Medical Oncologist
Rare Thoracic Tumours
Accurate diagnosis · Molecular characterisation · Individualised treatment planning
Rare thoracic tumours are a heterogeneous group of neoplasms that may arise from different anatomical structures, including the thymus, pleura, mediastinum and mesenchymal tissues. They include, among others, thymomas and thymic carcinomas, pleural mesothelioma and solitary fibrous tumour.
Management differs considerably between these diseases and depends on the precise histological diagnosis, the extent and location of the disease, whether complete surgical resection is feasible and, for some tumours, specific molecular or immunohistochemical features. Because these tumours are rare and the available clinical evidence is limited, multidisciplinary assessment is particularly important when planning the appropriate treatment strategy.
Which Rare Thoracic Tumours Are Included?
Rare thoracic tumours do not represent a single disease. Their anatomical origin and histological identity largely determine both their biological behaviour and the treatment approach.
Thymic Epithelial Tumours
Thymic epithelial tumours mainly include thymoma and thymic carcinoma. They usually arise in the anterior mediastinum and differ substantially in their biological behaviour and prognosis. The feasibility of complete surgical resection is a key factor in treatment planning.
Pleural Mesothelioma
Pleural mesothelioma is a malignancy that usually arises from the pleura, the membrane surrounding the lung. Accurate histological classification and assessment of the extent of disease are essential when choosing between systemic treatment and, in selected cases, a multimodality treatment approach.
Solitary Fibrous Tumour
Solitary fibrous tumour is a rare mesenchymal tumour that most commonly arises from the pleura within the thorax. Its clinical behaviour can vary considerably, from indolent disease to tumours with a risk of local recurrence or metastasis.
Other Rare Thoracic Tumours
Other very rare malignancies may also arise in the thorax, including primary sarcomas, NUT carcinoma and SMARCA4-deficient thoracic tumours. In these cases, accurate pathological and often molecular diagnosis is particularly important for treatment planning.
How Is the Diagnosis Made?
The diagnosis of a rare thoracic tumour is not based on a single test. The first step is to establish the anatomical structure from which the lesion arises—the thymus, pleura, mediastinum or another tissue—and then to select the appropriate method of tissue sampling and staging.
Imaging Guides the Initial Assessment
Contrast-enhanced CT of the chest is usually the main investigation for assessing the tumour’s location and size and its relationship to adjacent organs and vessels. MRI may be required in selected cases, particularly when possible involvement of the mediastinum, chest wall or other anatomical structures needs to be assessed.
PET/CT may provide additional information for certain tumours, but it does not have the same diagnostic value across all rare thoracic neoplasms. The choice of investigation is therefore tailored to the suspected tumour type.
Not All Tumours Require the Same Biopsy Approach
The method used to obtain tissue is particularly important. In a patient whose imaging features are consistent with a resectable thymoma, proceeding directly to surgical resection without a prior biopsy may be preferred in some circumstances.
By contrast, when thymic carcinoma, lymphoma, mesothelioma or another unresectable malignancy is suspected, histological confirmation before treatment is usually required.
In pleural mesothelioma, examination of fluid from a pleural effusion alone is often insufficient for a secure diagnosis. A pleural biopsy, obtained through a percutaneous or thoracoscopic approach, may be required to provide enough tissue for accurate classification.
Pathological Assessment May Substantially Alter the Diagnosis
For several rare tumours, morphology under the microscope is not sufficient. Immunohistochemistry and, in selected cases, molecular testing may be decisive.
In mesothelioma, specific immunohistochemical and molecular markers can help distinguish a malignant mesothelial neoplasm from a reactive process. In solitary fibrous tumour, nuclear STAT6 expression is a characteristic diagnostic finding and is associated with the NAB2–STAT6 rearrangement.
Similarly, exceptionally rare neoplasms may require specific tests, such as assessment of NUT expression or a NUTM1 rearrangement in NUT carcinoma, and assessment of SMARCA4/BRG1 loss in SMARCA4-deficient thoracic tumours.
Accurate Diagnosis Precedes the Treatment Decision
Distinguishing thymoma from thymic carcinoma, mesothelioma from other pleural neoplasms, or one mesenchymal tumour from another can completely change the treatment strategy.
