Dry Pleural Dissemination from Rectal Cancer: A Case Report.

Introduction: Malignant pleural disease is a manifestation of advanced malignancy and may present as malignant pleural effusion or solid pleural metastases. Dry pleural dissemination (DPD), defined as pleural metastases without pleural effusion, had been typically described in primary lung cancer but is rarely reported in metastatic malignancies. Case Presentation: A 79-year-old woman underwent laparoscopic low anterior resection for rectal cancer. On follow-up after 3 years and 5 months, computed tomography (CT) revealed two enlarging lung nodules along the right interlobar fissure without pleural effusion. Positron emission tomography-CT demonstrated mild uptake in these nodules, and there were no other lesions identified. Tumor marker levels showed increasing carcinoembryonic antigen and normal carbohydrate antigen 19-9. During thoracoscopic surgery for suspected pulmonary metastases, multiple small nodules were seen diffusely distributed on the parietal and visceral pleura without pleural effusion. Histopathological examination revealed metastatic rectal adenocarcinoma, leading to a diagnosis of DPD. Discussion: DPD is considered early-stage pleural metastasis and is difficult to detect due to the absence of pleural effusion. Most reports on DPD are limited to primary lung cancer, and its occurrence in metastatic colorectal cancer is extremely rare. In this patient, only a small number of lesions were detectable on imaging, whereas numerous pleural nodules were identified intraoperatively. This discrepancy highlighted the limitations of imaging modalities in detecting early pleural dissemination. Clinicians should consider the possibility of occult pleural metastasis, including DPD, in patients with rising tumor marker levels even when imaging findings are inconclusive. Conclusion: This case illustrated a rare presentation of rectal cancer recurrence as DPD. Awareness of this entity and consideration of thoracoscopic evaluation may facilitate early diagnosis and timely therapeutic intervention.