Normal thymic uptake of FDG on PET imaging

Imaging insights of FDG-PET from neonates to infants

In pediatric oncology, 18F-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT) is valuable as a tool for noninvasive imaging and monitoring. While many reports have reviewed the use of PET and PET/CT in pediatrics, considerable variations in age, body size, and metabolism are seen during different stages of childhood development. Neonates (from birth to one month old) and infants (from 1 month to 1 year) present unique challenges for FDG-PET/CT examination due to their small body size, the immaturity of organs, the need for specialized patient preparation, and support requirements during scanning. In addition, differences in metabolic activity can lead to distinct differences in patterns of physiological FDG uptake on PET/CT imaging between neonates and infants. These factors differ significantly from those encountered in older children, who may be treated similarly to adults during imaging procedures. This review, based on both the literature and clinical experience, explores the specific characteristics, challenges, and considerations for FDG-PET/CT imaging from neonates to infants, with a focus on optimizing imaging protocols and interpreting physiological variations in this growth period.

Evaluation of physiological Waldeyer's ring, mediastinal blood pool, thymic, bone marrow, splenic and hepatic activity with 18F-FDG PET/CT: exploration of normal range among pediatric patients

INTRODUCTION

While ¹⁸F-FDG PET/CT pediatrics applications have increased in number and indications, few studies have addressed normal maximum standardized uptake values (SUV_max) of referral organs in children. The purpose of this study is to assess these in a cohort of pediatric patients.

MATERIAL AND METHODS

285 ¹⁸F-FDG PET/CT scans in 229 patients were reviewed. SUV_max were assessed for mediastinal blood pool (MBP), thymus (T), liver (L), spleen (S), bone marrow (BM) and Waldeyer's Ring (Wald). L/MBP and S/L ratios were calculated. Same day complete blood counts (CBC) were available for 132 studies and compared to BM and S. Means, standard deviations and correlation coefficients with age, weight and body surface area (BSA) were calculated.

RESULTS

Weak correlation with age, weight or BSA was found for Wald. Strong correlations with weight/BSA more than with age were demonstrated for MBP, L and BM and moderate for S and T. After initial decrease between age 0 and 2, thymic activity peaked at age 11 years then involuted. No correlation was found between CBC ad BM or S. In 28 studies, L was less or equal to MBP. In 74 S was superior to L.

CONCLUSIONS

Referral organs ¹⁸F-FDG uptake varies in children more in relation with weight and BSA than with age for key referral organs, such as L, S and MBP. In a significant number of studies, L activity may impede evaluation of treatment response in comparison with MBP or inflammation/infection evaluation in comparison with S.

Increased Uptake of 68Ga-Pentixafor in Thymic Rebound After Chemotherapy in a Patient With Multiple Myeloma

A 38-year-old man with newly diagnosed multiple myeloma underwent Ga-pentixafor PET/CT for baseline assessment. Ga-pentixafor PET/CT demonstrated multiple focal bone and bone marrow lesions with intense radioactivity. After 9 cycles of chemotherapy against multiple myeloma, the patient experienced serologically complete remission of myeloma. Consistently, the follow-up Ga-pentixafor PET/CT for PET response showed significantly reduced uptake in the previously chemokine receptor 4-positive bone lesions. However, intense and homogeneous uptake of Ga-pentixafor was noted in the thymus with a smooth contour and slightly increased density, which is possibly caused by thymic hyperplasia after chemotherapy.

Differentiation of rebound and lymphoid thymic hyperplasia from anterior mediastinal tumors with dual-echo chemical-shift MR imaging in adulthood: reliability of the chemical-shift ratio and signal intensity index

PURPOSE

To prospectively evaluate (a) effectiveness and limits of dual-echo chemical-shift magnetic resonance (MR) imaging for distinguishing hyperplastic thymus from anterior mediastinal tumors in adulthood by using chemical-shift ratio ( CSR chemical-shift ratio ) and signal intensity index ( SII signal intensity index ), with proposal of optimal threshold value for each, and (b) whether age affects these indexes.

MATERIALS AND METHODS

Study was institutional review board approved, with informed consent obtained. Ninety-two subjects (53 men, 39 women; age range, 18-84 years) were divided into a rebound and lymphoid hyperplasia group (group A, 30 patients) and a tumor group (group B, 62 patients). MR images were assessed; interrater reliability was evaluated. Differences in CSR chemical-shift ratio and SII signal intensity index were tested with the Mann-Whitney U test and the Kruskal-Wallis test. Discrimination abilities of CSR chemical-shift ratio and SII signal intensity index were evaluated with logistic regression models, and optimal cutoff points were proposed. Quantitative parameters were correlated with age by using Pearson correlation coefficients.

