KL-6 as a Tumor Marker of Primary Peritoneal Cancer in Patients with Connective Tissue Diseases.

We herein report two patients with connective tissue disease who developed primary peritoneal cancer (PPC). Serum Krebs von den Lungen-6 (KL-6) levels increased when PPC was diagnosed, and these levels were correlated with the treatment and worsening of PPC in both cases. In one patient with systemic sclerosis, serum KL-6 levels increased despite stable interstitial lung disease (ILD), leading to a diagnosis of PPC. In the other patient with dermatomyositis and no ILD, PPC was diagnosed with elevated KL-6 levels four months post-treatment, without ILD development. Clinicians should be reminded that KL-6 is a tumor marker in various cancers.