When the diagnosis is rare, unusual or based on limited biopsy material, review of the histology slides and available investigations may therefore be of substantial value before a major treatment decision is made.
What Determines Treatment for Each Rare Thoracic Tumour?
Each of these neoplasms is managed differently. An accurate diagnosis is therefore not simply a name for the tumour; it directly determines the next steps.
Thymoma and Thymic Carcinoma
For thymic tumours, one of the first questions is whether the tumour can be completely removed surgically. In thymoma, surgical resection is a central part of treatment where feasible, while the need for additional treatment depends on the stage, involvement of adjacent structures and surgical margins.
Thymic carcinoma has different and generally more aggressive biological behaviour. Advanced or unresectable disease may require systemic treatment using a strategy that differs from that for thymoma.
Pleural Mesothelioma
In mesothelioma, confirming the histological subtype and accurately assessing the extent of disease are particularly important. For most patients with advanced disease, management is based mainly on systemic treatment, while the role of surgery is much more limited and applies to carefully selected cases within multidisciplinary planning.
The choice of systemic treatment is influenced, among other factors, by the histological subtype, the patient’s general condition and previous treatment.
Solitary Fibrous Tumour
For solitary fibrous tumour, treatment is primarily surgical when complete resection can be achieved. However, these tumours do not all behave in the same way.
Size, mitotic activity, necrosis and other pathological features are used to estimate the risk of recurrence or metastatic progression. Even after complete resection, follow-up may therefore differ substantially from one patient to another.
Extremely Rare Thoracic Tumours
For neoplasms such as NUT carcinoma, SMARCA4-deficient thoracic tumours or primary thoracic sarcomas, the treatment strategy depends to a very large extent on an accurate histological and molecular diagnosis. Access to specialist pathological review and, where available, clinical trials may be particularly important in these cases.
What Happens After Initial Treatment?
Completion of surgery or systemic treatment does not mean that all rare thoracic tumours follow the same surveillance schedule. The likelihood and timing of recurrence vary considerably according to the specific disease.
Thymoma: Long-Term Follow-up May Be Required
Thymomas may recur even several years after apparently complete surgical resection. Follow-up therefore usually continues for a long period and is not limited to the first few years after treatment.
The risk of recurrence is influenced mainly by the stage of the disease and whether complete surgical resection was achieved.
Thymic Carcinoma: Closer Follow-up in the First Few Years
Thymic carcinoma has different biological behaviour from thymoma and a greater likelihood of local or distant recurrence. Follow-up is therefore usually closer, particularly during the first few years after treatment.
Mesothelioma: Assessing More Than Tumour Size
In mesothelioma, response assessment can be more complex than in a conventional lung tumour because the disease often grows diffusely along the pleura rather than forming a single spherical mass.
Follow-up is based on imaging, the patient’s clinical condition and symptoms, using specific methods to assess the extent of pleural disease.
Solitary Fibrous Tumour: Recurrence May Occur Late
After complete surgical resection of a solitary fibrous tumour, the risk of recurrence is not the same for everyone. Factors such as tumour size, mitotic activity, age and the presence of necrosis may be used to estimate risk.
Importantly, some solitary fibrous tumours may recur many years after the initial diagnosis; patients at increased risk may therefore require long-term follow-up.
Frequently Asked Questions
Scientific Sources & Clinical Guidelines
Girard N, Ruffini E, Marx A, et al. Thymic epithelial tumours: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up. Ann Oncol. 2015;26(Suppl 5):v40-v55. doi:10.1093/annonc/mdv277. Popat S, Baas P, Faivre-Finn C, et al. Malignant pleural mesothelioma: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up. Ann Oncol. 2022;33(2):129-142. doi:10.1016/j.annonc.2021.11.005. Gronchi A, Miah AB, Dei Tos AP, et al. Soft tissue and visceral sarcomas: ESMO-EURACAN-GENTURIS Clinical Practice Guidelines for diagnosis, treatment and follow-up. Ann Oncol. 2021;32(11):1348-1365. doi:10.1016/j.annonc.2021.07.006. WHO Classification of Tumours Editorial Board. Thoracic Tumours. WHO Classification of Tumours, 5th ed. IARC; 2021. This source is particularly useful for the classification of rare thoracic tumours, including NUT carcinoma, SMARCA4-deficient thoracic tumours and solitary fibrous tumour.