This is an excerpt from our free eBook, “Prostate Cancer and Imaging in Clinical Trials.”
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Imaging is increasingly used in prostate cancer management. Indeed, when considering the low specificity of PSA levels and the complications that can arise with systematic biopsies (e.g., urinary symptoms, infection), imaging has a significant role to play as a sensitive and specific non-invasive exam for prostate cancer detection, staging, and monitoring.
Historically, two modalities are acquired for diagnosis:
- Transrectal ultrasound (TRUS): TRUS is the oldest and most widely used technique for initial prostate cancer detection. Today its primary purpose is to direct the needle for biopsies when investigating a raised PSA or abnormal findings on DRE.
- Magnetic Resonance Imaging (MRI): MRI displays a high resolution of the prostatic gland and surrounding structures. It has become the standard imaging modality for the diagnosis of prostate cancer and surgical planning.
In 2012, a Prostate Imaging Reporting and Data System (PI-RADS) was published by the European Society of Urogenital Radiology (ESUR) to provide standardized guidelines for acquisition, interpretation, and reporting of prostate MRI. It has since been revised and is still evolving. PI-RADS categories are based on the interpretation of multiparametric MRI (mpMRI) which combines anatomic and functional sequences:
- T1-weighted (T1W) and T2-weighted imaging (T2W): anatomic sequences showing excellent soft tissue contrast and a clear depiction of prostate anatomy. Altered anatomy will have an impact on signal intensity indicating the presence of cancer.
- Diffusion weighted imaging (DWI): functional sequence showing regionally altered cell density. High cellular density would be a sign of prostate cancer.
- Dynamic contrast-enhanced imaging (DCE): functional sequences highlighting altered vascular density. Prostate cancer tissue will show an early enhancement due to tumor angiogenesis.
When assessing mpMRI, radiologists will attribute a score to each lesion to represent the relative cancer risk (Table 2). PI-RADS’s objective is therefore not to increase the detection rate but to characterize the lesions and identify those likely to affect the patient’s overall survival.
Table 2 : PI-RADS score
|
PI-RADS 1 |
Very low (clinically significant cancer is highly unlikely to be present) |
|
PI-RADS 2 |
Low (clinically significant cancer is unlikely to be present) |
|
PI-RADS 3 |
Intermediate (the presence of clinically significant cancer is equivocal) |
|
PI-RADS 4 |
High (clinically significant cancer is likely to be present) |
|
PI-RADS 5 |
Very high (clinically significant cancer is highly likely to be present) |
mpMRI is reliable, accurate, and has become the primary imaging modality for prostate cancer detection. It is also being investigated as an alternative to PSA testing for screening purposes. Other anatomic modalities such as Computed Tomography (CT) are poor for detecting prostate cancer metastases or recurrences and are less used in clinical practice. However, high-resolution micro-ultrasound has emerged as a novel imaging modality combining the strength of both MRI (high resolution) and ultrasound (real time imaging).
While anatomic imaging is currently the gold standard, it has limitations, notably for the detection of small lesions and metastatic recurrence. Nuclear medicine modalities are therefore a major area of research in prostate cancer and are increasingly used for distant staging and assessment of recurrence.
Technetium 99m (99mTc) bone scans are widely used to evaluate bone metastases. The radiolabelled phosphonates such as methylene diphosphonate (MDP) bind to sites where there is intense bone remodelling (lytic or blastic). The tracer accumulates in bone metastases but also in degenerative, traumatic, and inflammatory lesions. Biomarkers have been developed for the quantitative assessment of bone scans such as the Bone Scan Index (BSI). Initially developed in the late seventies, the BSI now benefits from the advantages of artificial intelligence, making it a simple, reproducible, and practical method for quantifying bone metastases.
Positron Emission Tomography (PET)/CT is highly specific and can detect very small metastatic or recurrent malignant lesions even when PSA levels are low. PSMA PET/CT imaging is notably becoming the new gold standard in prostate cancer imaging (see section 3 for information on PET/CT tracers used in prostate cancer).
