Use of fluorine-18 fluorodeoxyglucose positron emission tomography in the detection of thymoma: a preliminary report

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.

Prognostic Significance of Metabolic Parameters by 18F-FDG PET/CT in Thymic Epithelial Tumors

Simple Summary

Thymic epithelial tumors have variable prognoses that depend on histological subtype, and ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) currently plays an important part in oncology images. Thus, we prosecuted a retrospective review of data from 83 patients with thymic epithelial tumors who underwent pretreatment ¹⁸F-FDG PET/CT and investigated the prognostic significance along with WHO classification, Masaoka stage, and volumetric ¹⁸F-PET parameters. Masaoka stage, histologic type, treatment modality, maximum standardized uptake values (SUV_max), average standardized uptake values (SUV_avg), metabolic tumor volume (MTV), and total lesion glycolysis (TLG) were significant prognostic factors for time-to-progression on univariate survival analysis. On multivariate analysis, SUV_avg and Masaoka stage were important independent prognostic factors for progression-free survival in thymic epithelial tumors.

Abstract

Background: Imaging tumor FDG avidity could complement prognostic implication in thymic epithelial tumors. We thus investigated the prognostic value of volume-based ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET)/CT parameters in thymic epithelial tumors with other clinical prognostic factors. Methods: This is a retrospective study that included 83 patients who were diagnosed with thymic epithelial tumors and underwent pretreatment ¹⁸F-FDG PET/CT. PET parameters, including maximum and average standardized uptake values (SUV_max, SUV_avg), metabolic tumor volume (MTV), and total lesion glycolysis (TLG), were measured with a threshold of SUV 2.5. Univariate and multivariate analysis of PET parameters and clinicopathologic variables for time-to-progression was performed by using a Cox proportional hazard regression model. Results: There were 21 low-risk thymomas (25.3%), 27 high-risk thymomas (32.5%), and 35 thymic carcinomas (42.2%). Recurrence or disease progression occurred in 24 patients (28.9%). On univariate analysis, Masaoka stage (p < 0.001); histologic types (p = 0.009); treatment modality (p = 0.001); and SUV_max, SUV_avg, MTV, and TLG (all p < 0.001) were significant prognostic factors. SUV_avg (p < 0.001) and Masaoka stage (p = 0.001) were independent prognostic factors on multivariate analysis. Conclusion: SUV_avg and Masaoka stage are independent prognostic factors in thymic epithelial tumors.

The usefulness of fluorodeoxyglucose-positron emission tomography as a preoperative diagnostic tool for thymic epithelial tumors

Background

To investigate the usefulness of ¹⁸F-fluorodeoxy glucose-positron emission tomography (¹⁸F-FDG PET) for the preoperative imaging diagnosis of malignant grade in thymic epithelial tumors (TETs) and the correlation between the maximum standardized uptake value (SUVmax) and tumor size in TETs.

Methods

We retrospectively investigated 51 patients with TETs performed ¹⁸F-FDG PET. The SUVmax was compared between thymic carcinomas and thymomas. We also evaluated the difference in the SUVmax limited to small TETs. In addition, the correlation between the SUVmax and the tumor size was evaluated.

Results

The mean SUVmax of thymic carcinomas (n=12) and thymomas (n=39) was 5.71±2.6 and 3.08±1.4, respectively. The SUVmax of thymic carcinomas was significantly higher than that of thymomas (P<0.001). The mean SUVmax of these small thymic carcinomas (n=3) and thymomas (n=13) was 2.97±0.24 and 1.79±0.47, respectively. The SUVmax of the small thymic carcinomas was significantly higher than that of the thymomas (P=0.001). We found a positive correlation between the SUVmax and the maximum tumor size of TETs (correlation coefficient: 0.632, P<0.001).

Conclusions

¹⁸F-FDG PET might be useful for evaluating the preoperative malignancy of TETs. Of note, the maximum tumor size should be considered when performing assessments by ¹⁸F-FDG PET.

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.

