Differential Diagnosis for Bilateral Knee and Leg Pain

Single Most Likely Diagnosis

• Statins-induced myopathy: Given the patient is taking statins (storvastatin), a well-known side effect is muscle pain, which can manifest as bilateral knee and leg pain. This condition is relatively common among statin users, especially in the context of physical activity or when combined with other medications that may increase the risk of myopathy.

Other Likely Diagnoses

• Chemotherapy-induced neuropathy: Chemotherapy can cause peripheral neuropathy, leading to pain, numbness, and weakness in the legs. This is a common side effect of various chemotherapy agents and could explain the bilateral leg pain.
• Lymphoma-related pain: Direct involvement of the lymphoma in the bones or soft tissues of the legs could cause pain. Although less common, it's a consideration given the patient's underlying condition.
• Bone metastasis: Although the primary diagnosis is lymphoma, the possibility of bone metastasis from another primary cancer or transformation of the lymphoma into a more aggressive form affecting the bones cannot be ruled out without further investigation.

Do Not Miss Diagnoses

• Deep Vein Thrombosis (DVT): Patients undergoing chemotherapy are at an increased risk of DVT due to the hypercoagulable state induced by cancer and its treatment. DVT can cause leg pain and swelling, and it's crucial to diagnose and treat promptly to prevent pulmonary embolism.
• Osteonecrosis: Also known as avascular necrosis, this condition can be a side effect of chemotherapy and corticosteroids (if used in the treatment regimen). It leads to the death of bone tissue due to a lack of blood supply and can cause severe pain.
• Infection: Immunocompromised patients, such as those undergoing chemotherapy, are at a higher risk of infections, including osteomyelitis or septic arthritis, which can cause severe leg pain.

Rare Diagnoses

• Vasculitis: Certain types of vasculitis, such as giant cell arteritis or polyarteritis nodosa, can cause leg pain but are less common and would typically be associated with other systemic symptoms.
• Paraneoplastic syndromes: These are rare disorders that are triggered by an altered immune system response to a neoplasm. They can cause a variety of symptoms, including muscle weakness and pain, but are relatively rare and often associated with specific types of cancer.