How can I tell if symptoms are masked vs the cancer improving?

The real way to tell is by checking what happens when the prednisone dose drops, and by following objective scans rather than how the dog feels.

Prednisone suppresses inflammation, pain, fever, and appetite loss by blocking the chemical signals that cause those symptoms [2]. When a dog on prednisone seems normal, you cannot tell whether the cancer has shrunk or is just being masked. The key test is the dose reduction. If symptoms return when the prednisone is tapered, you are seeing masking. If they stay away, the cancer may truly be improving.

In one human case, a patient had a good clinical response to prednisone, but symptoms returned as soon as the dose was lowered [1]. In a veterinary study of dogs with relapsed lymphoma on a prednisone-containing protocol, the median progression-free survival was only 56 days for dogs that responded at all [3]. The drug gave temporary symptom control but the cancer kept progressing.

The only reliable way to distinguish masking from real improvement is with objective imaging or blood work. For lymphoma in dogs, that means checking lymph node size by physical exam and, where available, by ultrasound or CT. For other cancers, repeat imaging at intervals tells you whether the tumor is stable, shrinking, or growing. If you are relying on how the dog acts alone, you cannot tell the difference.

My call: dose tapering and objective imaging are the only ways to tell masking from true improvement. Confidence in this is high.