Case Report: The "atoll sign": a case series on an unusual radiological pattern of immune-mediated pneumonitis.

Introduction: Immune checkpoint inhibitors (ICIs) have transformed the therapeutic landscape of advanced non-small cell lung cancer (NSCLC), offering durable survival in selected patients. However, ICIs can induce immune-related adverse events (irAEs), among which pneumonitis represents one of the most clinically significant due to its potential severity and diagnostic challenges. Different radiological patterns have been reported, although they may overlap with infectious or radiation-induced lung injury, complicating a timely diagnosis. Results: We report two cases of young patients with metastatic NSCLC and high PD-L1 expression who achieved long-term benefit from first-line pembrolizumab but were burdened by severe immune-related pneumonitis. Imaging findings in both patients were suggestive of organizing pneumonia associated with the presence of the so-called "atoll sign" (or reversed halo sign), a radiological feature rarely reported in the context of ICI-related pneumonitis. In both cases, the diagnosis was supported by imaging, microbiological tests, and histology. The subsequent administration of high-dose steroids led to rapid relief of symptoms associated with radiological improvement. Despite pembrolizumab discontinuation, both patients maintained durable systemic disease control. Conclusion: These cases emphasize the importance of recognizing uncommon radiological patterns such as the atoll sign in patients receiving ICIs, highlighting the need for vigilance even after prolonged therapy and underscoring the potential for sustained oncologic benefit despite treatment discontinuation.