Can 18F-FDG PET predict the grade of malignancy in thymic epithelial tumors? An evaluation of only resected tumors

Objective

Although 18-fluorine fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) is thought to be useful for predicting the histological grade of thymic epithelial tumors (TETs), it remains controversial. To date, just a few of many previous studies have included only resected cases. Therefore, we investigated ¹⁸F-FDG PET findings only in patients with resected TETs.

Patients and methods

A total of 112 patients with TETs consisting of 92 thymomas and 20 thymic carcinomas (TCs), resected at two institutes (Shizuoka Cancer Center [Shizuoka] and National Cancer Center Hospital [Tokyo]) between October 2002 and December 2015, were evaluated. Spearman rank correlation coefficient was used to assess the association between the maximum standardized uptake value (SUVmax) in the tumor and both the histological subtype and tumor stage. The cutoff value of SUVmax for differentiating thymoma from TC was calculated.

Results

The SUVmax was strongly related to both the World Health Organization (WHO) histological subtype and tumor stage based on the eighth edition of the tumor-node-metastasis (TNM) classification (Spearman rank correlation coefficient =0.485 and 0.432; p = 0.000 and 0.000, respectively). There was a significant difference between thymoma and TC. The optimal SUVmax cutoff value for differentiating thymoma from TC was 4.58 (sensitivity: 80% and specificity: 78.3%). In contrast, there was no significant difference between low-risk (type A, AB, and B1) and high-risk (type B2 and B3) thymoma, or between type B3 thymoma and the other subtypes.

Conclusion

Our results suggest that ¹⁸F-FDG PET is useful for differentiating thymoma from TC, but not for predicting the histologic grade of thymoma.

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.

Diffusion Weighted Imaging Can Distinguish Benign from Malignant Mediastinal Tumors and Mass Lesions: Comparison with Positron Emission Tomography

BACKGROUND

Diffusion-weighted magnetic resonance imaging (DWI) makes it possible to detect malignant tumors based on the diffusion of water molecules. It is uncertain whether DWI is more useful than positron emission tomography-computed tomography (PET-CT) for distinguishing benign from malignant mediastinal tumors and mass lesions.

MATERIALS AND METHODS

Sixteen malignant mediastinal tumors (thymomas 7, thymic cancers 3, malignant lymphomas 3, malignant germ cell tumors 2, and thymic carcinoid 1) and 12 benign mediastinal tumors or mass lesions were assessed in this study. DWI and PET-CT were performed before biopsy or surgery.

RESULTS

The apparent diffusion coefficient (ADC) value (1.51±0.46x10(-3) mm2/sec) of malignant mediastinal tumors was significantly lower than that (2.96±0.86x10(-3) mm2/sec) of benign mediastinal tumors and mass lesions (P<0.0001). Maximum standardized uptake value (SUVmax) (11.30±11.22) of malignant mediastinal tumors was significantly higher than that (2.53±3.92) of benign mediastinal tumors and mass lesions (P=0.0159). Using the optimal cutoff value (OCV) 2.21x10(-3) mm2/sec for ADC and 2.93 for SUVmax, the sensitivity (100%) by DWI was not significantly higher than that (93.8%) by PET-CT for malignant mediastinal tumors. The specificity (83.3%) by DWI was not significantly higher than that (66.7%) for benign mediastinal tumors and mass lesions. The accuracy (92.9%) by DWI was not significantly higher than that (82.1%) by PET-CT for mediastinal tumors and mass lesions.

CONCLUSIONS

There was no significant difference between diagnostic capability of DWI and that of PET-CT for distinguishing mediastinal tumors and mass lesions. DWI is useful in distinguishing benign from malignant mediastinal tumors and mass lesions.

