IntroductionInvasive Mucinous adenocarcinoma (IMA) is an unusual histological type of lung cancer. A primary pulmonary mucinous adenocarcinoma is an adenocarcinoma variant according to the current World Health Organization classification of lung cancers. Invasive mucinous adenocarcinomas are less prevalent than are invasive adenocarcinomas accounting for approximately 5% of lung adenocarcinomas.Case ReportA 64 year old female presented to Wake Forest Baptist Medical Center with a 2 month history of shortness of breath, nightly low grade fevers and cough with copious mucus production. She had received several courses of antibiotics at Outside medical facilities during that period of time without sustained improvement. CT scan revealed numerous nodular opacities with cavitation along with scattered areas of ground glass opacities and patchy consolidation. She was started on antibiotic treatment and underwent bronchoscopy with BAL that was negative for infectious etiologies and negative fluid cytology. She had progressive decline in her respiratory status with worsening hypoxia which led to a repeat bronchoscopy with trans-bronchial biopsies that revealed atypical columnar-cuboidal cells with abundant cytoplasmic mucin and small basally oriented nuclei lining the alveolar walls consistent with mucinous adenocarcinoma. Immunohistochemistry revealed positive CK7 and napsin along with negative CDX-2, consistent with primary lung tumor. She had a Positron Emission Topography scan done that showed metastatic disease to the bone. She was evaluated by oncology and radiation oncology who planned for palliative radiation therapy. Unfortunately patient had worsening respiratory and functional status and ultimately pursued hospice and palliative care.DiscussionInvasive mucinous adenocarcinoma is an uncommon subtype of lung cancer with clinical, radiological, pathological and genetic characteristics distinct from those of other lung adenocarcinomas. Discrimination between this type of cancer and pneumonia on initial presentation is challenging and requires a high index of suspicion for timely diagnosis and to prevent delays in treatment.