We performed Monte Carlo simulations in order to determine, by means of microdosimetry calculations, tumour control probability (TCP) curves for treatments withAc-PSMA of metastatic castration resistant prostate cancer (mCRPC). Realistic values of cell radiosensitivity, nucleus size and lesion size were used for calculations. As the cell radiosensitivity decreased, the nucleus size decreased and the lesion size increased, the absorbed dose to reach a given TCP increased. The highest variations occurred with regard to the cell radiosensitivity. For the Monte Carlo simulations, in order to address a non-uniform PSMA expression, differentAc-PSMA distributions were considered. The effect of these different PSMA distributions resulted in small variations in the TCP curves (maximum variation of 5%). Absorbed doses to reach a TCP of 0.9 for a uniformAc-PSMA distribution, considering a relative biological effectiveness (RBE) of 5, ranged between 35.0 Gy and 116.5 Gy. The lesion absorbed doses per administered activity reported in a study on treatments withAc-PSMA of mCRPC ranged between 1.3 Gy/MBq and 9.8 Gy/MBq for a RBE = 5. For a 70 kg-patient to whom 100 kBq/kg ofAc-PSMA are administered, the range of lesion absorbed doses would be between 9.1 Gy and 68.6 Gy. Thus, for a single cycle of 100 kBq/kg, a number of lesions would not receive an absorbed dose high enough to reach a TCP of 0.9.
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