RESULTS

Interreader agreement was excellent (intraclass correlation coefficient: CSR chemical-shift ratio , 0.893; SII signal intensity index , 0.898). Mean CSR chemical-shift ratio and SII signal intensity index ± standard deviation were 0.545 ± 0.162 and 46.29% ± 18.41 for group A and 1.045 ± 0.094 and -0.06% ± 4.89 for group B, respectively, with significant differences for both indexes between groups (P < .0001). No overlap was found for SII signal intensity index between groups; CSR chemical-shift ratio values overlapped in a few younger adults. Distinguishing hyperplastic thymus from tumors was better with SII signal intensity index than CSR chemical-shift ratio . Respective sensitivity, specificity, and cutoff points were 100%, 100%, and 8.92% for SII signal intensity index and 100%, 96.7%, and 0.849 for CSR chemical-shift ratio . Significant correlation was found for CSR chemical-shift ratio (r = -0.761) and SII signal intensity index (r = 0.821) with age in group A (P < .001). For group B, significant correlation with age was seen for CSR chemical-shift ratio (r = 0.702, P < .001) but not SII signal intensity index (r = -0.196, P = .127). All subjects but one in group A and none in group B had signal intensity decrease at chemical-shift MR imaging.

CONCLUSION

With dual-echo chemical-shift MR imaging, SII signal intensity index and CSR chemical-shift ratio have high accuracy to distinguish thymic hyperplasia from tumors, although overlapped CSR chemical-shift ratio values can occur in early adulthood.

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.

Watchful waiting for some children with a mediastinal mass: the potential role for ¹⁸F-fluorodeoxyglucose positron emission tomography: a case report and review of the literature

Background

Benign hyperplastic thymus is a rare but important differential diagnosis of anterior mediastinal lesions. Histological and radiological criteria are used to distinguish this benign condition from other malignant diseases but have their limitations, and biopsy of mediastinal masses can be risky. We report for the first time the diagnostic value of fluorodeoxyglucose ¹⁸ F positron emission tomography for patients with incidentally identified anterior mediastinal masses to avoid biopsy in some cases.

Case presentation

A 2 year old girl presented with new onset of emesis and constipation leading to the incidental discovery of an anterior mediastinal mass on radiograph. Chest computed tomography revealed cystic components within the mass concerning for a malignancy. Biopsy of the lesion and bone marrow aspiration and biopsy were negative but there was concern that the mediastinal biopsy may have missed the malignant component of the lesion. Hence, a positron emission tomography scan was obtained that showed mild homogeneous fluorodeoxyglucose ¹⁸ F avidity within the mass similar to that of normal thymus. The diagnosis of benign hyperplastic thymus was made.

Conclusion

The differential diagnosis of an incidentally found anterior mediastinal mass includes malignancy, but benign lesions such as benign hyperplastic thymus must also be considered, particularly when the complete blood count and biochemical profile are normal. Fluorodeoxyglucose ¹⁸ F positron emission tomography can help guide a clinician’s decision for further interventions and treatment.

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.

Thymic tissue is not evident on high-resolution computed tomography and [¹⁸F]fluoro-deoxy-glucose positron emission tomography scans of aviraemic HIV patients with poor recovery of CD4⁺ T cells

Some previously immunodeficient HIV patients responding to antiretroviral therapy display poor recovery of CD4⁺ T cells. Evaluation of the contribution of thymic function requires sensitive detection and quantitation of metabolically active thymic tissue. We describe patients with low but detectable thymopoiesis assessed as circulating CD4⁺ naive T cells expressing CD31. High-resolution computed tomography and PET scans found no residual thymic tissue even though metabolic activity was demonstrable by PET in lymph nodes.

Prediction of thymoma histology and stage by radiographic criteria

Thymic epithelial tumors, such as thymomas and thymic carcinomas, are the most common primary neoplasms of the mediastinum. In 1999, the World Health Organization (WHO) proposed a consensus classification of thymic epithelial tumors based on the morphology of the epithelial cells and the ratio of lymphocytes to epithelial cells, which was revised in 2004. The latest classification system stratifies thymic epithelial tumors into six categories: types A, AB, B1, B2, B3, and thymic carcinoma. This article describes the prediction of thymoma histology and stage on the basis of radiographic criteria by reviewing the following: the WHO histologic classification of thymic epithelial tumors, the clinical staging of thymomas based on prognosis, and the radiographic appearance of thymomas according to the WHO histologic classification.

Physiology and pathophysiology of incidental findings detected on FDG-PET scintigraphy

A routine feature of positron emission tomography/computed tomography (PET/CT) imaging is whole-body acquisition that results in many unexpected findings identified outside of the primary region of abnormality. Furthermore, (18)F-fluorodeoxyglucose (FDG) is a marker of glycolysis and does not specifically accumulate in malignancy. Understanding the physiology and pathophysiology of normal FDG distribution and common incidental findings is therefore essential to the physician interpreting whole-body FDG-PET/CT studies. Whereas many incidental findings are benign and of limited clinical significance, others represent uncommon manifestations of the primary malignancy, second malignancies, or various clinically significant pathologic processes. Patients with a single malignancy are at greater risk of developing synchronous or metachronous second malignancies, possibly related to exposure to shared carcinogenic agents or presence of prooncogenic mutations. The decision of how to pursue an intervention on the basis of an incidental finding is generally left to clinical judgment.

