Lymphocytic Interstitial Pneumonia Presenting as an Enlarging Pulmonary Nodule in a 65-Year-Old Woman: A Case Report.

Lymphocytic interstitial pneumonia (LIP) is a rare, benign lymphoid interstitial lung disease that frequently masquerades as malignancy on imaging and even frozen‑section histology. We report the case of a 65‑year‑old Vietnamese woman who presented with chronic right‑sided chest pain and a nonproductive cough. Baseline high‑resolution computed tomography (HRCT) revealed a 28 × 20 mm irregular opacity in the right upper lobe with scattered bilateral micronodules, all of which proved refractory to empirical antibiotics. Ten months later, the dominant lesion had enlarged to 36 mm, prompting video‑assisted right upper lobectomy for definitive diagnosis. Histopathology demonstrated diffuse lymphoplasmacytic infiltration of the alveolar septa with prominent lymphoid follicles. Immunohistochemistry showed CK7‑ and TTF‑1‑positive epithelial elements surrounded by CD20‑positive B‑cell follicles and scattered CD3‑positive T‑cells, with a Ki‑67 index < 5%. Although light chain restriction analysis was not performed, the histologic and immunophenotypic features supported a diagnosis of LIP. The patient recovered uneventfully and, at 12‑month follow‑up, remains asymptomatic without radiologic progression. This case underscores the importance of integrating advanced imaging with comprehensive immunopathology to distinguish LIP from carcinoma or lymphoma, thereby preventing unnecessary aggressive treatment, and it adds to the limited Asian literature on this uncommon entity.