Salvage Radiotherapy has been shown to be most effective when the PSA level is <0.5 ng/mL [21]; at these low PSA values, recurrence is expected to produce a tumor with very low volume, which is extremely difficult to detect by imaging modalities; for this reason, the current practice is that patients undergo local or systemic treatment based on a clinical probability evaluation [20]. This evidence concerns the gene KLK3 and neoplasm.