- Prostate: CT scans will be performed at 6, 12 and 24 months with imaging triggered by appropriate PSA rises. A rising PSA defined as 2 successive PSA rises from nadir, measured a minimum of 4 weeks apart. If the overall PSA rise has a doubling time of ≥ 3 months or the PSA level has doubled the original PSA value at trial entry or if clinically indicated, then restaging should be considered.