What is a rare tumour and how should it be dealt with clinically?

Abstract

The most common lung tumour is lung carcinoma, with adenocarcinoma, squamous cell carcinoma, large cell carcinoma and small cell carcinoma representing almost 99% of cases [1]. Excluding metastatic lesions in the lung, other histopathological categories account for a total of v1% of lung tumours, but these represent a wide range of diverse tumoral processes, with possible specific clinical and radiological imaging features permitting the clinician to suspect a rare tumour [2]. The role of the pathologist is obviously first to establish the diagnosis, and secondly to allow the clinician to determine and orientate the treatment and prognosis. Although diagnosis may be made on the basis of routinely stained sections, immunohistochemistry is often necessary to define the precise nature of the neoplasm and to evaluate its biological aggressiveness. More sophisticated techniques (flow cytometry and molecular and cytogenetic analysis) have a limited role in diagnosis, but may play a role in predicting clinical behaviour. Since rare pulmonary tumours are usually limited to the lung at the time of diagnosis, and frequently consist of benign or low-grade malignant lesions, the therapeutic strategy often consists of upfront surgical resection when the tumour is limited to the lung, ensuring both diagnosis and the first step of the treatment [3‐7]. Thus curability and prognosis may often be better than for nonsmall cell lung cancer (NSCLC) [2, 4]. As rare pulmonary tumours have been mostly described as single case report sa nd small series [3, 4], an exhaustive review of the literature was undertaken in order to obtain a rigorous overview of rare pulmonary tumours (table 1). As an exhaustive review of all types of rare lung tumour is outside the scope of the present chapter, five rare pulmonary tumours of particular interest to both the pathologist and the clinician, further illustrating the close cooperation between both specialists, were selected: myofibroblastic tumour, sarcomas, epithelioid haemangioendothelioma (EHE), mucoepidermoid carcinoma, and pulmonary lymphoma, especially mucosa-associated lymphoid tissue (MALT) lymphoma.