mediastinal masses in both pediatric and adult populations.

Thymoma of the Left Thymic Lobe with a Contralateral Small Pleural Implant Successfully Detected with Diffusion-weighted MRI

Thymoma is the most common primary neoplasm of the anterior mediastinum. At diagnosis, up to 40% of patients present with advanced disease. Because advanced thymomas receive neoadjuvant chemotherapy, diagnostic imaging is crucial to plan the correct treatment. For characterizing thymomas, CT is the first choice modality, whereas 18F-FDG/PET is reserved for questionable cases and MRI is not routinely employed. Hereby, we describe a case of thymoma with a single contralateral pleural implant in a 30-year-old woman. The small pleural thickening detected at CT was correctly interpreted as pleural seeding related to thymoma at diffusion-weighted (DW)-MRI after a negative 18F-FDG/PET scan, and was subsequently confirmed at surgery. Precise diagnosis and accurate preoperative staging are crucial in managing thymic epithelial tumours in order to design the appropriate treatment and improve prognosis. Indeed, when stage IVa for pleural seeding is diagnosed preoperatively, a multimodality approach including primary chemotherapy followed by surgery and postoperative radiotherapy/chemotherapy is recommended. This is the first report that used DW-MRI for the characterization of pleural seeding in thymoma and demonstrates that DW-MRI could be useful for the correct pre-operatory staging in thymoma patients, especially in cases with indeterminate pleural thickenings at CT, in order to define the correct management.

Diagnostic Utility of PET CT in Thymic Tumours with Emphasis on 68Ga-DOTATATE PET CT in Thymic Neuroendocrine Tumour - Experience at a Tertiary Level Hospital in India

INTRODUCTION

18 Fluorine-fluoro-2-deoxyglucose positron emis-sion tomography/computed tomography (18F- FDG-PET/CT) is of importance in assessing high-risk thymoma and thymic carcinomas. Detection of advanced thymoma versus thymic carcinoma by routine cross sectional anatomical imaging such as computed tomography (CT), magnetic resonance imaging (MRI) often poses a diagnostic dilemma. In this case series we observed the utility of FDG uptake to predict advanced thymoma and distinguish thymoma from thymic cancer.

MATERIALS AND METHODS

We reviewed 18F- FDG-PET/CT scans of 12 patients (8 males, 4 females); age 24-60yrs with thymic epithelial malignancy from January 2011 to May 2013. FDG activity in lesions was quantified using maximum standardised uptake value (SUVmax) and correlated with Masaoka staging and WHO classification. All patients fasted 4 hr prior to 18F-FDG PET/CT. Images from vertex to mid-thigh were acquired 60min post injection of 3.7 -4.7 MBq/kg (Mega Becquerel)/kilogram of18F-FDG and SUV max of each tumour was measured. One patient underwent DOTATATE scan, received 138MBq of 68Gallium (68Ga)-DOTATATE injection IV and imaging was done after 60 min.

RESULTS

Higher FDG uptake of SUVmax 7.35 was seen in type B3 thymoma. FDG uptake was higher in thymic carcinoma (20.45 in primary and 17.46 in the node) or neuroendocrine differentiation (NED) than in patients with thymomas (ranged 7.35 - 3.02). No significant association was observed between higher focal FDG uptake and advanced-stage disease in thymoma. In NED 68Ga - DOTATATE imaging identified more lesions than in FDG.

CONCLUSION

PET CT is a valuable diagnostic tool in evaluation of thymic tumours, to assess in initial workup, for treatment response and for prognostication. 68Ga-DOTATATE PET/CT is beneficial in assessing neuroendocrine thymic tumours. Focal FDG uptake cannot predict advanced thymoma but is helpful in distinguishing thymoma from thymic carcinoma, or the more aggressive thymoma B3.

May positron emission tomography reveal ectopic or active thymus in preoperative evaluation of non-thymomatous myasthenia gravis?

BACKGROUND

In myasthenia gravis (MG) both native and ectopic thymic tissue containing germinal centers should show greater metabolism compared to adjacent tissues. We evaluated whether preoperative standardized uptake value (SUV) of 18fluoro-deoxy-glucose on Positron Emission Tomography (PET) might be increased and correlated with the presence of native or ectopic germinal centers.

