From the Guidelines
The workup for residual thymus should typically involve imaging studies, particularly MRI of the chest, to evaluate the size, location, and characteristics of the thymic tissue, as MRI remains superior to CT for detection of invasion of the mass across tissue planes and involvement of neurovascular structures, secondary to its higher soft tissue contrast 1. The initial evaluation should include a thorough history and physical examination to assess for symptoms like chest pain, cough, or signs of myasthenia gravis.
- Laboratory tests may include complete blood count, basic metabolic panel, and specific antibody tests such as anti-acetylcholine receptor antibodies if myasthenia gravis is suspected.
- PET scans may be used to differentiate between normal thymic tissue and potentially malignant lesions based on metabolic activity, although a negative FDG-PET/CT has been shown to be helpful in excluding malignancy, while a positive FDG-PET/CT has little value for discrimination between benign and malignant lesions 1. In asymptomatic patients with incidentally discovered residual thymus, observation with serial imaging every 6-12 months is often recommended to monitor for any changes in size or characteristics.
- Biopsy may be necessary if there are concerning features on imaging or if the patient is symptomatic. This approach is based on the understanding that while the thymus normally involutes with age, residual thymic tissue can persist in adults and is often benign, though it requires evaluation to rule out thymoma or other pathologies, and MRI can distinguish normal and hyperplastic thymus from thymic tumors and lymphoma, whether by chemical-shift MRI in adults or by DWI with apparent diffusion coefficient (ADC) mapping 1. The use of MRI is preferred over CT due to its higher soft tissue contrast and ability to detect invasion of the mass across tissue planes, including the chest wall and diaphragm, and involvement of neurovascular structures 1.
From the Research
Imaging Modalities for Residual Thymus
- Computed Tomography (CT) is the imaging modality of choice for detecting thymic abnormalities, including residual thymus 2
- Magnetic Resonance Imaging (MRI) can be useful in equivocal cases, especially in differentiating focal thymic lymphoid hyperplasia from thymoma using chemical-shift sequences 2, 3
- Diffusion-weighted (DW)-MRI can differentiate lipid-poor normal/hyperplastic thymus from thymoma and may be useful in differentiating non-advanced from advanced thymomas 2
- Positron Emission Tomography (PET)-CT is not helpful in distinguishing early from advanced thymoma but can be used to differentiate thymic carcinoma from thymoma 2
Surgical Options for Residual Thymus
- Robotic-extended rethymectomy may be considered as a safe and beneficial treatment option for patients with refractory myasthenia gravis and suspected residual thymic tissue 4
- Thymectomy can lead to clinical improvement in patients with lymphoid hyperplasia or thymoma, but the presence of residual thymus can lead to recurrence of myasthenia gravis 5, 6
Clinical Outcomes and Residual Thymus
- The presence of residual thymus can lead to recurrence of myasthenia gravis and poor clinical outcomes 5
- Clinical improvement following thymectomy is more likely in patients with lymphoid hyperplasia or thymoma than in those with normal histologic examination 6
- The use of imaging modalities such as CT and MRI can help identify patients with residual thymus and guide surgical management 2, 3, 6