Metastasis without malignancy! the diagnostic challenges of fluid cytology from a rare case of Benign metastasizing leiomyoma

Metastatic mesenchymal neoplasms presenting as pleural effusions are exceptionally rare, accounting for under 1% of malignant effusions, and pose severe diagnostic challenges in fluid cytology due to overlapping cellular morphology and deceptively bland cytological features. Benign metastasizing leiomyoma (BML) is an elusive clinicopathological entity characterized by histologically benign smooth muscle tumors metastasizing to extrauterine sites, predominantly the lungs, typically occurring in women with a prior history of uterine leiomyomas. The primary identification of BML via pleural fluid cytology remains practically undocumented. We report a 29-year-old female presenting with progressive dyspnoea, dry cough, pleuritic chest pain, bilateral pulmonary nodules, and a massive right pleural effusion one year post-hysterectomy for a uterine fibroid. Thoracocentesis revealed cohesive spindle cell whorls and 'cannonball-like' aggregates with minimal atypia. Masson's trichrome stain confirmed smooth muscle differentiation. Subsequent histopathological review of the primary uterine tumor revealed infiltrative growth, reclassifying it as a smooth muscle tumor of uncertain malignant potential (STUMP). The patient rapidly deteriorated from systemic disease progression. This case highlights that cytological blandness in fluid specimens can mask an aggressive tumor trajectory, necessitating thorough multi-block primary sampling and tight clinico-radiological correlation.