Spontaneous Muscle Bleeding During Oral Anticoagulation Therapy: When Should We Suspect an Underlying Tumor?

Spontaneous intramuscular hematomas (SMHs) are rare but potentially serious complications of oral anticoagulation therapy. Although often attributed solely to anticoagulant use, such lesions may mask underlying soft tissue sarcomas or paraneoplastic conditions. We report the case of an 80-year-old man on warfarin who presented with a painful thigh mass initially interpreted as a hematoma but ultimately diagnosed as a malignant fibrous histiocytoma (MFH). In addition, we provide a narrative review of published cases, focusing on clinical presentation, diagnostic challenges, imaging strategies, and outcomes. Key pitfalls leading to delayed diagnosis include attribution bias, inadequate imaging, and premature management decisions. Epidemiological data show that while the incidence of SMHs is estimated at 0.5–1.5% among patients on vitamin K antagonists, clinically significant cases are increasingly reported with direct oral anticoagulants (DOACs). Suggested measures include clinical algorithms to prompt imaging and biopsy in persistent masses, validation of magnetic resonance imaging (MRI) criteria, and the establishment of prospective registries, aimed at facilitating earlier recognition of malignant lesions and improving patient outcomes. These strategies may improve early detection of malignancy and optimize care in anticoagulated patients presenting with soft tissue lesions.