Imaging of a case of metaplastic thymoma on 18F-FDG PET/CT

Metaplastic thymoma is a rare primary thymic epithelial tumor. This report presents the 18F-FDG PET/CT appearance of a metaplastic thymoma developed in a 61-year-old female patient. The FDG uptake of the mediastinal mass showed a high maximum SUV, thus suggesting the possibility of thymic cancer. Surgical resection was performed for both a definitive diagnosis and treatment of the mediastinal tumor. A histological examination of the resected specimen revealed metaplastic thymoma. Although rare, thymoma should be carefully considered in the differential diagnosis in cases of mediastinal masses presenting with highly positive findings on 18F-FDG PET/CT to avoid any inappropriate patient management.

18F-fluorodeoxyglucose positron emission tomography in the management of patients with thymic epithelial tumors

PURPOSE

There are limited data regarding the role of (18)F-fluorodeoxyglucose positron emission tomography ([(18)F]-FDG PET) imaging in management of patients with thymic epithelial tumors (TET). The primary objective of this study was to assess the usefulness of early [(18)F]-FDG PET to monitor treatment efficacy and its correlation with Response Evaluation Criteria in Solid Tumors (RECIST) in patients with TETs.

EXPERIMENTAL DESIGN

[(18)F]-FDG PET/computed tomographic (CT) scans were conducted at baseline and after 6 weeks of treatment in patients enrolled in two phase II and one phase I/II clinical trials. On the basis of data from other solid tumors, metabolic response was defined as a reduction of [(18)F]-FDG uptake by more than 30% as assessed by average standardized uptake values (SUV) of up to five most metabolically active lesions.

RESULTS

Fifty-six patients with unresectable Masaoka stage III or IV TETs were included. There was a close correlation between early metabolic response and subsequent best response using RECIST (P < 0.0001-0.0003): sensitivity and specificity for prediction of best response were 95% and 100%, respectively. Metabolic responders had significantly longer progression-free survival (median, 11.5 vs. 4.6 months; P = 0.044) and a trend toward longer overall survival (median, 31.8 vs. 18.4 months; P = 0.14) than nonresponders. [(18)F]-FDG uptake was significantly higher in thymic carcinoma than in thymoma (P = 0.0004-0.0010).

CONCLUSION

In patients with advanced TETs, early metabolic response closely correlates with outcome of therapy. [(18)F]-FDG PET may be used to monitor treatment efficacy and assess histologic differences in patients with advanced TETs.

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.

Thymic neoplasms: a clinical update

Thymoma is a unique neoplasm of the anterior mediastinum that is frequently associated with indolent growth and a variety of paraneoplastic syndromes. One third of cases are detected during the evaluation of myasthenia gravis. Classification systems of thymoma have limited ability in accurately predicting prognosis and course of disease. Thus, staging is the only way to predict clinical behavior. Encapsulated tumors that are surgically resected carry the best prognosis. Adjuvant radiotherapy is recommended for incompletely excised and most invasive thymomas. Chemotherapy in anthracycline-based chemotherapy remains the most effective chemotherapy for neoadjuvant, adjuvant or palliative treatment.

The role of [¹⁸F]fluorodeoxyglucose positron emission tomography in thymic epithelial tumors

The purpose of this study was to systemically review the available literature regarding the diagnostic performance of positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) in patients with thymic epithelial tumors. We reviewed 13 studies that evaluated the diagnostic role of thymic epithelial tumors with [18F]FDG-PET. [18F]FDG-PET is a useful radiological modality for differentiating between thymomas and thymic carcinoma. However, [18F]FDG-PET may not be useful for differentiating low-risk thymoma and high-risk thymoma. One paper reported that [18F]FDG-PET has a predictive significance for treatment and prognosis in thymic epithelial tumors. Two papers reported that the degree of [18F]FDG uptake in thymic epithelial tumors is based on glucose metabolism. [18F]FDG-PET may have a further use for radiological differential diagnosis of thymomas and thymic carcinomas.

Screening for tumours in paraneoplastic syndromes: report of an EFNS task force

BACKGROUND

paraneoplastic neurological syndromes (PNS) almost invariably predate detection of the malignancy. Screening for tumours is important in PNS as the tumour directly affects prognosis and treatment and should be performed as soon as possible.