[What is the role of FDG-PET in thoracic oncology in 2010?]

18F-Fluorodeoxyglucose-Positron Emission Tomography (FGD-PET) has been considered to have a major impact on the management of lung malignancies since the beginning of this century. Its value has been demonstrated by many publications, meta-analysis and European/American/Japanese recommendations. PET combined with computed tomography has provided useful information regarding the diagnosis and staging of lung cancer and allows for the delivery of adaptive radiotherapy. In its more common uses, PET has been shown to be cost-effective. With the widespread use of new radiotracers, PET will play an increasing role in the evaluation of response to treatment.

Time-dependent changes in 18F-FDG activity in the thymus and bone marrow following combination chemotherapy in paediatric patients with lymphoma

PURPOSE

To investigate the time-dependent changes in (18)F-FDG uptake by the thymus and marrow following combination chemotherapy for lymphoma in a paediatric study population.

METHODS

Included in the study were 27 paediatric patients who were in complete metabolic remission after chemotherapy and who underwent off-therapy follow-up with serial whole-body PET-CT scans. A total of 142 PET-CT scans were recorded. (18)F-FDG uptake by the thymus and marrow was assessed both visually and semiquantitatively. Visual uptake was scored on the three-dimensional maximum intensity projection of the whole-body PET image according to a three-point scale. For the semiquantitative assessment, standard uptake values were measured. To find a pattern in the (18)F-FDG uptake by the thymus and marrow a moving average technique was applied.

RESULTS

Our time series analysis indicated that the marrow activity was highest at cessation of chemotherapy and declined thereafter. During an off-chemotherapy period of on average 6 months, marrow activity decreased quickly. From 6 months onward, the activity declined more slowly. The posttherapy changes in (18)F-FDG uptake by the thymus were quite different from the changes in uptake by the marrow. The lowest thymic FDG uptake was found at cessation of chemotherapy. Thereafter, thymic activity steadily increased, reached a peak on average 10 months after therapy, and then slowly decreased.

CONCLUSION

Knowledge of the time-dependent changes in metabolic activity in the thymus and marrow is important to avoid misinterpretation of increased (18)F-FDG uptake as disease in the off-therapy setting.

Positron emission tomography/computed tomography potential pitfalls and artifacts

With the recent use of 18-fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) for tumor staging and treatment response, it is important to recognize many pitfalls, artifacts, and benign uptakes that are commonly encountered. Normal physiology can explain many regions of increased FDG activity, as well as incidental benign tumors and benign metabolic conditions. Recognition of characterization of benign causes and physiologic variants for FDG uptake are discussed to avoid improper characterization as a malignancy. A basic understanding of PET/computed tomographic physics is also discussed, in relation to attenuation correction artifacts caused by metallic implants and contrast agents in the gastrointestinal tract, as well as artifacts caused in fused images due to patient motion. Also presented is the rationale for expected, benign uptake in various metabolic diseases, as well as pharmacologic methods for decreasing the artifacts caused by metabolic diseases. PET/computed tomographic evaluation of the thyroid, thymus, adrenal adenomas, uterus and ovaries, infection/inflammatory changes, and postradiation/chemotherapy changes are also discussed, with expected normal changes, as well as pitfalls and artifacts.

Physiologic thymic uptake of 18F-FDG in children and young adults: a PET/CT evaluation of incidence, patterns, and relationship to treatment

(18)F-FDG uptake in the thymus, mainly related to hyperplasia after chemotherapy, has been described. Thymic uptake can challenge the accurate assessment of cancer patients by (18)F-FDG imaging. The present study defines the incidence, patterns, and intensity of thymic (18)F-FDG uptake in relationship to age and time after treatment in a large cohort of patients.

METHODS

A total of 559 consecutive (18)F-FDG PET/CT studies in 160 patients (86 men, 74 women; age, 3-40 y) performed at baseline, during treatment, at the end of treatment, and during follow-up were retrospectively reviewed. PET/CT studies were assessed for the presence or absence (T+ or T-, respectively), pattern, and intensity (SUVmax) of increased (18)F-FDG uptake in the anterior mediastinum, localized by the CT component to the thymus. The overall incidence of (18)F-FDG avidity in the thymus in relationship to the patient's age and time after treatment administration were statistically evaluated.

