RT Journal Article SR Electronic T1 Eligibility for 177Lu-PSMA Therapy Depends on the Choice of Companion Diagnostic Tracer: A Comparison of 68Ga-PSMA-11 and 99mTc-MIP-1404 in Metastatic Castration-Resistant Prostate Cancer JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 227 OP 231 DO 10.2967/jnumed.122.264296 VO 64 IS 2 A1 Gary J.R. Cook A1 Wai-Lup Wong A1 Bal Sanghera A1 Stephen Mangar A1 Amarnath Challapalli A1 Amit Bahl A1 Paul Bassett A1 Darren Leaning A1 Christian Schmidkonz YR 2023 UL http://jnm.snmjournals.org/content/64/2/227.abstract AB 177Lu-prostate-specific membrane antigen-617 (177Lu-PSMA-617) is an effective therapy for metastatic castration-resistant prostate cancer (mCRPC), with evidence of improved survival over standard care. The VISION trial inclusion criteria required a metastatic lesion-to-liver ratio of greater than 1 on 68Ga-PSMA-11 PET scans. We aimed to determine whether an equivalent ratio is suitable for a SPECT tracer, 99mTc-MIP-1404, and to compare lesion and lesion–to–normal-organ ratios between the 2 radiotracers. Methods: Two cohorts of patients with mCRPC matched for age, prostate-specific antigen level, and total Gleason score, with either 99mTc-MIP-1404 SPECT/CT (n = 25) or 68Ga-PSMA-11 PET/CT (n = 25) scans, were included for analysis. Up to 3 lesions in each site (prostate/prostate bed, lymph nodes, bone and soft-tissue metastases) as well as normal liver, parotid gland, spleen, and mediastinal blood-pool SUVmax were measured. Results: 99mTc-MIP-1404 SPECT lesion SUVmax was not significantly different from 68Ga-PSMA-11 PET (median, 18.2 vs. 17.3; P = 0.93). However, 99mTc-MIP-1404 liver SUVmax was higher (median, 8.5 vs. 5.8; P = 0.002) and lesion-to-liver ratios were lower (median, 2.7 vs. 3.5; P = 0.009). There was no significant difference in parotid gland or splenic SUVmax or lesion–to–parotid gland ratios between the 2 tracers although there was a small difference in lesion-to-spleen ratios (P = 0.034). Conclusion: There are differences in biodistribution and, in particular, liver activity, between 68Ga-PSMA-11 and 99mTc-MIP-1404. Therefore, if 99mTc-MIP-1404 is used to assess eligibility for 177Lu-PSMA-617 therapy, a lower adjusted lesion-to-liver ratio should be used.