Challenges in Diagnosing Isolated Rib Metastasis From Low‐Grade Endometrial Carcinoma: Utility of USG‐FNAC and Molecular Characterisation

Bone metastasis from endometrial carcinoma represents a rare phenomenon, affecting fewer than 1.0% of cases. Although common metastatic locations include the pelvis and lungs, solitary rib metastasis remains exceptionally uncommon, especially in low and intermediate-grade EC. We describe an unusual case of mismatch repair-deficient endometrioid endometrial carcinoma manifesting as an isolated osteolytic rib lesion. A 50-year-old woman had undergone complete abdominal hysterectomy for FIGO Grade 2, stage IIA endometrioid endometrial carcinoma exhibiting squamous differentiation and a MELF invasion pattern. Six months following surgery, PET scanning revealed a solitary osteolytic lesion in the left fifth rib. Given the lesion's cardiac proximity, USG-FNAC was utilised for expedited diagnostic confirmation. Cytomorphological examination demonstrated a highly cellular specimen with papillary structures, acinar formations, and scattered keratinocytes without atypia. Retrospective molecular testing identified MMR-deficiency (MLH1/PMS2 loss), wild-type POLE, and wild-type p53. The disease was reclassified as 2023 FIGO Stage IVC, showing a favourable response to chemotherapy with consideration for immunotherapy. This case demonstrates the diagnostic value of USG-FNAC as a reliable and safe method for confirming metastasis in anatomically challenging locations. Additionally, it emphasises the importance of incorporating molecular profiling and recognising histological features such as squamous differentiation and MELF patterns as markers of aggressive behaviour in a subset of low-to-intermediate grade EC.