RESULTS

There were 137 of 559 T+ studies (25%), with equal sex distribution. T+ studies were found in significantly younger patients (20.6 +/- 9.3 y vs. 27.4 +/- 8.4 y, P < 0.001). Most T+ patients (60%) showed an inverted V pattern of thymic uptake, with additional unilateral mediastinal extension in 24% and focal midline uptake in 16% of studies. T+ studies were encountered in 80% of patients younger than 10 y, compared with 8% of patients in the 31- to 40-y age group. There were 17% T+ studies at baseline, 6% during treatment, 8% at the end of treatment, and 27%-40% during follow-up. The average SUVmax of thymic (18)F-FDG uptake was 3.73 +/- 1.22.

CONCLUSION

Thymic (18)F-FDG uptake was found in 28% of the present study population, more frequently after treatment. T+ patients were significantly younger. Thymic uptake was found in 73% of untreated patients up to the age of 13 y and in 8% of patients in the fourth decade of life. Knowledge of this age- and treatment-related incidence of physiologic thymic (18)F-FDG avidity can reduce the number of potential pitfalls in reporting PET/CT studies in cancer patients.

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.

Human T cell reconstitution in DiGeorge syndrome and HIV-1 infection

The thymus is essential for proper development and maintenance of a broad T cell repertoire capable of recognizing a wide-range of foreign antigens. Recent advances in multicolor flow cytometry, non-invasive imaging techniques, and molecular assessments of thymic function have enabled a more comprehensive characterization of human thymic output in clinical settings than in the past. These techniques have been particularly valuable in monitoring human T cells after therapeutic thymic grafting for complete DiGeorge syndrome and during HIV-1 infection and AIDS. By defining the degree and mechanisms of T cell reconstitution in these settings, clinical investigators and primary caregivers have been able to better diagnose, treat and care for individuals with congenital or acquired immune deficiencies associated with loss of thymic function.

PET and PET/CT in pediatric oncology

18F-fluorodeoxyglucose positron emission tomography (FDG-PET) and FDG-PET/computed tomography (CT) are becoming increasingly important imaging tools in the noninvasive evaluation and monitoring of children with known or suspected malignant diseases. In this review, we discuss the preparation of children undergoing PET studies and review radiation dosimetry and its implications for family and caregivers. We review the normal distribution of 18F-fluorodeoxyglucose (FDG) in children, common variations of the normal distribution, and various artifacts that may arise. We show that most tumors in children accumulate and retain FDG, allowing high-quality images of their distribution and pathophysiology. We explore the use of FDG-PET in the study of children with the more common malignancies, such as brain neoplasms and lymphomas, and the less-common tumors, including neuroblastomas, bone and soft-tissue sarcomas, Wilms' tumors, and hepatoblastomas. For comparison, other PET tracers are included because they have been applied in pediatric oncology. Multiple multicenter trials are underway that use FDG-PET in the management of children with neoplastic disease; these studies should give us greater insight into the impact FDG-PET can make in their care. PET is emerging as an important diagnostic imaging tool in the evaluation of pediatric cancers. The recent advent of dual-modality PET-computed tomography (PET/CT) imaging systems has added unprecedented diagnostic capability by revealing the precise anatomical localization of metabolic information and metabolic characterization of normal and abnormal structures. The use of CT transmission scanning for attenuation correction has shortened the total acquisition time, which is an especially desirable attribute in pediatric imaging. Moreover, expansion of the regional distribution of the most common PET radiotracer, FDG, and the introduction of mobile PET units have greatly increased access to this powerful diagnostic imaging technology. Here, we review the clinical applications of PET and PET/CT in pediatric oncology. General considerations in patient preparation and radiation dosimetry will be discussed.

Detection of age-related changes in thoracic structure and function by computed tomography, magnetic resonance imaging, and positron emission tomography

It is useful to understand the normal changes in structure and function in the thorax that occur with age. Thus, we present the following quantitative preliminary data obtained from retrospective quantitative analysis of computed tomography (CT) and positron emission tomography (PET) examinations in subjects 0 to 90 years of age: Mean lung standard uptake values were found to significantly increase with increasing age and with increasing body mass index (BMI). Mean lung attenuation was seen to statistically significantly decrease with increasing age in subjects who had a CT scan, had a nonsignificant tendency to decrease with increasing age in subjects with a PET/CT scan, had a nonsignificant tendency to increase with increasing BMI, and was seen to significantly increase with increasing mean lung standard uptake values. Mean lung volumes were not noted to significantly change with increasing age in adult subjects whether or not they were normalized to the craniocaudal thoracic lengths, although mean lung volumes significantly increased with increasing age in pediatric subjects. Mean lung volumes had a nonsignificant tendency to decrease with increasing BMI, although normalized mean lung volumes significantly decreased with increasing BMI. Lung metabolic volumetric products were not noted to significantly change with increasing BMI or with increasing age. In this work, we also review the literature regarding normal structural and functional changes in the thorax with age.