OBJECTIVES

an overview of the screening of tumours related to classical PNS is given. Small cell lung cancer, thymoma, breast cancer, ovarian carcinoma and teratoma and testicular tumours are described in relation to paraneoplastic limbic encephalitis, subacute sensory neuronopathy, subacute autonomic neuropathy, paraneoplastic cerebellar degeneration, paraneoplastic opsoclonus-myoclonus, Lambert-Eaton myasthenic syndrome (LEMS), myasthenia gravis and paraneoplastic peripheral nerve hyperexcitability.

METHODS

many studies with class IV evidence were available; one study reached level III evidence. No evidence-based recommendations grade A-C were possible, but good practice points were agreed by consensus.

RECOMMENDATIONS

the nature of antibody, and to a lesser extent the clinical syndrome, determines the risk and type of an underlying malignancy. For screening of the thoracic region, a CT-thorax is recommended, which if negative is followed by fluorodeoxyglucose-positron emission tomography (FDG-PET). Breast cancer is screened for by mammography, followed by MRI. For the pelvic region, ultrasound (US) is the investigation of first choice followed by CT. Dermatomyositis patients should have CT-thorax/abdomen, US of the pelvic region and mammography in women, US of testes in men under 50 years and colonoscopy in men and women over 50. If primary screening is negative, repeat screening after 3-6 months and screen every 6 months up till 4 years. In LEMS, screening for 2 years is sufficient. In syndromes where only a subgroup of patients have a malignancy, tumour markers have additional value to predict a probable malignancy.

Biologic Correlation of 2-[18F]-Fluoro-2-Deoxy-D-Glucose Uptake on Positron Emission Tomography in Thymic Epithelial Tumors

Purpose

The usefulness of 2-[18F]-fluoro-2-deoxy-D-glucose ([18F]FDG) positron emission tomography (PET) can help predict the grade of malignancy and staging in thymic epithelial tumors. However, no satisfactory biologic explanation exists for this phenomenon. The aim of this study was to investigate the underlying biologic mechanisms of [18F]FDG uptake.

Patients and Methods

Forty-nine patients with thymic epithelial tumors who underwent [18F]FDG PET were included in this study. Tumor sections were stained by immunohistochemistry for glucose transporter 1 (GLUT1), glucose transporter 3 (GLUT3), hypoxia-inducible factor-1 α (HIF-1α), vascular endothelial growth factor (VEGF), microvessels (CD31 and CD34), cell cycle control marker (p53), and apoptosis marker (bcl-2). We also conducted an in vitro study of [18F]FDG uptake in a thymic tumor cell line.

Results

There was a positive correlation between [18F]FDG uptake and GLUT1 (P < .0001), HIF-1α (P = .0036), VEGF (P < .0001), microvessel density (MVD; P < .0001), and p53 (P = .0002). GLUT3 and bcl-2 showed no positive correlation with [18F]FDG uptake. The expression of Glut1, HIF-1α, VEGF, CD31, CD34, and p53 was also significantly associated with the grade of malignancy and poor outcome in thymic epithelial tumors, whereas this relationship was not observed in GLUT3 and bcl-2. Our in vitro study demonstrated that upregulation of GLUT1 and HIF-1α was closely associated with [18F]FDG uptake into thymic tumor cell.

Conclusion

[18F]FDG uptake in thymic epithelial tumors is determined by the presence of glucose metabolism (GLUT1), hypoxia (HIF-1α), angiogenesis (VEGF and MVD), and cell cycle regulator (p53).

(18)F-FDG PET/CT is Useful for Pretreatment Assessment of the Histopathologic Type of Thymic Epithelial Tumors

PURPOSE

This study was performed to assess the usefulness of (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET) or PET/computed tomography (CT) for distinguishing thymic epithelial tumors according to World Health Organization (WHO) classifications.

