Which imaging modality is used to assess metabolic response to therapy?

• A. MRI
• B. PET/CT (FDG-PET)
• C. Ultrasound
• D. CT with contrast only

Answer: (B)

Metabolic response reflects how actively tumor cells are using glucose after therapy. FDG-PET uses a radioactive form of glucose, so regions with high metabolic activity light up. When therapy works, tumor cells reduce their glucose uptake, and the PET signal decreases, signaling response even if the tumor’s size hasn’t changed yet. PET/CT combines this metabolic information with precise anatomical localization, making it especially powerful for assessing response.

Imaging that focuses on anatomy and structure—MRI, ultrasound, CT with contrast—shows where or how big a lesion is but doesn’t directly measure metabolic activity. Size changes can lag behind metabolic changes, so they’re less sensitive for early response. Inflammation or infection can also cause increased FDG uptake, so results must be interpreted in context. Some tumors aren’t highly FDG-avid, which is another limitation.