Abstract Introduction Invasive mucinous adenocarcinoma (IMA) is a rare subtype of lung adenocarcinoma (2-10%) that frequently presents a diagnostic challenge due to its common mimicry of pneumonia with persistent unilobar or pneumonic-type consolidation. Radiographically, IMA often appears as ground-glass or dense consolidation, and is histologically characterized by columnar or goblet cells with abundant intracellular mucin. We report an uncommon presentation of IMA: a small right-lower-lobe (RLL) nodule with progressive interstitial infiltrate in an otherwise asymptomatic patient, highlighting the need for vigilance in diagnosis. Case Presentation Our patient is an 85-year-old man with an 18-pack-year smoking history and treated prostate cancer (prostatectomy 2010). During routine PSMA PET/CT surveillance (2022-2024) for prostate adenocarcinoma recurrence, a 5mm right-lower-lobe (RLL) nodule and non adjacent mild interstitial infiltrate were incidentally detected but not followed. Retrospectively, over a two-year period, this finding progressed radiographically to diffuse interstitial thickening and solid consolidations strictly confined to the RLL, while the patient remained completely asymptomatic. Given this progressive pattern, a robotic-assisted bronchoscopy with cryobiopsy was performed. Pathology revealed a CK7+/TTF-1+, NKX3.1−, NKX3.1− genotype, ruling out metastatic prostate cancer and confirming IMA. Subsequent lymph node biopsies (11L, 7, 11R) along with FDG PET/CT (Oct 2025) showed hypermetabolic RLL consolidation with mild lymphangitic extension and no lymphadenopathy, supported a localized stage (cT3/T4 N0, IIIA). Multidisciplinary discussion favored definitive radiotherapy without chemotherapy. Discussion This case is notable for its atypical presentation radiographically, histologically, and genetically. Its indolent and single lobar presentation is unusual for IMA. IMA is more typical of acinar histology which is associated with a worse prognosis. IMA typically expresses CK7, may variably express CK20 or CDX2, but seldom shows TTF-1 positivity; reported expression occurs in fewer than 10-15 % of cases, making this a rare variant. Recognizing such patterns prevents misclassification as non-mucinous or metastatic adenocarcinoma. Accurate interpretation of immunohistochemistry requires correlation with imaging and histology, particularly in patients with prior malignancy, as NKX3.1 negativity excluded prostate origin. Although TTF-1 positivity is uncommon in IMA, it does not exclude diagnosis when radiologic and histologic findings align, emphasizing the need for early diagnosis and genotyping. Late IMA diagnosis leads to poorer response to therapy and prognosis overall when compared to non-mucinous adenocarcinoma (NMA). Given that early diagnosis of IMA is correlated with better outcomes compared to NMA, a high index of suspicion and low threshold for workup is vital to prevent diagnostic delay and improve patient morbidity and mortality. This abstract is funded by: None
Tanaya et al. (Fri,) studied this question.