FDG uptake of thymus measured by PET/CT is an effective indicator for the differentiation of mediastinal lymphoma from normal thymus in pediatric patients, according to a study published online Dec. 12 in Radiology.
Heike E. Daldrup-Link, MD, of Stanford University School of Medicine in Stanford, Calif., and colleagues wanted to evaluate the use of PET/CT in diagnosing malignant lymphomas, which are the most common pediatric malignant masses of the anterior and middle mediastinum. More than two-thirds of pediatric patients with Hodgkin lymphoma and half of pediatric patients with lymphoblastic non-Hodgkin lymphoma present with a mediastinal mass, according to the study’s background information.
The researchers retrospectively reviewed 282 FDG-PET/CT studies in 75 pediatric oncology patients, who were divided into four groups: 16 with anterior mediastinal lymphoma (group a), five with anterior mediastinal lymphoma with subsequent recurrence (group b), 16 with lymphoma outside the mediastinum (group c) and 38 with other malignant tumors outside the thymus (group d). The analysis included the measurement of maximum standardized uptake values of anterior mediastinal mass, thymus (SUVt), on PET images.
Results of the study showed a mean prechemotherapy SUVt of 4.82 for group a, 8.45 for group b, 2.00 for group c and 2.09 for group d. Postchemotherapy SUVt for group b was 4.74 and thymic rebound was seen in 44 percent of patients at an average 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,” wrote the authors. “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 enlargement can be difficult to discern from tumor recurrence, though various imaging criteria have been used in an attempt to characterize thymic rebound. Despite the existing strategies, the researchers noted that additional criteria were warranted due to the lack of specificity for current criteria.
The authors added that other imaging modalities may still be helpful in distinguishing thymic rebound from recurrent disease, pointing to additional research that showed chemical shifts seen on MRI can help in demonstrating the presence of fat in normal thymic tissue.