Pretreatment considerations on mpMRI

Success in focal therapy is contingent on accurate pre-treatment patient selection and precise disease localization.

The 2017 International Delphi Consensus Project on patient selection for prostate focal therapy provides recommendations on ideal cancer foci for focal therapy (Figure 4) [3].

Fig 4: The 2017 International Delphi Consensus Project on patient selection for prostate focal therapy provides recommendations on ideal cancer foci for focal therapy.

Before focal therapy, patients must undergo prostate mpMRI and targeted and systematic mapping biopsy to delineate the index lesion and exclude non-MRI visible clinically significant lesions.

In pretreatment planning for focal therapy, radiologists should identify even the non-dominant cancers for accurate risk stratification and must accurately map the location and extent of all cancers as this impacts treatment planning and selection of appropriate focal therapy modality (Figure 5).

Fig 5: Prostate size and tumour location are essential considerations in determining the most appropriate focal treatment modality.

Limitations of mpMRI in pretreatment assessment include challenges in assessing extraprostatic extension and inability to identify MRI-occult low volume clinically significant prostate cancer. Literature suggests enhancement on dynamic contrast-enhanced sequences (DCE) may more accurately reflect true tumour extent when correlated with histology (Figure 6) [4].

Fig 6: DCE findings may alter the choice of lesion to be targeted for focal therapy.

MpMRI protocol for scanning the post focal therapy prostate

MpMRI protocol for scanning the post focal therapy prostate is summarised in Figure 7 [5].

Fig 7: MpMRI protocol for scanning the post focal therapy prostate.

There is substantial variability in the literature on the timing and frequency of surveillance mpMRI post focal therapy (Figure 8) [2,5]. At the authors’ institution, first surveillance mpMRI is performed 12 months after focal therapy.

Fig 8: Timing of surveillance mpMRI following focal therapy

Expected post focal therapy changes on mpMRI

Immediate and early (< 6 months) post-treatment appearance of the prostate varies based on the focal ablative modality used. However, all focal therapy procedures eventually lead to fibrosis of the treated area. By 6-12 months following treatment, the post treatment prostate demonstrates similar appearance regardless of modality used (Figures 9, 10). Therefore, similar principles may be employed in the detection of tumour recurrence in the post focal therapy prostate.

Fig 9: Early and late changes post cryotherapy. Regardless of focal therapy modality, late post treatment changes are similar. Hence, similar principles may be employed in detection of post focal therapy tumour recurrence.

Fig 10: Early and late changes post high intensity focused ultrasound (HIFU). Regardless of focal therapy modality, late post treatment changes are similar. Hence, similar principles may be employed in detection of post focal therapy tumour recurrence.

Being familiar with expected post focal therapy changes also allows radiologists to use mpMRI to assess treatment adequacy (Figures 11, 12, 13).

Fig 11: Adequate treatment with expected post focal therapy changes.

Fig 12: Treatment failure with in-field residual tumour.

Fig 13: MpMRI features indicative of treatment adequacy and treatment failure.

Detection and scoring of post focal therapy tumour recurrence  

MRI assessment of the post focal therapy prostate is challenging as the appearance of post treatment fibrosis may overlap with tumour recurrence.

International consensus incorporates DCE as the major sequence and DWI and T2WI as joint minor sequences in assessing recurrent tumour in the post focal therapy setting. Focal nodular strong early contrast enhancement is most suspicious for tumour recurrence (Figure 14) [2,5].

Fig 14: Appearance of post focal therapy tumour recurrence on mpMRI

Tumour recurrence following focal therapy can be categorized into in-field and out-of-field recurrence (Figure 15) [2].

Fig 15: Tumour recurrence following focal therapy can be categorized into in-field and out-of-field recurrence.

Following focal therapy, PI-RADS v2.1 is used to grade likelihood of malignancy for out-of-field lesions in the untreated prostate gland (Figure 16). However, PI-RADS v2.1 is inappropriate for scoring in-field lesions. Standardised scoring systems such as Prostate Imaging after Focal Ablation (PI-FAB) (Figure 17) or Transatlantic Recommendations for Prostate Gland Evaluation with MRI after Focal Therapy (TARGET) should be used instead  (Figure 18). This is because post treatment fibrosis and tumour recurrence both demonstrate hypointense signal on T2WI and ADC map – the dominant sequences according to PI-RADS guidelines. DCE hence assumes a dominant role for detection of tumour recurrence post focal therapy [6].

Fig 16: Standardised approach in scoring likelihood of tumour recurrence post focal therapy.

Fig 17: Prostate Imaging after Focal Ablation (PI-FAB) system for scoring the likelihood of in-field tumour recurrence post focal therapy.

Fig 18: Transatlantic Recommendations for Prostate Gland Evaluation with MRI after Focal Therapy (TARGET) system for scoring the likelihood of in-field tumour recurrence post focal therapy.

We briefly compare the PI-FAB and TARGET scoring systems (Figure 19) [5,6].

Fig 19: Comparison of the PI-FAB and TARGET scoring systems.

Fig 20: A recent study by Esengur et al comparing the use of PI-FAB and TARGET for detecting tumor recurrence on mpMRI after primary focal therapy suggests that both scoring systems exhibit similar performance in diagnosing recurrent cancer.

Case review of histologically proven tumour recurrence

In the following cases, we showcase our experience of post focal therapy histopathology proven clinically significant recurrent prostate cancer determined via targeted and systematic MRI-ultrasound fusion biopsy. The cancers are scored using the proposed PI-FAB and TARGET systems for in-field recurrence (Figure 21) and PI-RADS v2.1 for out-of-field tumour recurrence / residual tumour (Figures 22, 23).

Fig 21: In-field tumour recurrence.

Fig 22: Out-of-field tumour recurrence.

Fig 23: Out-of-field residual tumour.

Medium and long term surveillance of the post focal therapy prostate

To date, there is no consensus on how many years patients should receive protocol MRI surveillance for, although this duration should be dependent on patient’s clinicopathological disease characteristics and further imaging is recommended in event of new triggering factors (Figure 24) [2,5].

Fig 24: Medium and long term surveillance of the post focal therapy prostate.

In our experience, mpMRI continues to be a crucial component of medium and long term active surveillance following focal therapy. With continued follow up, post treatment changes have stabilised and small tumour foci not detected on prior MRI or systematic biopsy have had time to progress and manifest. Furthermore, biopsy after focal therapy may be challenging as scarring and fibrosis may cause sampling error. Close imaging follow up is a good complement to periodic histologic sampling in detecting recurrence (Figure 25).

Fig 25: Following focal therapy, mpMRI continues to be a crucial component of medium and long term active surveillance.

Management of in-field persistence and out-of-field tumors following focal therapy

Besides initial treatment, discussion has also revolved around management options of biopsy-proven tumour recurrence, which are summarised in Figure 26 [2].

Fig 26: Management of tumour recurrence post focal therapy.