Thymic Hyperplasia and Thymus Gland Tumors

Thymic Hyperplasia and Thymus Gland Tumors: Differentiation with Chemical Shift MR Imaging Tsutomu Inaoka, Koji Takahashi, etc Department of Radiology, Asahikawa Medical College, Asahikawa, Japan. Radiology 2007 243: 869-876 Introduction • Difficult to make specific diagnosis of thymus abnormalities – Overlapping imaging features – Age-related and reactive alterations of thymus • Minimally invasive surgical techniques – Risk of local recurrence • Chemical shift MR imaging – Useful for identifying normal and hyperplastic thymus • Normal fat infiltration within the tissues – No study assessing systematically various thymic lesions Purpose • To prospectively evaluate chemical shift MR imaging in the differentiation of thymic hyperplasia and tumors of the thymus gland Materials and Methods - Patients • 46 patients : thymic abnormalities at CT  prospectively, MR – Patients ≤ 15 years, thymic cyst : excluded – 41 patients (17 male, 24 female, Mean 47.1 yrs, 16-78) • 2 groups – Hyperplasia group (n=23) • 18 hyperplastic thymus a/w Graves dz, 5 rebound thymic hyperplasia, 41.1 yrs (p <.001) • Rebound : breast ca (2), soft-tissue sa (2), ovarian ca (1) – Tumor group (n=18) • 7 thymoma, 4 invasive thymoma, 5 thymic cancer, 2 malignant lymphoma, 54.7 yrs • Myasthenia gravis : 2 thymoma, 2 invasive thymoma Materials and Methods – Proof of Diagnosis • Hyperplasia group (n=23) – Pathologic proof : 1 rebound – Clinical and CT findings : 4 rebound, 18 hyperplasia with Graves dz • Tumor group (n=18) – Core biopsy (n=3), surgical excision (n=15) Materials and Methods – MR imaging • 1.5 T • Transverse T2W FSE, T1W GE in-phase and opposed-phase • Respiratory gating, not cardiac gating • 2 radiologists • Independently, blinded to patient’s identification & clinical information • Final decisions by consensus Materials and Methods – MR Image Analysis • Shape - diffuse enlargement (biconvex margin) with or without lobulation, round, or irregular • Size – maximum thickness on transverse images • Signal intensity – compared with that of paraspinal muscle • Heterogeneity – homogenous, heterogenous (dominant SI) • Decrease in SI on opposed-phase relative to in-phase image • Chemical shift ratio (CSR) – (tSI op/mSI op)/(tSI in/mSI in) – comparing SI of thymus gl (tSI) with that of paraspinal muscle (mSI) on both in-phase (in) and opposed-phase (op) images Results - MR Image Assessment Results - MR Image Assessment • Maximum thickness of thymus gland – Hyperplasia • 14 ~ 26 mm (mean, 17.2) – Tumor group • Round (n=15), irregular (n=1) : 18 ~ 50 mm (mean, 32.6) • Diffuse enlargement (n=2) : 14 mm, 22 mm Hyperplastic thymus in a 28-year-old woman with Graves disease T2W-FSE In-phase GE T1W Opposed-phase CSR 0.670 Thymoma in a 48-year-old woman with myasthenia gravis. T2W-FSE In-phase GE T1W Opposed-phase CSR 1.080 Results – CSR Calculation •Statistically significant differences between the hyperplasia and tumor groups (P<.001) •No overlap in range Rebound thymic hyperplasia in a 41-year-old woman with left breast cancer (after postoperative radiation therapy). T2W-FSE In-phase GE T1W Opposed-phase CSR 0.613 Malignant lymphoma of the thymus gland in a 49-year-old woman. T2W-FSE In-phase GE T1W Opposed-phase CSR 0.989 Discussion – Chemical shift MR imaging • Much more sensitive than other fat-suppressed MR sequences for detecting microscopic fat within tissue – Unique difference in resonance frequency between protons in water and those in triglyceride molecules – Loss of SI on opposed-phase relative to in-phase image • Differentiation of hyperplasia from thymoma – Cf. morphologic assessment, esp. myasthenia gravis • Differentiation of rebound thymic hyperplasia from lymphoma recurrence – Cf. increase in thymus size after chemoTx a/o radiation Tx Discussion • Thallium 201 scintigraphy – Low spatial resolution, high cost, low throughput, irradiation, considerable overlap in diagnosis criteria • PET with 18-FDG – Normal thymus, hyperplastic thymus, rebound thymic hyperplasia, thymoma, thymic cancer, metastasis – Considerable overlap in ranges of standard uptake values Discussion - Limitation • No pathologic proof of the diagnosis for most patients in hyperplasia group • Small sample size – Malignant lymphoma (n=2) – No evaluation of thymic lymphoma after treatment • Only 4 myasthenia gravis • ROI may have microscopically included the boundary or fat of paraspinal muscles Conclusion • Signal intensity of thymus gland at chemical shift MR imaging – Hyperplasia group : apparent decrease – Tumor group : no decrease • Mean CSR – Hyperplasia group considerably lower than that in tumor group, no overlap in range • Chemical shift MR imaging may help differentiate thymic hyperplasia from tumors – In particular, thymus : homogenous soft-tissue structure at CT/MR