Benign Category in the WHO Reporting System for Lung Cytopathology: Diagnostic Difficulties and Differential Diagnosis

There are numerous benign cellular changes that occur in the lung due to diverse underlying diseases that can be encountered in exfoliative samples and transbronchial and CT-guided samples. Entities such as bronchial cell hyperplasia, pneumocyte type II proliferation, repair, etc. should be recognised and diagnosed. Yet, many pathologists encounter difficulties in their interpretation and frequently either diagnose them as ‘Atypical’ resulting in more invasive procedures or misdiagnose them as ‘Malignant’ with drastic consequences to the patient. Other entities may be dismissed as benign lung elements, resulting in misclassification as non-diagnostic/non-representative, resulting in repeated or more invasive procedures. In this review, we will present an overview of many of these diagnostic pitfalls and mimickers and describe clues to their diagnosis and how to differentiate them from malignant conditions. As in all cytologic diagnoses, both false positive and false negative errors can occur and both types of errors can have grave consequences to patient management, resulting in increased unnecessary morbidity such as pneumonectomy and, at times, litigation.