METHODS

We analyzed a total of 45 patients (range, 29-75 years of age; mean, 55 years) with pathologically confirmed thymic epithelial tumors who underwent pretreatment (18)F-FDG PET or PET/CT between November 2003 and October 2009. The size, visual grading of uptake value, peak standardized uptake value (SUVpeak), uptake pattern, and contour of each tumor, and associated findings on PET or PET/CT, were analyzed relative to the three simplified WHO subgroups: less-invasive thymomas (types A and AB), more-invasive thymomas (types B1, B2, and B3) and thymic carcinomas. We statistically assessed the relationship of (18)F-FDG PET or PET/CT findings with these simplified subgroups.

RESULTS

Of the 45 patients, ten had less-invasive thymomas, 23 had more-invasive thymomas, and 12 had thymic carcinomas. The SUVpeak of the less- and more-invasive thymomas were significantly lower than those of thymic carcinomas (p < 0.000), but there was no difference in SUVpeak between less- and more-invasive thymomas. The visual grading scale (p < 0.000), uptake pattern (p = 0.001), and contour (p < 0.000) of the tumors differed significantly among the three simplified subgroups.

CONCLUSION

The image findings of (18)F-FDG PET or PET/CT differed significantly by histologic subgroups. Pre-treatment evaluation with (18)F-FDG PET or PET/CT might be helpful in differentiating subgroups of thymic epithelial tumors.

Comparison of patterns of relapse in thymic carcinoma and thymoma

OBJECTIVE

Thymic carcinomas are considered to be more aggressive than thymomas and carry a worse prognosis. We reviewed our recent experience with the surgical management of thymic tumors and compared the outcomes and patterns of relapse between patients with thymic carcinoma and those with thymoma.

METHODS

We performed a single-institution retrospective cohort study. Data included patient demographics, stage, treatment, pathologic findings, and postoperative outcomes.

RESULTS

During the period 1995-2006, 120 patients with thymic tumors underwent surgical intervention, including 23 patients with thymic carcinoma and 97 patients with thymoma, as classified according to the World Health Organization 2004 histologic classification. The overall 5-year survival was significantly different between patients with thymic carcinoma and those with thymoma (thymic carcinoma, 53%; thymoma, 89%; P = .01). Data on relapse were available for 112 patients. The progression-free 5-year survival was also significantly different between patients with thymic carcinoma and those with thymoma (thymic carcinoma, 36%; thymoma, 75%; P < .01). Using multivariate analysis, thymic carcinoma and incomplete resection were found to be independent predictors of progression-free survival. Relapses in patients with thymic carcinoma tended to occur earlier, and occurred significantly more frequently at distant sites than in patients with thymoma (60% vs 13%, P = .01).

CONCLUSIONS

Patterns of relapse differ significantly between patients with thymic carcinoma and those with thymoma, with lower progression-free survival, earlier onset, and more distant relapses in patients with thymic carcinoma. Given the greater propensity for distant failures, the inclusion of systemic therapy in the treatment of thymic carcinoma might take on greater importance. Despite significantly higher rates of distant relapse, good overall survival in patients with thymic carcinoma can be achieved.

Utility of positron emission tomography in the mediastinum: moving beyond lung and esophageal cancer staging

PET and PET-CT are rapidly evolving as modalities of thoracic imaging. In the mediastinum, PET can provide information to distinguish thymic hyperplasia from neoplasia, although the use of this imaging for this purpose is not accepted uniformly as necessary. PET is the standard of care in staging and follow-up of mediastinal lymphoma and in follow-up of metastatic seminomas after chemotherapy. Mycobacterial/fungal infections, sarcoidosis, and brown fat can mimic malignant findings on PET in the mediastinum.

Role of flourine-18 fluorodeoxyglucose positron emission tomography in thymic pathology

OBJECTIVE

To evaluate the utilization of positron emission tomography (PET) scan with fluorine-18 fluorodeoxyglucose (FDG) in thymic pathology.

METHODS

Twenty-five consecutive patients with thymic pathology underwent FDG-PET after being evaluated by computed tomography (CT). The indication for CT was myasthenia gravis in 10, anterior mediastinal mass in 7, and recurrent thymic tumor after surgical excision in 8 patients. The results of PET were compared with results obtained by CT, and histopathologic examination of the surgical specimens.

