Oncologic basis of focal therapy
In this era of personalised precision medicine, focal therapy is an emerging middle ground between active surveillance and radical whole gland therapy for treatment of localised primary prostate cancer (Figure 1). 
Evidence suggests although prostate cancer is often multifocal, it is the pathologic characteristics of the largest or most aggressive cancer focus (index lesion) that determines tumour progression and metastasis risk [1]. It may hence be sufficient to treat the index lesion and adopt active surveillance for the remaining low risk lesions.
Focal therapy focuses treatment on clinically significant cancer while minimising injury to the adjacent structures of the prostate, particularly the neurovascular bundles, bladder neck and urethral sphincter.
Role of multiparametric MRI (mpMRI) in prostate cancer surveillance post focal therapy
Serum prostate-specific antigen (PSA) level is widely used for surveillance after whole-gland treatment, but it is less reliable following focal therapy because of significant amount of residual prostatic parenchyma which continues to produce PSA.
International multidisciplinary consensus statements agree that mpMRI is the preferred imaging modality to evaluate treatment response following focal therapy, as it is the most established (Figure 2). They acknowledge that this is an evolving field however, and other imaging modalities may prove beneficial [2]. 
Cryotherapy - An example of focal therapy
We briefly consider cryotherapy as an example of focal therapy (Figure 3). 