Focal therapy has emerged as a promising alternative to whole-gland treatments for prostate cancer, aiming to balance oncologic control with reduced adverse effects. Traditional methods, like radical prostatectomy and radiation therapy, provide well-documented long-term outcomes but often lead to significant morbidities such as urinary incontinence and erectile dysfunction, impacting quality of life.
To minimise overtreatment, active surveillance is available for patients with low-risk or favourable intermediate-risk disease, although it comes with challenges like repeated imaging, patient anxiety and potential costs. There is also debate over whether intermediate-risk patients should be included in this strategy due to possible disease progression.
Focal therapy targets only the most aggressive lesion, preserving surrounding prostate tissue. Its success depends on accurately identifying the index lesion, with multiparametric MRI (mpMRI) being vital for pre-treatment selection, post-treatment assessment and long-term surveillance. Radiologists must recognise expected imaging changes after treatment to differentiate between normal post-therapy effects and potential tumour recurrence.
The Role of Multiparametric MRI in Focal Therapy
Prior to focal therapy, mpMRI is essential for identifying the dominant cancer lesion, determining tumour volume and accurately mapping the cancerous areas within the prostate. Since focal therapy aims to treat only the index lesion, it is crucial for radiologists to identify all possible cancer foci to ensure appropriate risk stratification. Although focal therapy may preserve genitourinary function more effectively than whole-gland treatments, its success depends on the ability to achieve complete ablation of the targeted tumour without missing clinically significant lesions elsewhere in the gland.
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Despite its importance, mpMRI is not without limitations. It can underestimate tumour burden and may fail to detect MRI-occult low-volume clinically significant prostate cancer. These challenges highlight the need for a cautious approach in treatment planning, as missing a significant lesion may result in disease progression. To address this, recently developed MRI scoring systems—such as the Prostate Imaging after Focal Ablation (PI-FAB) and the Transatlantic Recommendations for Prostate Gland Evaluation with MRI after Focal Therapy (TARGET)—aim to standardise post-treatment assessment. These systems provide structured criteria for evaluating tumour recurrence and help improve consistency in radiologic interpretation. However, further validation studies are needed to assess their long-term reliability in clinical practice.
Post-Treatment MRI Surveillance
Following focal therapy, mpMRI plays a crucial role in post-treatment monitoring, as prostate-specific antigen (PSA) testing alone is often unreliable. Since residual prostate tissue continues to produce PSA, fluctuations in PSA levels may occur even in the absence of recurrence, making imaging a necessary adjunct for surveillance.
In the months following focal therapy, the treated area undergoes dynamic changes and the ability to distinguish expected post-treatment effects from residual or recurrent cancer is critical. Early post-treatment changes vary depending on the focal therapy modality used. However, by six to twelve months after treatment, most therapies result in similar imaging findings, including fibrosis, volume loss at the treatment site and decreased signal intensity on T2-weighted imaging. These changes reflect the body’s healing response and should not be mistaken for active disease.
Despite these expected changes, distinguishing benign post-treatment effects from recurrent cancer remains a significant challenge. Inflammation, granulation tissue and fibrosis can obscure tumour recurrence, while post-treatment haemorrhage may mimic cancer on imaging. Comparison with pre-treatment scans and careful evaluation of imaging sequences are necessary to ensure accurate diagnosis. Dynamic contrast-enhanced (DCE) MRI is often the most reliable sequence for detecting residual disease, with recurrent tumours typically exhibiting strong focal enhancement. The integration of standardised MRI scoring systems further assists radiologists in assessing the likelihood of recurrence and determining whether further investigation, such as targeted biopsy, is required.
Tumour Recurrence and Imaging Challenges
Tumour recurrence following focal therapy is classified as either in-field (within the treated area) or out-of-field (elsewhere in the prostate). In-field recurrence suggests incomplete tumour ablation, indicating that the initial treatment was not fully effective. Out-of-field recurrence, on the other hand, may represent previously undetected clinically significant cancer, highlighting the limitations of pre-treatment imaging in capturing all disease sites.
mpMRI is the preferred modality for detecting tumour recurrence, but accurate interpretation requires experience and familiarity with post-treatment imaging characteristics. DCE imaging plays a dominant role in identifying recurrent tumours, as contrast enhancement patterns can help differentiate between treatment-induced changes and true malignancy. However, reliance on DCE imaging has limitations, particularly in patients with renal impairment where contrast administration may not be feasible. Additionally, post-treatment fibrosis can complicate MRI interpretation, as fibrotic tissue can mimic tumour recurrence by demonstrating restricted diffusion on diffusion-weighted imaging (DWI).
Scoring systems such as PI-FAB and TARGET aid in standardising post-treatment assessment. PI-FAB assigns lesions a three-point score based on enhancement patterns and lesion size, guiding clinicians in deciding whether surveillance or biopsy is warranted. TARGET, on the other hand, employs a five-point scoring system that incorporates DCE, DWI and T2-weighted imaging findings. Both systems provide valuable frameworks for evaluating recurrence, but further research is needed to determine their optimal use in routine clinical practice.
mpMRI is an indispensable tool in focal therapy for prostate cancer, supporting patient selection, treatment planning and post-treatment surveillance. However, its effectiveness is dependent on careful interpretation and an understanding of the expected post-treatment changes that can mimic tumour recurrence. Challenges such as MRI-occult lesions, imaging artefacts and treatment-induced changes necessitate the use of standardised scoring systems and multidisciplinary collaboration to optimise patient outcomes.
As focal therapy continues to evolve, ongoing research into MRI assessment protocols, scoring systems and long-term patient outcomes will be crucial in refining this approach. Radiologists, urologists and oncologists must work together to improve diagnostic accuracy, reduce unnecessary biopsies and ensure that focal therapy remains a viable and effective treatment option for selected patients with prostate cancer.
Source: Radiology: Imaging Cancer
Image Credit: iStock