RESULTS

All mediastinal abnormal thymic tissue showed FDG uptakes. FDG-PET managed to differentiate between thymic hyperplasia and thymoma in myasthenia gravis group (n=10) in which CT images were questionable in two patients. There was one case of ectopic thymic tissue which was not diagnosed preoperatively. There were no false-negative results for both CT and FDG-PET in seven patients with thymoma presented as anterior mediastinal mass. However, PET scan predicted thymic carcinoma in one patient. PET was superior to CT scan in localization of recurrent thymoma in two patients, and equal to CT in detecting metastatic lesions in six patients during the follow-up after thymoma excision.

CONCLUSIONS

In myasthenia gravis, selective use of FDG-PET is useful in differentiating thymoma from hyperplasia, especially when CT scan is controversial, but fails to recognize ectopic thymic tissue. FDG-PET may differentiate thymoma from thymic carcinoma. FDG-PET is also useful in follow-up patients, who underwent thymoma excision, when there is suspicion of recurrence or metastasis.

Characteristics of Thymoma Successfully Resected by Videothoracoscopic Surgery

PURPOSE

The inclusion criteria were established for a videothoracoscopic resection of early-stage thymoma. We retrospectively evaluated the validity of these criteria in the treatment of early-stage thymoma.

METHODS

The computed tomography (CT) image characteristics and clinical information comprised these criteria. The image considerations included the location of the tumor in the anterior mediastinum, a distinct fat plane between the tumor and vital organs, unilateral tumor predominance, tumor encapsulation, the existence of residual normal-appearing thymic tissue, and no mass compression effect. All enrollees were expected to be free of pleural effusion, pericardial effusion, paralysis of the hemidiaphragm, and the encasement of great vessels. An elevation of either the serum alpha-fetoprotein or beta-human chorionic gonadotropin levels, severe chest pain, superior vena cava syndrome, hoarseness, and age less than 20 years excluded the patient from enrollment. The heterogeneous content of the tumor was not an exclusion criterion, and the tumor size was not considered important. According to the above criteria, 44 patients were enrolled between November 1999 and November 2005.

RESULTS

Twenty-seven patients had stage I thymoma and 17 had stage II thymoma. All patients successfully underwent a complete tumor resection using a three-port endoscopic technique. There was no open conversion.

CONCLUSIONS

Based on these criteria, we can select suitable patients to confidently perform a thoracoscopic resection of early-stage thymoma.

Thymoma: Benign Appearance, Malignant Potential

Thymoma is a rare tumor with a largely indolent growth pattern. It does, however, have malignant potential as a result of its ability to invade locally and metastasize regionally. Often associated with a number of immune- and nonimmune-mediated paraneoplastic syndromes, patient outcomes are directly related to stage of disease and the ability to achieve a complete surgical resection. Surgery is the mainstay of treatment, with adjuvant radiation recommended for invasive thymoma. Sensitive to both chemotherapy and radiation, durable responses are achievable in incompletely resected and inoperable patients. We present two cases of thymoma followed by a general discussion with an emphasis on treatment for both early and advanced-stage disease.

A high yield robotic synthesis of 9-(4-[18F]-fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG) and 9-[3-[18F]fluoro-1-hydroxy-2-propoxy)methyl]guanine([18F] FHPG) for gene expression imaging

