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Primary pleural thymoma mimicking signs of another tumor

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A team of Italian researchers recently reported an unusual case of the tumor of the thymus, manifesting test results that are similar to another tumor, thus underscoring the imperative need for a better awareness of clinical results among physicians in order to avoid misdiagnosing their cancer patients. 

The thymus gland, located in the anterior mediastinum (under the breastbone in the chest cavity) is an organ that produces infection-fighting lymphocytes – subsequently called T-cells – and it is most active during childhood and then atrophies in adulthood. The cancer of the thymus, called thymoma, is an uncommon form of neoplasm; according to the American Society of Clinical Oncology, less than one person per 1.5 million people develops thymoma. Moreover, “[t]he occurrence of thymoma outside the mediastinum is a rare event,” described a case report published in this issue of AMOR by Roberto Monaco, Antonietta D’Alterio, Raffaella Lucci, and Oscar Nappi from the pathology departments of Cardarelli Hospital and University of Naples Federico II in Naples, Italy. While thymomas occurring at unusual sites such as the neck, lung, and thyroid have been occasionally observed, existing literature rarely reports thymomas affecting the pleural, i.e. the thin and delicate layers of membranes that wrap, cushion, and protect the lungs. 

In the case of their patient, the authors described, “Diagnostic imaging showed that the pleural tumor encased the entire left lung.” The specimen biopsied from the tumor showed signs that were consistent with thymoma. However, the surgical exploration of the anterosuperior mediastinum found no evidence of a thymic tumor, which led Monaco and his fellow researchers to believe that the thymoma may have originated from an abnormal thymic tissue in the pleura, and then likely developed as a separate lesion that spreads along the pleural in ways similar to a diffuse mesothelioma afflicting the linings of the lungs. “Owing to their peculiar location and a variety of manifested histologic patterns, pleural thymomas may be confused with other neoplasms and may cause diagnostic problems clinically, radiologically, and morphologically,” the researchers explained. 

In their case report, Monaco and his co-authors highlighted several instances of lesions that need to be examined more closely by physicians during differential diagnosis in order to distinguish primary pleural thymoma from other neoplasms, especially malignant mesotheliomas, as in their case study. In rare instances, according to the authors, malignant mesotheliomas may have a significant infiltration of the lymphoid, thus giving rise to diagnostic mistakes. In addition, they pointed out that close similarities may exist between thymic tumors and malignant mesothelioma in small biopsies, thus making analysis problematic. “An awareness of these interpretative pitfalls is important to prevent misdiagnosis of malignant pleural mesothelioma,” they cautioned. 

“Because of their ectopic site and variety of histologic patterns, as well as the occurrence of an ‘invasive’ border, diagnosis of primary pleural thymomas via pleural biopsy can be very difficult, especially if the pathologist is not aware of this entity or if the typical features of thymoma are lacking,” the researchers said. Owing to the possibility of thymomas mimicking both radiological and microscopical lesions, Monaco and fellow authors stressed upon the consideration of a primary pleural thymoma in the diagnostic approach concerning pleural pathology. Their diagnosis of pleural thymoma in their patient, outlined in their case report, was established based on careful examination of histological features, combined with radiographic, and surgical findings — which they argued “should be considered essential for excluding any mediastinal involvement.” Hence, “[a] combination of clinical information, histopathologic appearance of the tumor, and immunohistochemical studies will often help to distinguish primary pleural thymoma from other neoplasms,” they concluded.

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