B80-5-26 Low Grade Foetal Adenocarcinoma of the Lung in an Adult Woman: A Rare Variant of Pulmonary Adenocarcinoma

Abstract Introduction Low-grade foetal adenocarcinoma (L-FLAC) of the lung is an exceptionally rare neoplasm, accounting for 0.1-0.5% of pulmonary adenocarcinomas. It is recognized by the World Health Organization as a distinct subtype with characteristic histological and molecular features. Recognition is essential, as L-FLAC typically affects younger, non-smoking women and carries a favourable prognosis with complete surgical resection. Case Report A 44-year-old woman with no past medical history presented with a six-month history of non-productive cough and no systemic symptoms. She was a never-smoker, and physical examination was unremarkable. Computed Tomography (CT) of the thorax demonstrated a large left lower lobe mass obstructing the origin of the left lower lobe bronchus. Fluorodeoxyglucose (FDG) positron emission tomography (PET) showed a lobulated, intensely avid mass extending to the left hilum without nodal or distant metastasis. Bronchoscopy revealed complete occlusion of the left lower lobe bronchus, but biopsies were non-diagnostic. A CT-guided biopsy demonstrated adenocarcinoma with enteric differentiation and thyroid transcription factor-1 (TTF-1) positivity. Testing for ALK and ROS1 rearrangements was negative, and staging was pT4N0M0. The patient underwent a left pneumonectomy. Histology showed adenocarcinoma with cribriform, papillary, and solid architecture composed of cells with uniform nuclei, cytoplasmic vacuolation, and scattered morules. Immunohistochemistry demonstrated diffuse nuclear and cytoplasmic β-catenin expression, confirming low-grade foetal adenocarcinoma of the lung. Discussion Low-grade foetal adenocarcinoma resembles the pseudoglandular stage of foetal lung development and demonstrates nuclear β-catenin staining with morule formation. Unlike the high-grade variant, L-FLAC is usually localized, lacks common oncogenic driver mutations, and is associated with excellent long-term outcomes following complete resection. Necrosis, although less commonly described, may occur without indicating aggressive behaviour. The five-year survival rate exceeds 75%. This case emphasizes the importance of recognizing L-FLAC as a distinct pathological entity, as accurate diagnosis influences surgical planning, prognostic counselling, and contributes to the limited published experience with this rare tumour. This abstract is funded by: NA