LAUSR

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.