METHODS

From 2005 to 2012 we performed extended thymectomy in 68 patients with non-thymomatous MG. All patients underwent PET-scan preoperatively and one-year postoperatively. SUVs were assessed in thymic and perithymic regions. Then it was matched with same-age, non-MG and non-neoplastic control group and finally correlated with presence of germinal centers in native thymus or in the ectopic tissue found in the surgical specimens.

RESULTS

Mean SUV was significantly increased in MG patients compared to control group. Thymic SUV was significantly higher in presence of thymic germinal centers [3.5 (2.4-5.0) Vs 2.1 (1.4-2.5), p = 0.021] while perithymic SUV was significantly higher in presence of ectopic germinal centers [3.1 (2.7-3.5) Vs 1.3 (0.9-1.7), p = 0.001]. SUV was significantly correlated with MG score (rho = 0.289, p = 0.017) and marginally with antibodies anti-acetylcholine receptors (rho = 0.129, p = 0.05). At Kaplan Meier analysis, ectopic thymic tissue (p = 0.045) and ectopic germinal centers (p = 0.036) were significant predictors of complete stable remission, but preoperative dichotomized thymic (3.5 or more Vs less) (p = 0.083) and perithymic (2.1 or more Vs less) (p = 0.052) SUVs did not.

CONCLUSIONS

Thymic and perithymic SUVs were significantly higher in patients with MG than non-MG and non-neoplastic patients. Thymic SUV was significantly correlated with the presence of germinal centers. Perithymic SUV resulted significantly correlated with the discovery of ectopic active thymic tissue. Neither thymic nor perithymic high SUVs predicted remission.

Advances in thymic carcinoma diagnosis and treatment: a review of literature

Thymomas account for up to 50 % of anterior mediastinal neoplasms with an incidence of 0.13 per 100,000 person-years in the USA. Thymic carcinoma is a rare malignancy of the thymus gland distinguished from thymomas as it has a more invasive and metastasizing potential conferring poor prognosis. Due to the rarity of thymic carcinoma and the great variety of its histological subtypes, there is no solid evidence on optimal staging, imaging and treatment guidelines. Herein, we systematically review the literature on current clinical practice with regard to diagnostic evaluation, histopathological assessment, management and treatment of squamous thymic carcinoma.

Diffusion-weighted MR imaging in thymic epithelial tumors: correlation with World Health Organization classification and clinical staging

PURPOSE

To assess thymic epithelial tumors with diffusion-weighted magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Informed consent from patients and institutional review board approval were obtained. Prospective study was conducted on 30 consecutive patients (21 men and nine women; age range, 35-71 years) with thymic epithelial tumors. They underwent true fast imaging with steady-state precession and single-shot echo-planar diffusion-weighted MR imaging of the mediastinum with b values of 0, 400, and 800 sec/mm(2). Apparent diffusion coefficient (ADC) of the thymic epithelial tumors was calculated by the same observer at two settings and was correlated with World Health Organization classification and clinical staging.

RESULTS

There was significant difference in longest diameter (P = .001) and necrotic part of the tumor (P = .014) between low-risk thymoma, high-risk thymoma, and thymic carcinoma. Mean ADC value of both readings of thymic epithelial tumors (n = 30) was 1.24 × 10(-3) mm(2)/sec and 1.22 × 10(-3) mm(2)/sec, with good intraobserver agreement (κ = 0.732). There was significant difference in both readings (P = .01 and .20) of low-risk thymoma (1.30 × 10(-3) mm(2)/sec and 1.29 × 10(-3) mm(2)/sec), high-risk thymoma (1.16 × 10(-3) mm(2)/sec and 1.14 × 10(-3) mm(2)/sec), and thymic carcinoma (1.18 × 10(-3) mm(2)/sec and 1.06 × 10(-3) mm(2)/sec). Cutoff ADC values of both readings used to differentiate low-risk thymoma from high-risk thymoma and thymic carcinoma were 1.25 and 1.22 × 10(-3) mm(2)/sec with area under the curve of 0.804 and 0.851, respectively. There was significant difference in both readings of ADC value of early (stage I, II) and advanced stages (stage III, IV) of thymic epithelial tumors (P = .006 and .005, respectively).

CONCLUSION

ADC value is a noninvasive, reliable, and reproducible imaging parameter that may help to assess and characterize thymic epithelial tumors.