Imaging of an Invasive Malignant Thymoma on PET Scan: CT and Histopathologic Correlation

Thymomas are common primary mediastinal neoplasms. They can be benign or malignant. The diagnosis of malignant thymomas is based on capsular invasion, cellular atypia and distant metastases. We present a case of invasive malignant thymoma as diagnosed on PET and CT scan with histologic correlation and review of literature. A seventy year old Asian man presented with cough and dyspnea. A CT scan showed an anterior mediastinal mass, and a PET scan showed a hypermetabolic mass in the corresponding location. Resection of the mass revealed a minimally invasive thymoma based on evidence of capsular invasion. PET scan proved invaluable in correctly diagnosing, staging the lesion, and excluding any extramediastinal involvement.

Positive gallium-67 and thallium-201 scans in thymic rebound after chemotherapy for lymphoma

It is a diagnostic problem to distinguish thymic rebound or rebound thymic hyperplasia from thymic malignancy, but it is frequently made more difficult because most patients have had previous malignancies. Recently we evaluated a six-year-old girl with thymic rebound after chemotherapy for lymphoma, by both gallium-67 and thallium-201 scans. On gallium-67 scan, intensive uptake was seen in the anterior mediastinum. CT revealed a triangular-shaped, homogeneous mass in the anterior mediastinum. On early scan of thallium-201 study, slight accumulation was seen in the anterior mediastinum and was enhanced in delayed scans. Considering the clinical state and imaging results, thymic rebound after chemotherapy was the most likely diagnosis, and follow-up observation was done without therapy. During the course, there were no signs of relapse. Some reports have described both positive and negative thallium-201 accumulation in thymic rebound. Although more experience with similar cases is necessary, it is likely that thallium-201 also tends to accumulate in thymic rebound as well as gallium-67.

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.

Intense 18-fluorodeoxyglucose uptake by the thymus on PET scan does not necessarily herald recurrence of thyroid carcinoma

This is the first report of intense fluorodeoxyglucose positron emission tomography (FDG-PET) uptake secondary to thymic hyperplasia during follow-up for thyroid carcinoma. A 36-yr-old woman underwent thyroidectomy for a papillary carcinoma measuring 4 cm in diameter. After two doses of radioiodine, thyroglobulin (Tg) remained detectable following recombinant human TSH (rhTSH) stimulation. A whole body scan (WBS) was negative. On computed tomography (CT) scan, a slightly lobulated thymus was visualized. PET scan showed intense thymic uptake. Following resection, anatomo-pathological analysis showed homogenous hyperplastic thymic gland without neoplastic cells. Two months later, under levothyroxin (L-T4) substitution, Tg was no longer detectable and PET scanning did not show any 18-FDG uptake. This observation suggests that thymic FDG uptake does not necessarily herald recurrence of thyroid carcinoma and must be interpreted with caution in such a setting. Other conditions associated with abnormal uptake by hyperplastic thymus must also be envisaged.

FDG-PET in the management of germ cell tumor

Germ cell tumor is the most common malignancy in young men. The cure rate of these patients has tremendously increased in the cisplatin era, and recent results have indicated that the management of patients with GCT is still improving. The use of FDG-PET in the management of patients with GCT has been recently investigated. This report attempts to comprehensively review new advances and delineate the potential applications of FDG-PET in GCT.

Evolving role of positron emission tomography in the management of patients with inflammatory and other benign disorders

Fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) has evolved from a research imaging modality assessing brain function in physiologic and pathologic states to a pure clinical necessity. It has been successfully used for diagnosing, staging, and monitoring a variety of malignancies. FDG-PET imaging also is evolving into a powerful imaging modality that can be effectively used for the diagnosis and monitoring of a certain nononcological diseases. PET has been shown to be very useful in the diagnosis of osteomyelitis, painful prostheses, sarcoidosis, fever of unknown etiology, and acquired immunodeficiency syndrome. Based on recent observations, several other disorders, such as environment-induced lung diseases, atherosclerosis, vasculitis, back pain, transplantation, and blood clot, can be successfully assessed with this technique. With the development and the introduction of several new PET radiotracers, it is expected that PET will secure a major role in the management of patients with inflammatory and other benign disorders.

Imaging Evaluation of Pediatric Mediastinal Masses

Mediastinal masses in children are a heterogeneous group of asymptomatic to potentially life-threatening congenital, infectious, or neoplastic lesions that can present complex diagnostic and therapeutic dilemmas. This article presents the imaging features of the common mediastinal masses seen in the pediatric population. Classification of the masses is performed according to the traditional mediastinal compartment model, consisting of the anterior, middle, and posterior mediastinum. This scheme facilitates differentiation of the variety of disorders.

Evaluation of normal fatty replacement of the thymus with chemical-shift MR imaging for identification of the normal thymus

PURPOSE

To clarify the influences of age and gender on normal fatty replacement of the thymus in childhood, adolescence, and early adulthood using chemical-shift MRI.

