An otherwise healthy man in his 40s presented with fever, cough and abnormal chest imaging. Initial CT demonstrated bilateral peripheral ground-glass opacities (figure 1A). Bronchoalveolar lavage fluid remained bloody, raising concern for diffuse alveolar haemorrhage. Steroid pulse therapy produced transient radiologic improvement; however, new pulmonary nodules emerged, rapidly enlarging (figure 1B).

Given the progressive, atypical radiologic evolution, a thoracoscopic lung biopsy was performed. Thoracoscopy revealed a striking appearance on the lung surface, which we termed the ‘garnet mosaic sign’, characterised by sharply demarcated, dark-red discolouration of secondary pulmonary lobules (figure 2A). This lobule-based pattern suggested a vascular process rather than an inflammatory or infectious disease.

Histopathological examination of the resected specimen showed proliferation of spindle-shaped tumour cells with nuclear atypia and mitotic activity (figure 2B). Immunohistochemistry demonstrated strong membranous CD31 positivity, confirming endothelial differentiation and supporting the diagnosis of pulmonary angiosarcoma (figure…