Differential Diagnosis for New Onset Edema in a 55-year-old Man

Single Most Likely Diagnosis

• Lymphedema secondary to cancer treatment: Given the patient's history of prostate cancer, even though it's in remission, the new onset of edema in multiple limbs could be related to previous treatments such as radiation therapy or surgery that might have affected lymph nodes, leading to lymphedema.

Other Likely Diagnoses

• Deep Vein Thrombosis (DVT): Although the CT shows no abnormalities, DVT can cause edema in the affected limbs. The absence of findings on CT does not rule out DVT, as it might not be visible, especially if it's an acute event.
• Chronic Venous Insufficiency: This condition can lead to edema, particularly in the lower limbs, due to the incompetence of venous valves.
• Heart Failure: Although not directly indicated by the CT, new onset heart failure could lead to edema, particularly in the lower limbs. The involvement of the right arm could be due to other factors or less common presentations.

Do Not Miss Diagnoses

• Superior Vena Cava Syndrome: This is a medical emergency where the superior vena cava is obstructed, which can cause edema in the upper body, including the right arm. Given the patient's history of cancer, there's a possibility, however small, of a new or recurrent tumor causing this obstruction.
• Pulmonary Embolism: While the primary symptom is usually respiratory, a large pulmonary embolism can lead to increased pressure in the pulmonary circulation, potentially causing edema in unusual distributions due to increased right heart pressures.
• Nephrotic Syndrome: Although less likely, nephrotic syndrome can cause significant edema. It's a condition that would need to be considered, especially if there are other signs such as significant proteinuria.

Rare Diagnoses

• Lipedema: A rare condition characterized by the abnormal growth of fat cells in the legs, which can cause edema and swelling.
• Fabry Disease: A genetic disorder that can lead to edema among other symptoms due to the accumulation of a particular type of fat in the body's cells.
• Sarcoidosis: A condition that can cause inflammation in various parts of the body, potentially leading to edema in unusual distributions, although this would be less common.

Each of these diagnoses has a different level of likelihood based on the patient's presentation and history. A thorough diagnostic workup, including further imaging, laboratory tests, and potentially a biopsy, would be necessary to determine the underlying cause of the edema.