Thymoma is the most common primary neoplasm of the anterior mediastinum. At diagnosis, up to 40% of patients present with advanced disease. Because advanced thymomas receive neoadjuvant chemotherapy, diagnostic imaging is crucial to plan the correct treatment. For characterizing thymomas, CT is the first choice modality, whereas 18F-FDG/PET is reserved for questionable cases and MRI is not routinely employed. Hereby, we describe a case of thymoma with a single contralateral pleural implant in a 30-year-old woman. The small pleural thickening detected at CT was correctly interpreted as pleural seeding related to thymoma at diffusion-weighted (DW)-MRI after a negative 18F-FDG/PET scan, and was subsequently confirmed at surgery. Precise diagnosis and accurate preoperative staging are crucial in managing thymic epithelial tumours in order to design the appropriate treatment and improve prognosis. Indeed, when stage IVa for pleural seeding is diagnosed preoperatively, a multimodality approach including primary chemotherapy followed by surgery and postoperative radiotherapy/chemotherapy is recommended. This is the first report that used DW-MRI for the characterization of pleural seeding in thymoma and demonstrates that DW-MRI could be useful for the correct pre-operatory staging in thymoma patients, especially in cases with indeterminate pleural thickenings at CT, in order to define the correct management.
Tumori 2015; 101(1): e13 - e17
Article Type: CASE REPORT
AuthorsAdriano Massimiliano Priola, Sandro Massimo Priola
- • Accepted on 16/04/2014
- • Available online on 24/01/2015
- • Published online on 20/03/2015
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Thymoma is the most common primary neoplasm of the anterior mediastinum. It arises from the thymic epithelium and represents 20% of all mediastinal tumors in adults (1-2-3-4). Although usually asymptomatic, 15% of patients present with myasthenia gravis (5, 6). At diagnosis, approximately 15%-40% of patients have advanced disease (Masaoka-Koga stage III-IV) for invasion of adjacent structures, pleural/pericardial seeding, and metastatic disease (5-6-7). The preoperative distinction between non-advanced and advanced thymomas is essential to plan the proper treatment, because patients with advanced disease receive neoadjuvant chemotherapy before surgery (8). Hence, diagnostic imaging has a crucial role in the assessment of patients with thymoma. Computed tomography (CT) is the first-choice modality in the identification, staging, and follow-up of patients with thymoma, although it has limited value in differentiating among the various histological subtypes and in detecting residual or recurrent disease after treatment (9-10-11-12-13). In addition, CT has limitations in the characterization of indeterminate tiny pleural thickenings, especially when ispilateral to the primary mediastinal tumour, since it may be difficult to report these findings to pleural implants or non-calcified benign plaques (6, 14). 2-[18F]-fluoro-2-deoxy-D-glucose (18F-FDG)/positron emission tomography (PET) is very helpful for the characterization of thymomas, particularly in suggesting the aggressiveness of thymoma, in detecting metastatic lesions, and in evaluating residual or recurrent disease (15). Currently, magnetic resonance imaging (MRI) is not routinely employed in the assessment of patients with thymoma and is generally reserved for further evaluation of equivocal CT findings of local invasion because of its better contrast resolution, or in case with suspected vascular invasion in which iodine contrast cannot be administered (6, 16-17-18-19). Nevertheless, newer MRI techniques like diffusion-weighted (DW) imaging could help in differentiating non-advanced from advanced thymomas in order to plan the correct management in a multimodality approach (4, 6, 8). Hereby, we present a case of advanced thymoma in which the single small pleural implant, contralateral to the primary tumour, was detected at DW-MRI after a negative 18F-FDG/PET.
A 30-year-old woman with recent onset of myasthenia gravis underwent CT. Chest CT (64-row MDCT; section-thickness, 1.5 mm) revealed a left anterior mediastinal mass with soft tissue attenuation, that measured 65 mm in maximal diameter, resembling a thymoma (
A-B) Unenhanced and contrast-enhanced chest CT scan obtained at the level of the aortic arch (soft tissue window, axial plane) demonstrates an unilateral oval-shaped soft-tissue mass (thin arrows) that arises in the left lobe of the thymus, anterior to the aorta and superior vena cava. The mass exhibits smooth and well-circumscribed borders, slightly heterogeneous attenuation on unenhanced scan, and noticeable diffuse heterogeneous enhancement with hypodense areas consistent with necrotic areas within the lesion (thick arrow). C) Contrast-enhanced CT scan (mediastinal window, axial plane) shows a small nodular pleural thickening of the diaphragm in the anterior recess of the right lung, contralateral to the primary tumour.