MATERIALS AND METHODS

A total of 95 normal subjects (52 males and 43 females, mean age = 15.6 years, range = 7-25 years) who underwent chemical-shift MRI of the thymus were assessed prospectively. Signal intensity loss (SIL) of the thymus was determined by dividing the thymus/muscle ratio on the opposed-phase image by that on the in-phase image. We evaluated SIL for its correlation with age and gender, and assessed SIL of the thymus with uncommon morphological features.

RESULTS

A significant correlation was found between SIL and age (r = 0.750, P < 0.001). There was no significant difference in SIL between the genders. No significant SIL was identified in any of the subjects aged 10 years or less. However, significant SIL was found in 70.8% of those aged 11-20 years, 100% of those aged 21 years or more, and 46.2% of subjects with uncommon morphological features of the thymus.

CONCLUSION

Chemical-shift MRI can depict physiologic fatty infiltration within the normal thymus in subjects over 11 years of age. It is crucial to correlate these normal age-related findings with clinical cases in order to avoid misinterpretation.

PET in pediatric diseases

FDG-PET is being increasingly applied to pediatric conditions, particularly in oncology. PET and PET/CT scanning in children are not currently supported by Centers for Medicare and Medicaid Services unless the disease condition coincides with a reimbursed adult condition. The recent merger of the Children's Cancer Group and the Pediatric Oncology Group to form the Children's Oncology Group creates an opportunity to examine the use of FDG-PET in the management of childhood tumors in multi-institutional, cooperative efforts. The interest in incorporating PET imaging technology in pediatric medicine has been evidenced by several recent review articles summarizing the ongoing progress in this area. Future data will show that FDG-PET provides useful diagnostic information and can play a pivotal role in the clinical management and care of children with disease.

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.

Evidence of thymic reconstitution after highly active antiretroviral therapy in HIV-1 infection

OBJECTIVES

We aimed to provide evidence of thymic reconstitution after highly active antiretroviral therapy (HAART) in HIV-1 infected patients and to correlate this with the restoration of peripheral naïve T cells.

METHODS

Positron emission tomography (PET) enables definitive evidence of thymic activity, indicating functional potential. In this case study, a single patient who initiated HAART demonstrated reconstitution of the naïve T-cell pool and underwent thymic PET scans at baseline and 2 and 6 months following initiation of therapy. Two patients who failed to demonstrate such reconstitution acted as controls. These patients (mean age 27 years) had chronic HIV infection with low CD4 T-cell counts (mean 82, range 9-160 cells/microL blood). Increased function of the thymus visualized by PET was correlated with phenotypic changes in CD4 and CD8 T cells in the periphery measured by flow cytometry, and with numbers of recent thymic emigrants measured by quantification of the numbers of T-cell receptor excision circles (TRECs) in peripheral cells.

RESULTS

In one patient, clear correlations could be drawn between visible activity within the thymus, as measured by increased [F18]fluorodeoxyglucose (FDG) uptake, and regeneration of naïve CD4 (CD45RA/CD62L) T cells, increased numbers of CD4 T cells, controlled viraemia and increased numbers of recent thymic emigrants. A second patient displayed no increase in peripheral CD4 count and no increase in thymic activity. The third patient elected to stop therapy following the 2-month time point.

CONCLUSIONS

The use of PET suggests that thymic activity may increase after HAART, indicating that the thymus has the potential to be functional even in HIV-1 infected persons with low CD4 T-cell counts.

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.

Thymic enlargement and FDG uptake in three patients: CT and FDG positron emission tomography correlated with pathology

OBJECTIVE

Our purpose was to describe three adult patients in whom we found increased thymic uptake of FDG on positron emission tomography and thymic enlargement with convex lateral margins on CT. Subsequent biopsy or resection showed normal thymic tissue.

CONCLUSION

In three adults, we found a physiologic uptake of FDG by the thymus with standardized uptake values in the range of thymic neoplasia.

Early detection of relapse by whole-body positron emission tomography in the follow-up of patients with Hodgkin's disease

BACKGROUND

Relapse after treatment of Hodgkin's disease (HD) is usually identified as a result of the investigation of symptoms. We undertook this study to examine the value of whole-body positron emission tomography (PET) for the detection of preclinical relapse.

PATIENTS AND METHODS

Thirty-six patients underwent 2-[fluorine-18]fluoro-2-deoxy-D-glucose ((18)F-FDG) PET at the end of treatment and than every 4-6 months for 2-3 years after the end of polychemotherapy and/or radiotherapy. In those cases of abnormal (18)F-FDG accumulation a confirmatory study was performed 4-6 weeks later.

