A 57-year-old female nonsmoker presented with cough and left-sided chest pain.
She underwent a chest radiograph and CT scan. Chest CT suggested several possible differential diagnostic considerations, including invasive thymoma, thymic carcinoma, thymolipoma, germ cell tumor and metastatic disease.
She had a remote history of left breast cancer and had undergone left total mastectomy, chemotherapy, radiation therapy, and reconstruction with breast implant in 1995. The CT scan also revealed a small left pleural effusion, the cytology sampling of which was negative for malignancy.
FDG PET/CT study revealed a 13-cm by 9-cm left anterior mediastinal mass demonstrating intense circumferential FDG activity, with maximum standard uptake value (SUVmax) of 13.4, and central low FDG activity probably related to necrosis.
FDG PET/CT also revealed the rightward shift of the mediastinal structures secondary to displacement by the mass. There was compression of the left upper lobe bronchi, as well as the left main pulmonary artery. Small left pleural effusion was unchanged. Stable, small pericardial effusion was seen, as well. The left breast implant was in place. There was no abnormal FDG uptake in neck, abdomen, and visualized axial and proximal appendicular skeleton.
The patient underwent median sternotomy, left thoracotomy and resection of the mediastinal tumor. Intraoperatively, the tumor was outside of the atrophied thymus.
The pathology revealed a necrotic sarcomatoid carcinoma involving the lung and mediastinal soft tissues and molecular study confirmed the thymic origin.
The common differential diagnostic considerations for anterior mediastinal masses include — but are not limited to — lymphoma, thyroid carcinoma, germ cell tumor and thymoma.
FDG PET/CT, depending on the histology of the tumor, demonstrates a variable degree of FDG uptake in these masses. High FDG avidity in the primary index lesion is very useful for detection of regional nodal and distant metastases with high sensitivity.
Tissue sampling of these masses is essential for accurate diagnosis and for further management. However, FDG PET/CT — if performed prior to tissue sampling — may help guide an optimal location for tissue sampling by identifying the site of high degree of focal FDG uptake and by avoiding necrotic regions. This may be very useful in the assessment of tumor aggressiveness, chemotherapy response and identification of viable tumor in residual masses upon treatment with chemotherapy.
Although the differentiation between various thymic neoplasms is difficult based on imaging alone, the degree of FDG uptake can help differentiate between low- and high-grade thymic tumors. For example, SUVmax of low-risk thymoma — like types A, AB and B1 — were significantly lower than those of high-risk thymic tumors, such as B2 and B3 thymomas or thymic carcinomas.
Sharma and colleagues studied the role of 18F-FDG PET/CT in thymic carcinoma and reported that SUVmax of more than or equal to 7 in the presence of lobulated masses with areas of necrosis and calcification, encasement of mediastinal structures, pleural thickening, mediastinal lymphadenopathy and distant metastases was suggestive of thymic carcinoma. Prior research also reported a significant relationship between SUVmax and WHO classification and Masaoka stage.
Luzzi and colleagues suggested that any anterior mediastinal mass with SUVmax greater than 5 “not be resected without biopsy because of the possibility that it may be more aggressive than thymoma or even a nonsurgical disease, such as lymphoma.”
In this patient, a large soft tissue mass with variable degree of FDG uptake but with SUVmax in the most intense region of 13.4 correlated with the final diagnosis of sarcomatoid carcinoma.
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For more information:
Munir Ghesani, MD, FACNM, is assistant professor of radiology and director of PET/CT fellowship at NYU Langone Medical Center. He also serves as a HemOnc Today Editorial Board member. He can be reached at firstname.lastname@example.org.
Ajit Karakbelkar, MD, is a PET/CT fellow at NYU Langone Medical Center in New York.
Disclosure: Ghesani and Karakbelkar report no relevant financial disclosures.