The aim of this study was to develop an automated synthesis of 9-(4-[(18)F]-fluoro-3-hydroxymethylbutyl)guanine ([(18)F]FHBG) and 9-[(3-[(18)F]fluoro-1-hydroxy-2-propoxy)methyl]guanine ([(18)F]FHPG) using a Scanditronix Anatech RB III robotic system. [(18)F]HF was produced via (18)O(p, n)(18)F using a Scanditronix MC17F cyclotron. On average, a typical run produced [(18)F]FHBG and [(18)F]FHPG with an uncorrected radiochemical yield of 19% and 16%, respectively, at end of synthesis (EOS) from irradiation of 95% enriched [(18)O]water. The total synthesis time was 80 min. The retention time of [(18)F]FHBG and [(18)F]FHPG (the radio-peak) was 3.9 and 4.0 min, respectively, which was consistent with the [(19)F]FHBG and [(19)F]FHPG ultraviolet peak. The radiochemical purity was greater than 97%. A robotic, automated method for [(18)F]FHBG and [(18)F]FHPG radiosynthesis is therefore feasible. The radiation burden for the operator can be reduced as much as possible. Sufficient radioactivities of [(18)F]FHBG and [(18)F]FHPG could be obtained for non-invasive monitoring the expression of transfected gene in vivo with positron emission tomography (PET).

False-positive FDG PET uptake--the role of PET/CT

Positron emission tomography (PET) is a powerful molecular imaging technique for the human body-imaging applications currently available. As altered glucose metabolism is characteristic for many malignancies, FDG-PET is mostly used in oncology for staging and therapy control. Although PET is a sensitive tool for detecting malignancy, FDG uptake is not tumor specific. It can also be seen in healthy tissue or in benign disease as inflammation or posttraumatic repair and could be mistaken for cancer. The experienced nuclear medicine physician mostly manages to differentiate malignant from non-malignant FDG uptake, but some findings may remain ambiguous. In these cases, the difficulties in differentiating physiologic variants or benign causes of FDG uptake from tumor tissue can often be overcome by combined PET and CT (PET/CT) as anatomic information is added to the metabolic data. Thus, PET/CT improves the diagnostic accuracy compared to PET alone and helps to avoid unnecessary surgery/therapy. However, PET/CT involves other sources of artifacts that may occur when using CT for attenuation correction of PET or by patient motion caused by respiration or bowel movements.

Imaging of thymoma

Thymomas are the most common mediastinal tumors, usually diagnosed initially by CT. CT scanning can aid in characterization of thymoma and can successfully distinguish thymomas from other benign mediastinal tumors and from lymphoma. However, many of the primary mediastinal masses demonstrate overlap between imaging features and frequently resection is advisable for definitive characterization.

Spectrum of Thymic Uptake at18F-FDG PET

Fluorine-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) has emerged as a strong diagnostic tool for the diagnosis and staging of neoplasms. Uptake in the thymus at (18)F-FDG PET complicates the assessment of mediastinal involvement by tumor in children and young adults. Increased thymic (18)F-FDG uptake may represent normal physiologic uptake but may also indicate the presence of thymic hyperplasia, lymphomatous infiltration, primary thymic neoplasm, or metastatic disease. Familiarity with the patterns of (18)F-FDG uptake that characterize these pathologic conditions is crucial to the interpretation of PET findings in the thymus. In addition, awareness of the subsets of patients in whom physiologic uptake may be seen and of the normal morphologic features and (18)F-FDG PET appearance of the thymus, along with a general sense of the upper limits of metabolic activity for physiologic thymic uptake, will aid in differentiating between physiologic thymic uptake and mediastinal disease. In equivocal cases, correlation with morphologic data from computed tomography or magnetic resonance imaging will likely continue to play a key role in diagnosis and will aid in differentiating benign thymic uptake from malignancy.

Thymic uptake of radiopharmaceuticals

The thymus, as the site of T-cell differentiation, plays a critical role in the development of the immune system. The gland increases in weight until puberty and then is slowly replaced by fat. Various radiopharmaceuticals can localize to the thymus before it involutes. This pictorial presentation reviews the appearance and proposed mechanisms of thymic uptake of Ga-67 citrate, F-18 fluorodeoxyglucose, radioiodine, and In-111 pentetreotide.