RESULTS

One patient had residual tumor and four patients relapsed during a follow-up of 5-24 months. All five events were correctly identified early by (18)F-FDG PET. Residual tumor or relapse was never first diagnosed based on clinical examination, laboratory findings or computed tomography (CT) studies. Two patients presented B symptoms and the three others were asymptomatic at the time of residual disease or relapse. Confirmation of residual disease or relapse was obtained by biopsy in four patients 1, 1, 5 and 9 months after PET and by unequivocal clinical symptoms and CT studies in one patient 3 months after PET. False-positive (18)F-FDG PET studies incorrectly suggested possible relapse in six other patients, but the confirmatory PET was always negative. Our study also provides important information about physiological (18)F-FDG uptake in the thymus.

CONCLUSIONS

Our data suggest the potential of (18)F-FDG PET to detect preclinical relapse in patients with HD. This could help identify patients requiring salvage chemotherapy at the time of minimal disease rather than at the time of clinically overt relapse. Further studies are warranted to determine the impact of PET on treatment management and outcome. In fact, the aim of follow-up procedures is not only to detect preclinical relapse but mainly to obtain better results by starting salvage treatment earlier. A cost-benefit analysis will also be necessary before (18)F-FDG PET can be used routinely in the follow-up of patients with HD.

18FDG PET versus high-dose 67Ga scintigraphy for restaging and treatment follow-up of lymphoma patients

To date, only one published study has directly compared 67Ga scintigraphy (low dose, planar) with planar dual-head gamma camera 18F-fluorodeoxyglucose (18FDG) imaging for the purpose of treatment follow-up monitoring in lymphoma patients, and no data on restaging are available. The present study reports the direct comparison of high-dose (297-370 MBq) 67Ga planar and single photon emission computed tomography (SPECT) imaging and conventional 18FDG positron emission tomography (PET) for restaging and treatment follow-up of lymphoma patients versus a gold standard consisting of morphological imaging, including plain radiography and computed tomography (CT) scanning, bone marrow examination and long-term follow-up (<12 months). Sixteen patients, 10 with non-Hodgkin's lymphoma and six with Hodgkin's disease, were included (10 men, six women; median age, 43 years; range, 16-64 years). The median follow-up time was 27 months (range, 12-34 months). In two patients, 67Ga and 18FDG PET (370 MBq) were performed twice, resulting in 18 cross-sectional episodes. In 11 episodes, the results obtained by both imaging modalities were in agreement with regard to the presence or absence of disease when compared with the gold standard. However, the abnormalities found on 18FDG PET were always more extensive. In two episodes, 67Ga imaging normalized after treatment, whereas PET showed significant regression followed by subsequent normalization. In four additional episodes, 67Ga images were negative, whereas 18FDG PET visualized non-tumour-related pathology, such as lung infection, rib fracture or dense thymic tissue. In one gold standard-negative patient, the underlying cause of sternal FDG uptake remained undetermined. The data presented, although limited in number, suggest that 18FDG PET performs better than Ga imaging in monitoring lymphoma disease status. However, a correlation with clinical history and a knowledge of the characteristics of benign lesions are mandatory. Further studies are recommended.

Positron emission tomography imaging in nonmalignant thoracic disorders

The role of the fluorodeoxyglucose (FDG) technique positron emission tomography (PET) is well established in the management of patients with lung cancer. Increasingly, it is becoming evident that FDG-PET can be effectively employed to diagnose a variety of benign pulmonary disorders. Knowledge of such applications further expands the domain of this powerful modality and further improves the ability to differentiate benign from malignant diseases of the chest. We describe pertinent technical factors that substantially contribute to optimal imaging of the thoracic structures. Particularly, the complementary role of attenuation correction (AC) to that of non-AC images is emphasized. We further outline the need for and the state of the art for co-registration of PET and anatomic images for diagnostic and therapeutic purposes. We then review patterns of physiologic uptake of FDG in thoracic structures, including the lung, the heart, the aorta and large arteries, esophagus, thymus, trachea, thoracic muscles, bone marrow, and joints and alterations following radiation therapy to the thorax. A great deal of information is provided with regard to differentiating benign from malignant nodules and in particular, we emphasize the role of dual time point imaging and partial volume correction for accurate assessment of such lesions. Following a brief review of the diagnostic issues related to the assessment of mediastinal adenopathies, the role of FDG-PET imaging in environment-induced lung diseases, including pneumoconiosis, smoking, and asthma are described. A large body of information is provided about the role of this technology in the management of patients with suspected infection and inflammation of the lungs such as acquired immunodeficiency syndrome, fever of unknown origin, sarcoidosis, chronic granulomatous disease and monitoring the disease process and response to therapy. Finally, the value of FDG-PET in differentiating benign from malignant diseases of the pleura including asbestosis-related disorders is described at the conclusion of this comprehensive review.

