Research use only

This tool implements the SPARE algorithm for research and educational purposes. It has not been validated for clinical use and must not guide patient management.

Response assessment and treatment decisions in [177Lu]Lu-PSMA therapy remain the responsibility of the treating multidisciplinary team.
SPARE

Making more of the cycle-2 SPECT/CT.

After the second cycle of [177Lu]Lu-PSMA radioligand therapy (RLT) for metastatic castration-resistant prostate cancer (mCRPC), every patient already has a post-therapy SPECT/CT. SPARE, a SPECT-guided, PET-sparing Algorithm for PSMA Radiopharmaceutical therapy Evaluation, reads that scan with RECIP 1.0, weighs the 12-week PSA trend, and calls for PSMA or FDG PET only where they change the decision. Developed at Institut Jules Bordet; prospective validation is pending.

See the algorithm Try it Read the paper

The SPARE algorithm

Every branch is anchored to a published finding. Select any box to see the evidence behind that step. Entering a case further down highlights its path here. Not yet prospectively validated; research use only.

  • Continue RLT
  • Perform FDG PET/CT
  • Perform PSMA PET/CT
  • Discontinue RLT
        • PSMA PET/CTRECIP 1.0
SPARE cohort, n = 142Ref 6
84.5%interim PSMA PETs avoided (120/142)95% CI 77.7–89.5
91.1%agreement with real-world continue/discontinue callsκ 0.81 (0.69–0.91)

Try the algorithm

Choose a category for each finding after cycle 2, or calculate one from measurements. The result updates as you go.

Findings

Interim assessment, about 12 weeks, after cycle 2

Examples
SPECT/CT, RECIP 1.0 Step 1
Calculate from total tumor volume (TTV)
ΔTTV — → —
RECIP 1.0: PR = TTV ≤ −30% without new lesions · PD = TTV ≥ +20% with new lesions · otherwise SD.
PSA trend Step 2
Reference PSA: day of cycle 1 (pre-injection).
No PSA progression before 12 weeks. An early rise (flare) does not count.
Calculate % change from PSA values
ΔPSA — → —
≤ −50% → response · ≥ +25% → progression · otherwise stable.
PSMA PET/CT, RECIP 1.0 Step 3, if reached
Calculate from total tumor volume (TTV)
ΔTTV — → —
Same RECIP 1.0 rule as the SPECT, now on its native modality.
Unlocks when SPECT is stable and PSA is stable.

Evidence base

The papers behind each step. Every citation in the algorithm links back to one of these.

  1. 1Gafita A, Rauscher I, Weber M, Hadaschik B, Wang H, Armstrong WR, et al. Novel framework for treatment response evaluation using PSMA PET/CT in patients with metastatic castration-resistant prostate cancer (RECIP 1.0): an international multicenter study. J Nucl Med. 2022;63(11):1651–8.
  2. 2Kassas M, Jaafari A, Shagera QA, Devriendt L, Léger MA, Arçay Öztürk A, et al. SPECT versus PET for [177Lu]Lu-PSMA response assessment using quantitative RECIP 1.0. J Nucl Med. 2026;67(6):903–9.
  3. 3Unterrainer LM, De Leiris N, Unterrainer M, Delker A, Hempel L, Ells Z, et al. Evidence-based clinical protocols to monitor efficacy of [177Lu]Lu-PSMA radiopharmaceutical therapy in metastatic castration-resistant prostate cancer using real-world data. J Nucl Med. 2025;66(7):1054–60.
  4. 4Kassas M, Artigas C, Refalo N, Mallia A, Flamen P. Added value of interim FDG PET/CT in resolving PSA–PSMA discordance during PSMA-targeted radioligand therapy. Clin Nucl Med. 2026;51(2):191–3.
  5. 5Kassas M, Léger M-A, Poenaru R, Tahmi D, Zeiter RL, Shagera QA, et al. FDG PET/CT beyond mismatch exclusion in [177Lu]Lu-PSMA therapy: baseline prognosis and response assessment. The EANM Journal. 2026;4:100234.
  6. 6Kassas M, Zekri M, Poenaru R, Kristanto P, Karfis I, Flamen P, Artigas C. A SPECT-guided, PET-sparing algorithm for PSMA radiopharmaceutical therapy evaluation (SPARE). J Nucl Med. 2026;67(11). Published online November 12, 2026. doi:10.2967/jnumed.126.273398

Contact

Questions about SPARE, or a possible collaboration? Get in touch.

mutaz.kassas@hubruxelles.be carlos.artigas@hubruxelles.be