Thymoma and myotonic dystrophy: successful treatment with chemotherapy and radiation: case report and review of the literature

We present the case of a 42-year-old woman with myotonic dystrophy and thymoma. She was treated with combination chemotherapy followed by external beam radiation, and remains in remission 19 months after thymoma was diagnosed. The myotonic dystrophy is unchanged. Only six cases of this nature have been reported in the literature, and this patient is the first to be successfully treated with combined modality therapy.

Thymic lesions in patients with myasthenia gravis: characterization with thallium 201 scintigraphy

PURPOSE

To assess thallium 201 ((201)Tl) single photon emission computed tomography (SPECT) for evaluation of thymic lesions associated with myasthenia gravis (MG), including lymphoid follicular hyperplasia (LFH) and thymoma.

MATERIALS AND METHODS

(201)Tl SPECT and computed tomography (CT) were performed preoperatively in 46 patients with MG who had undergone thymectomy. SPECT was conducted 15 (early image) and 180 (delayed image) minutes after (201)Tl injection. Results were visually assessed, and (201)Tl uptake ratios (thymic lesion count density/lung count density) were measured for quantitative analysis. Uptake was analyzed among the normal thymus, LFH, and thymoma patient groups.

RESULTS

Histopathologic results indicated a normal thymus, LFH, and thymoma in 19, 16, and 11 patients, respectively. Mean uptake ratios in the normal thymus, LFH, and thymoma were 0.96 (95% CI: 0.90, 1.03), 1.14 (95% CI: 1.04, 1.25), and 1.87 (95% CI: 1.56, 2.25), respectively, on early images and 1.09 (95% CI: 1.00, 1.18), 1.65 (95% CI: 1.48, 1.85), and 2.03 (95% CI: 1.65, 2.50), respectively, on delayed images. Thymoma showed more intense (201)Tl accumulation than did the normal thymus (P <.001) and LFH (P <.001) on early images. Both thymoma (P <.001) and LFH (P <.001) displayed more intense uptake than did the normal thymus on delayed images.

CONCLUSION

(201)Tl SPECT can enable differentiation between normal thymus, LFH, and thymoma in patients with MG.

FDG uptake in the morphologically normal thymus: comparison of FDG positron emission tomography and CT

The purpose of this study was to evaluate the correlation between fluorine-18 fluorodeoxyglucose (FDG) thymic uptake and a normal appearing thymus on CT. Non-attenuation corrected FDG positron emission tomography (PET) data from 94 young persons (mean age 25.4 years, range 18-29 years) with a normal thymus diagnosed on CT were retrospectively evaluated. No subject had clinical symptoms suggestive of thymus-related disease or mediastinal tumour (follow-up period 6-69 months). PET images were visually assessed and the count ratio between the thymus and the lung (T/L ratio) was calculated. Increased FDG uptake occurred in 32 (34%) subjects. In these 32 cases, the T/L ratio was 2.86+/-0.49 (range 2.02-3.99). In 86 subjects whose CT images were available to calculate the CT attenuation of the thymus (CAT), the CAT value was -17.5+/-45.7 HU (range -103.6 HU to 79.9 HU). The T/L ratio correlated with the CAT value (r=0.58). CAT values in subjects with positive PET findings were significantly higher than CAT values in subjects with negative PET findings (p<0.001, unpaired t-test). These results suggest that even in young adults, if the thymus has a relatively high CT attenuation value, the presence of physiological thymic uptake in FDG-PET is a normal variant. In this study, the diagnosis of normal thymus was based on CT appearance and clinical course. Further studies are needed to clarify the relationship between histopathology and FDG uptake in the thymus.

Mediastinal tumors

Mediastinal tumors are comprised of various benign and malignant neoplasms that share the same anatomic location within the thorax. The mediastinum is traditionally divided into three compartments: the anterior, middle, and posterior mediastinum. This division, based on lateral chest radiographs, helps clinicians establish appropriate differential diagnoses and plan further imaging, diagnostic, and treatment strategies. With the continued and complex advances in imaging, medical treatment, and surgery, we recommend a multidisciplinary approach to the management of mediastinal tumors. This discussion is intended to guide the pulmonary specialist through this potentially complex approach.