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Erlotinib is a first-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) approved in the first-line treatment of advanced non-small-cell lung cancer (NSCLC) patients with sensitizing epidermal growth factor receptor (EGFR) mutations. The response rate to erlotinib is ~60% and the incidence of erlotinib-induced interstitial lung disease (ILD) is ~1-4%. The Response Evaluation Criteria in Solid Tumours (RECIST) tool is commonly used to assess response to erlotinib; however, evaluation of response and subsequent progression in the presence of atypical cystic lung changes may be challenging. We herein present a rare case of diffuse cystic lung changes secondary to erlotinib treatment in a patient with EGFR mutation-positive metastatic NSCLC. Challenges in assessing atypical tumour response to erlotinib, pitfalls in using RECIST and differential diagnosis of TKI-related ILD are discussed in detail.