A80-3-30 A Mass-less Malignancy: Diagnostic Pitfalls in Lepidic Adenocarcinoma

Abstract Introduction Malignant pleural effusions secondary to lung cancer are commonly seen in advanced stages, and often with an associated lung mass visible on CT scan. Lepidic adenocarcinoma is a distinct subtype of pulmonary adenocarcinoma characterized by a noninvasive growth pattern along intact alveolar septa. Unlike other subtypes of lung adenocarcinoma, lepidic-predominant tumors may not form a discrete mass. Their imaging appearance can mimic inflammatory or infectious processes, making radiologic detection challenging. We report a unique case of a patient who presented with a massive pleural effusion and no identifiable lung mass on imaging after thoracentesis, ultimately diagnosed with lepidic adenocarcinoma via lung biopsy. Case Presentation A 72-year-old woman with a remote history of tobacco use presented with a persistent dry cough and progressively worsening dyspnea on exertion. Chest X-ray revealed a massive right-sided pleural effusion. Thoracentesis was performed, and pleural fluid studies were indicative of metastatic adenocarcinoma. However, a contrast-enhanced CT scan of the chest did not identify a discrete parenchymal lung mass to explain the findings. Tumor marker evaluation revealed elevated CA 19-9 levels (7000 U/mL), raising suspicion for gastrointestinal or pancreatic malignancy. An MRI of the abdomen demonstrated a lesion in the pancreatic head with signal dropout on opposed-phase imaging, consistent with focal fatty infiltration, rather than malignancy. A whole-body PET scan revealed multiple FDG-avid lymph nodes in the mediastinum, right hilar, and right pericardial regions, but again, no dominant lung mass was detected. Given persistent diagnostic uncertainty, a wedge biopsy of the lung was performed, which revealed moderately differentiated adenocarcinoma with mixed lepidic and invasive acinar growth patterns. Tumor involvement was noted at the parenchymal specimen margin and visceral pleura. Discussion Lepidic adenocarcinoma is typically an indolent form of lung cancer, but when mixed with invasive growth patterns such as acinar, it can behave more aggressively, leading to pleural dissemination. Notably, a chest radiograph performed just one month prior was unremarkable, underscoring the abrupt and aggressive progression of the disease. Markedly elevated CA 19-9 level further complicated the diagnosis, as it is commonly associated with pancreaticobiliary malignancies. However, lung adenocarcinomas, especially those with lepidic features, can also produce CA 19-9, leading to potential diagnostic confusion. This case emphasizes the importance of considering atypical presentations of lung cancer, particularly in patients with unexplained pleural effusions, and underscores the need for tissue diagnosis when imaging findings are inconclusive but cytology is positive for malignancy. This abstract is funded by: none