Flare Phenomenon
When effective treatment makes the scan look worse before it looks better
Quick Answer
The flare phenomenon is a transient apparent worsening of an imaging study early after starting effective therapy, caused by the healing response rather than by disease progression. On bone scintigraphy, responding osteoblastic metastases become more avid — or previously occult lesions become visible — as reparative bone forms; on FDG-PET, an inflammatory response to treatment can transiently raise uptake. Misreading flare as progression can lead to stopping a therapy that is actually working. Timing and follow-up, not a single early scan, resolve it.
The bone-scan flare
After starting effective systemic therapy (classically hormonal therapy for prostate or breast cancer, but also chemotherapy), osteoblastic metastases heal by laying down new bone, which avidly binds diphosphonate. For roughly the first ~3 months, this reparative activity can make lesions look more intense, and can unmask lesions that were present but not yet visible — an appearance indistinguishable from progression on a single scan. The defining feature is what happens next: on a repeat scan ≈3–6 months later, a true flare improves, whereas genuine progression continues to worsen.
The practical rule: do not call progression on a bone scan performed within ~3 months of starting a new effective therapy in a patient who is otherwise responding (falling PSA, improving symptoms). Correlate with tumour markers and clinical status, and confirm with a follow-up scan.
The PET / FDG flare
A comparable pitfall exists on FDG-PET: treatment can provoke an inflammatory response — post-radiation change, immune-related inflammation (notably with immunotherapy, where pseudoprogression describes lesions that enlarge or become more avid before responding), and post-chemotherapy marrow rebound or G-CSF-stimulated marrow uptake. As with bone flare, the discriminator is timing and trajectory: interim inflammatory uptake that later resolves versus true metabolic progression. Response criteria (e.g. PERCIST, and immunotherapy-specific frameworks) build in timing rules and confirmatory follow-up for exactly this reason.
Distinguishing flare from progression
- Trajectory over time is decisive: flare improves on the next interval scan; progression continues.
- Clinical and biochemical concordance: flare occurs while markers and symptoms are improving; discordance (worsening scan with worsening markers) favours true progression.
- Timing window: highest suspicion within the first ~3 months of a new effective therapy.
- Pattern: flare intensifies known osteoblastic lesions and unmasks reparative ones; the appearance of new lesions in new territories, especially with clinical decline, favours progression.
Why it matters
Calling flare "progression" can trigger premature discontinuation of a working therapy; calling progression "flare" delays a needed change. The safeguard is to anchor response assessment to trajectory and clinical context, not to a single early post-treatment scan — and to schedule the confirmatory interval study.
Self-Check
Q1. What causes the bone-scan flare, and in what time window does it occur?
Answer: Responding osteoblastic metastases heal by forming new bone that avidly binds diphosphonate, transiently increasing uptake and unmasking lesions — typically within the first ~3 months of starting effective therapy.
Q2. How do you distinguish flare from true progression?
Answer: Trajectory and clinical concordance: flare improves on a follow-up scan ~3–6 months later and occurs while markers/symptoms improve; progression continues to worsen and is usually accompanied by rising markers and new-territory lesions.
Q3. What is the practical rule for reading an early post-treatment bone scan?
Answer: Do not diagnose progression on a bone scan within ~3 months of starting a new effective therapy in an otherwise-responding patient; correlate with markers/symptoms and confirm with an interval scan.
Q4. What is the FDG-PET analogue of flare, and with which therapy is it especially relevant?
Answer: Treatment-related inflammatory uptake, including pseudoprogression with immunotherapy (lesions transiently enlarge/intensify before responding), plus post-radiation change and G-CSF marrow uptake — resolved by timing and follow-up.
Key References
- Reviews of the osteoblastic flare phenomenon on bone scintigraphy after systemic therapy for prostate and breast cancer.
- PERCIST and immunotherapy response-assessment literature addressing pseudoprogression and treatment-related inflammation on FDG-PET.
Evidence & sources
https://nuc.med/flare-phenomenon/ Report an issue