Myofibroblastic tumor with referred or localized pain: This is the most straightforward explanation, where the myofibroblastic mass in the thigh could be causing pain that is referred to or directly affects the knee due to its proximity and potential involvement of surrounding tissues or nerves.

Soft tissue sarcoma: Although less common, a soft tissue sarcoma could present similarly to a myofibroblastic mass and cause pain in the knee if it involves or compresses nearby structures.
Nerve compression or neuropathy: The mass could be compressing a nerve that supplies the knee, leading to knee pain.
Bursitis or tendinitis: Inflammation of the bursae or tendons around the knee could be caused by or exacerbated by the presence of a mass in the thigh, leading to knee pain.

Malignant transformation of the myofibroblastic tumor: Although myofibroblastic tumors are typically benign, there is a small chance of malignant transformation, which would significantly alter the treatment and prognosis.
Deep vein thrombosis (DVT): Knee pain in the setting of a thigh mass could also be due to a DVT, especially if the mass is large and compressing veins, which is a medical emergency.
Osteosarcoma or other bone tumors: A tumor originating in the bone could cause both a mass in the thigh and knee pain, and missing this diagnosis could have severe consequences.

Desmoid tumor: A type of rare, benign tumor that can occur in the soft tissues and cause significant local destruction and pain.
Inflammatory myofibroblastic tumor: A rare tumor that can occur in various parts of the body, including the soft tissues of the thigh, and is characterized by an inflammatory component.
Neurofibroma or schwannoma: Benign nerve sheath tumors that could cause a mass in the thigh and lead to knee pain if they compress or involve nearby nerves.