LUtetium-177 DOsimetry as a Predictive Biomarker of Response in Metastatic Prostate Cancer Patients Treated With PSMA Radioligand THerapy.

A substantial proportion of patients with mCRPC do not respond to 177Lu-PSMA-RLT. The PSA response to Lu-PSMA was observed in nearly 46% of patients included in VISION trial and 66% in LuPSMA trial (4,5). The response to treatment can be evaluated after two cycles using the PSA or PSMA PET/CT scan. Gafita et al. Data have shown that PSA and PSMA perform equally in assessing response to 177Lu-PSMA treatment, and their changes after two cycles are related to patient survival. After two cycles, patients with no PSA or PSMA response had worse outcomes than those with partial response or stable disease . That means PSA and PSMA changes after two cycles can be used as a surrogate of patient outcome. However, the explanation of disease resistance to 177Lu-PSMA-RLT is not yet fully understood. Inappropriate dose administration might be one of the possible explanations. A dose-response relationship has been established in radiotherapy , making dosimetry a standard of care in conventional radiotherapy. In the radionuclide therapy settings, the dose-response relationship has been reported in a multi-center phase 2 trial on the selective internal radiotherapy in hepatocellular carcinoma. In this context, calculating the absorbed dose to tumour lesions could be an excellent method to individualize radionuclide therapy to achieve a maximal response to treatment. If dosimetry calculations could predict which patients would ultimately respond or not respond to treatment, administered dose and number of 177Lu-PSMA-RLT cycles could be adapted early during the treatment course. In this context, our study aims to analyze if absorbed tumour dose obtained by dosimetry calculations could be used as a biomarker to predict non-response to treatment early after one cycle, as the first step towards treatment dose adaptation of a personalized radionuclide treatment approach.