18-FDG PET scan in the staging of recurrent melanoma: additional value and therapeutic impact

Staging of melanoma patients by means of whole body functional imaging in a single evaluation session using positron emission tomography (PET) with fluorine-18- labelled deoxy-d-glucose (FDG) as a metabolic tracer has created much interest over the last decade. After enthusiastic pilot studies, more attention has been paid to the false-negative and false-positive results of this technique than to its true therapeutic impact. This study aimed to evaluate (1) the sensitivity and specificity of this technique at a single lesion level compared with conventional screening procedures (CSP) - both of these accompanied by careful clinical examination; and (2) the additional value of the PET scan at the level of the individual patient and its therapeutic impact for different types of melanoma recurrence. A consecutive series of 100 PET scans performed on 84 melanoma patients with regional or distant recurrence according to CSP (89 PET scans) or suspicion of recurrence, i.e. inconclusive CSP (11 PET scans), were retrospectively analysed and compared with the CSP results. At the single lesion level, PET scan and CSP showed a sensitivity of 85 and 81%, a specificity of 90 and 87% and an accuracy of 88 and 84%, respectively. PET provided false-negative results for small skin metastases and brain involvement; false-positive results were associated with unrelated benign or malignant tumours and peripheral soft tissue and bone uptake. PET scan showed an additional value over and above CSP at the individual patient's level by true upstaging in 10 cases, true downstaging in 24 cases and depiction of more lesions within the same stage of disease in 15 cases. The overall therapeutic impact reached 26%: 17 out of 71 (24%) cases with regional recurrence, one out of 18 cases (5.5%) with distant metastasis and eight out of 11 cases (73%) with suspicion of recurrence where CSP remained doubtful. However, in 19 cases comparison between CSP and PET resulted in discordant findings, suggesting upstaging in one area and downstaging at another site within the same patient. In conclusion, PET scan has an additional value in the staging of recurrent melanoma, providing it is accompanied by careful clinical examination and specific brain imaging. However, in the absence of evidence of metastasis or unrelated conditions at the same site on CSP, PET spots may represent false-positive images, which would falsely upgrade a patient to an incurable state, or they may be early true-positive findings, which will become evident during close follow-up.

Applications of nuclear medicine in pediatric oncology

Nuclear medicine is important in the diagnosis, staging, and long-term surveillance of a number of pediatric cancers. Skeletal scintigraphy is used to evaluate primary skeletal cancers, such as osteosarcoma and Ewing sarcoma, and nonskeletal cancers such as neuroblastoma, lymphoma, medulloblastoma, rhabdomyosarcoma, and retinoblastoma. Metaiodobenzylguanidine scintigraphy is valuable in examinations of children with neuroblastoma. The therapeutic response of primary bone and brain tumors can be assessed using Tl-201 and Tc-99m MIBI scintigraphy. Imaging strategies for staging and monitoring the therapeutic response of Hodgkin's lymphoma include Ga-67 citrate scintigraphy. Pediatric oncologic applications of positron emission tomography are being investigated extensively.

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.

Similar Pelvic Abnormalities on FDG Positron Emission Tomography of Different Origins

Fluorodeoxyglucose positron emission tomography (FDG PET) has been used extensively to detect and stage various cancers. However, normal variation and inflammatory lesions may lead to false-positive interpretations of PET findings. The authors report three cases of increased pelvic FDG uptake with differing origins. Although the findings are similar, a postpartum uterus, lymphoma, and a bleeding uterus caused pelvic FDG uptake in these patients. Interestingly, of these three patients, the patient with lymphoma had the lowest level of FDG uptake. Clinical correlation is needed for the accurate interpretation of FDG-PET findings.

18F-FDG Uptake in the Anterior Mediastinum. Physiologic Thymic Uptake or Disease?

Purpose: We reviewed 18F-fluorodeoxyglucose (18F-FDG) studies to determine the frequency of thymic visualization, to characterize this uptake to facilitate differentiation from disease, and to ascertain effects of therapy on visualization.Methods: Hybrid positron emission tomography images performed on 14 patients with known or suspected malignancy before therapy, and on six of the patients after treatment, were reviewed. Mediastinal uptake was characterized by location, contour and intensity.Results: Thymic uptake seen in five patients, 13-16-years-old, was characterized by an anterior midline location, regular contours, and a mean uptake ration of 2.5, and involved large glands. In five patients with mediastinal disease, 16-23-years-old, uptake was more posterior and eccentric in location with irregular borders and a mean uptake ratio of 4.3. One patient had anterior (thymic) and posterior (disease) mediastinal uptake. Three patients, 15-22-years-old, without disease and small thymus glands had no uptake. Five patients, including two with baseline uptake, showed no thymic uptake subsequently. One patient, negative initially, had thymic uptake five months after therapy.Conclusions: Thymic 18F-FDG uptake occurs in younger patients, before or after treatment, and is associated with larger glands. Its midline anterior mediastinal location and mild intensity should facilitate discrimination from disease.