Ultrasound

Since the introduction of grayscale transrectal ultrasound imaging for prostate cancer in the late 1960s, technical developments have improved image quality. However, in conjunction with biopsy, it remains a test with widely variable sensitivity and specificity, ranging from 50% to 92% and from 46% to 91%, respectively. Furthermore, it has been shown that the positive predictive value of transrectal ultrasound-guided biopsies may be as low as 15.2%, compared with 28% for digital rectal examination. This is attributed to variable tumor echogenicity, the multifocal nature of disease, concomitant inflammatory or pathologic processes, and operator inexperience.

Although three-dimensional ultrasonography, color Doppler, and microbubble contrast agents have been shown to improve sensitivity, specificity, and accuracy to varying degrees, ultrasound remains primarily a cost-effective imaging modality to guide transrectal biopsy. Ultrasound has an adjunct role during seed placement in brachytherapy, and targeted therapies such as magnetic resonance imaging (MRI)-guided focused ultrasound and cryoablation of focal lesions.

Elastography relies on detecting variance in tissue compliance, generated by compression and relaxation, used in conjunction with an imaging modality such as ultrasound or MRI. With ultrasound, Elastography has a sensitivity and specificity of more than 75%, and a positive predictive value of up to 88%.

Computed tomography

Although computed tomography (CT) has a limited role in the detection of prostate cancer, in patients with highly elevated prostate-specific antigen (PSA) levels, it may be a useful modality to assess nodal involvement, capable of scanning the entire body in a short period of time. However, MRI and dedicated bone scans have been shown to be superior in their assessment of both nodes and bone metastases.

Positron emission tomography (PET) relies on increased cellular metabolism of radiotracer by tumor cells, to identify loci of tumor or recurrence. PET has not been widely used in prostate cancer, however a potential role is emerging for locoregional nodal staging, detection of recurrent and metastatic disease in biochemical relapse, and assessment of tumor response to therapy.

Computed tomography

Although computed tomography (CT) has a limited role in the detection of prostate cancer, in patients with highly elevated prostate-specific antigen (PSA) levels, it may be a useful modality to assess nodal involvement, capable of scanning the entire body in a short period of time. However, MRI and dedicated bone scans have been shown to be superior in their assessment of both nodes and bone metastases.

Positron emission tomography (PET) relies on increased cellular metabolism of radiotracer by tumor cells, to identify loci of tumor or recurrence. PET has not been widely used in prostate cancer, however a potential role is emerging for locoregional nodal staging, detection of recurrent and metastatic disease in biochemical relapse, and assessment of tumor response to therapy.

Magnetic resonance imaging

Since its introduction into clinical practice, MRI has provided a previously unparalleled opportunity to visualize prostate tissue detail without patient exposure to ionizing radiation. The myriad of refinements that have been necessary to increase signal-to-noise ratio and achieve higher spatial, spectral, and temporal resolution are beyond the scope of this article, however consensus is being reached within the literature that a 3-Tesla strength magnetic field, and use of a pelvic phased-array coil and/or endorectal pelvic phased-array coil represent the current gold standard.

MRI encompasses various sequences, each suited to expose a particular anatomic or pathophysiologic feature of disease. Multiparametric imaging is therefore necessary to fully use the potential of MR and to accurately stage and monitor disease. Standard T1- and T2-weighted images are used in concert to define morphology and distinguish between areas of signal drop arising from foci of cancer as opposed to artifact related to hemorrhage or inflammatory change from recent biopsy.

Additional functional sequences such as dynamic contrast enhanced (DCE), diffusion-weighted imaging (DWI) and magnetic resonance spectroscopy (MRS) are used, each of which provides unique information on tissue characteristics. DCE acquires data on tissue perfusion characteristics and tumor wash-in and wash-out contrast. These rely on pathophysiologic principles that tumors display increased angiogenesis, thus are expected to show early and increased enhancement. Graphic representations of the data are generated, from which computer-assisted quantitative analysis is derived.

DWI records the microscopic motion of water molecules within tissue, based on the theory that poorly differentiated cancers show marked tissue heterogeneity and decreased water movement. An apparent diffusion coefficient (ADC) map is generated, and ADC values then acquired, assisting in detection of foci of disease. Spectroscopy examines cellular metabolism within single or multiple voxels, using high levels of choline and low levels of citrate as likely areas of cancer. Metaanalyses have shown in certain patient populations that MRS has high specificity, but low sensitivity, suggesting a role as a rule-in test for low-risk patients.

Various novel radiotracers and positron-emitting radioisotopes have been proposed, including [ ¹¹ C]choline, [ ¹⁸ F]fluorocholine and [ ¹¹ C]acetate, and [ ¹⁸ F]fluoride, [ ¹¹ C]methionine and [ ¹¹ C]tyrosine, respectively. These together with radiolabeled monoclonal antibodies against specific cancer cell surface antigens, may represent more sensitive means of tumor detection, either for staging purposes or evaluating biochemical recurrence.

Currently, state of the art imaging of prostate cancer involves multiparametric MRI at either 3-Tesla or 1.5 Tesla, incorporating T1- and T2-weighted sequences, together with DWI and DCE. It has been proposed that prebiopsy MRI may obfuscate the potential confusion generated by residual blood products, potential distortion of native tissue, and local inflammation. However, given the increasing frequency of this disease and the potential cost burden of obtaining an MRI because of increased PSA levels and/or abnormal prostate digital examination, it is unlikely to be a viable solution. However, increasing evidence suggests an important role for MRI prior to, or during, a biopsy can be very useful, allowing targeted biopsies. This should lead to increased accuracy and assist in therapy planning. Ongoing research will attempt to better delineate foci of disease and achieve greater sensitivity and specificity, with the use of more sophisticated imaging techniques, postprocessing software, and novel biomolecular markers.