Maximum intensity projection 18F-FDG PET images (coronal views; upper images) demonstrate a large area of increased 18F-FDG in the mediastinum on the left side, superior to the heart (arrows), corresponding to the anterior mediastinal lesion detected at CT. No other area of abnormal uptake was seen, especially in the location of the small nodular pleural thickening of the right diaphragm appreciable on CT (arrows; axial views; lower images). The small area of increased 18F-FDG uptake (dotted arrow; left axial image) represents an upper calyx of the left kidney.
A) MR images (axial plane) confirm the large unilateral soft-tissue mass of the anterior mediastinum (thin arrows), adjacent to the aortic arch, that shows intermediate signal intensity on T1-weighted images (left image), similar to that of muscle, and a high signal intensity on T2-weighted images that remains unchanged on fat suppressed T2-weighted sequence (right image). T2-weighted image clearly detects a high signal intensity region which corresponds to cystic area (thick arrow), that is not clearly detectable on T1-weighted images (T1-W = T1-weighted; T2-W FS = fat suppressed T2-weighted). B) Diffusion-weighted MRI (axial plane) at the level of the mediastinal lesion was obtained at different b values of 0, 150, 500, and 800 s/mm2, respectively. The transverse image obtained at a b value of 0 s/mm2 demonstrates a high signal intensity within the lesion (arrow). At higher b values, the lesion presents a progressive slight attenuation of the signal intensity (arrows) that is not completely suppressed, even at the b800 gradient image. This appearance suggests a restricted diffusion within the tissue. C) At ADC-MAP, the lesion exhibits heterogeneous low signal intensity with a low mean ADC value of 0.56 ± 0.15 x 10-3mm2s-1, consistent with the hypothesis of a soft tissue lesion with high cellularity, suspicious for a thymic epithelial tumour.
A) Diffusion-weighted chest MRI (axial plane) at the level of the anterior recess of the right lung in the location of the nodular thickening of the right diaphragm was obtained at different b values of 0, 150, 500, and 800 s/mm2. The transverse image obtained at a b value of 0 s/mm2, that results in conventional anatomic T2-weighted images with no vessels signal attenuation (arrowheads), demonstrates a high signal intensity lesion in the anterior recess of the right lung, close to the anterior margin of the liver (arrow), corresponding to the site of the nodular pleural thickening on CT. At higher b values, where vessels signal attenuation is seen (arrowheads), the lesion presents similar signal intensity (arrows); especially, the b800 gradient image shows a high signal intensity that means restricted diffusion of the nodule and suggests the diagnosis of a pleural implant. B) At ADC-MAP, the low signal of the nodule and the low mean apparent diffusion coefficient value of 0.52 ± 0.11 x 10-3mm2s-1, similar to that of the primary tumor, demonstrates restricted diffusion within the tissue and strengthens the hypothesis of metastatic pleural lesion related to thymoma.
CT is currently considered the cross-sectional imaging modality of choice in the identification, staging, as well as in the follow-up of patients with thymic epithelial tumors (1-6, 20, 21). However, CT has limitations in differentiating among the various histological subtypes of thymomas and in defining locally advanced disease (stage III and IVa), especially for stage IVa when pleural or pericardial implants are represented by a very small amount of tissue, as in our case (9-10-11-12-13). In addition, CT does not allow to distinguish between pleural implants related to thymoma (that may appear as discrete soft tissue nodules or as circumferential diffuse pleural thickening) and non-calcified benign pleural plaques (1-2-3-4-5-6). 18F-FDG/PET has been reported as an accurate tool in preoperative staging of thymic epithelial tumours, although it has limits in detecting very small lesions and imparts a substantial dose of radiation (15). In our case, 18F-FDG/PET did not demonstrate an increased uptake of the contralateral small pleural implant, maybe due to the limited size of the pleural nodule. In addition, considering the negative finding obtained at 18F-FDG/PET, and since pleural seeding typically appears ipsilateral to the primary mediastinal tumour, the contralateral pleural thickening was wrongly interpreted as a benign pleural thickening (4-5-6).
In the last years, DW-MRI has come as a novel radiologic modality for assessment of disease extent in various malignancies and is already being incorporated into general oncologic imaging practice for various tumours (22-23-24-25-26). DW-MRI, that is sensitive to thermally driven molecular water motion (which in vivo is impeded by cellular packing, intracellular elements, membranes and macromolecules), is able to characterize malignant tumours and metastases that show restricted diffusion by appearing as high signal intensity lesions even at high b values, with a low ADC (