Tumor-to-Tumor Metastasis: Pulmonary Carcinoid Metastasizing to Solitary Fibrous Tumor

Objective: Rare disease Background: Tumor-to-tumor metastasis is an uncommon phenomenon in which a primary tumor metastasizes into another tumor. Case Report: An 81-year-old Asian woman was referred to our hospital for evaluation and treatment of a solid mass in the right middle lung lobe that had rapidly enlarged for 1.5 years compared to that observed over the last 5 years. On computed tomography (CT), the mass was 68x60 mm, and 2 different tumors appeared to exist in the upper portion of the mass. Blood examination findings revealed high serum levels of progastrin-releasing peptide and neuron-specific enolase. Based on the radiographic course of the tumor and elevated levels of tumor markers, we suspected that a new malignant tumor, such as a neuroendocrine tumor, had developed dorsally adjacent to the benign tumor. CT-guided percutaneous needle biopsy of the lung indicated a solitary fibrous tumor (SET), which did not lead to the diagnosis of another tumor adjacent to the original tumor. Therefore, a right middle lobectomy was performed. The resected specimen contained 2 different tumors: an SET and a typical carcinoid without mitosis or necrosis. On microscopic examination, they were separated from each other by normal alveolar tissue. In addition, a typical carcinoid was also observed inside the SET lesion, completely enclosed by the SET tissue. These findings suggested that the carcinoid metastasized to the SET in the same lung lobe. Conclusion: To the best of our knowledge, this is the first case of a pulmonary typical carcinoid metastasizing to an intraparenchymal SET.