The authors have done an excellent job of preparing a complete
and unbiased review of various imaging modalities that are currently available
or being developed for the evaluation of patients with carcinoma of the
prostate. In addition to the review of the literature, the authors have
succinctly summarized the technical details that allow the uninitiated to
understand the basic principles of these imaging technologies.
PSA and Multiple-Needle Biopsies vs Imaging Modalities
As the authors point out, at the present time, there is no
standard imaging modality that can reliably be used for the diagnosis or
accurate staging of prostatic cancer. Transrectal ultrasound (TRUS) has several
limitations, and the interpretation of its images is heavily influenced by the
expertise and experience of the user. As noted by the authors, several studies
have failed to demonstrate the superiority of TRUS over digital rectal
examination for the detection of local tumor extension. Likewise, from a
large study of 618 patients, Rifkin et al concluded that while Doppler
ultrasound imaging improves the positive predictive value (PPV) of TRUS, it
shows no demonstrable superiority over TRUS alone in diagnosing prostatic
Although some studies mentioned by the authors indicate that
tumor vascularization may correlate with rapid tumor growth and a higher
incidence of distant metastases, similar prognostic information can be obtained
from pretreatment prostate-specific antigen (PSA) levels and the pathologic
Gleason score in prostate biopsies. Furthermore, the use of ultrasound contrast
agents has no practical application at the present time; however, some are being
studied for use in prostate imaging (see Table 1 of the article by El-Gabry et
Other advanced ultrasound imaging methods using contrast
enhancementsuch as harmonic prostate imaging or intermittent ultrasound with
microbubble contrast agentsare in the early stages of development and are
certainly not ready for clinical use. More refinements of techniques and
appropriate clinical evaluations are in order.
Applications of MRI and CT Scans
Endorectal surface coils have been shown to be superior to
conventional body coil magnetic resonance imaging (MRI) in the staging of
prostate cancer; although the overall accuracy of staging has been found to be
only about 70%. Fast spin-echo imaging with a pelvic phased-array multicoil
may improve the accuracy of endorectal coil imaging along with fat-suppressed
MRI spectroscopy; however, these need to be properly evaluated in a clinical
The use of MRI spectroscopy with correlation of low citrate and
high choline levels associated with malignant disease were shown by Kaji et al
to improve the accuracy of MRI from 52% to 75% and specificity from 26% to 66%
for tumor detection. However, these predictive values are not better than
those obtained with simpler methods using pretreatment PSA levels and a Gleason
score upon prostate biopsy.
As Oesterling and Perez have shown, computed tomography
(CT) of the pelvis and periaortic lymph nodes and bone scans have limited value
in the evaluation of patients with prostate cancer who have pretreatment PSA
levels lower than 10 ng/mL and a Gleason score of 6 or lower. It has been noted
that eliminating these tests in the routine evaluation of patients with
favorable or intermediate prognostic risk factors will result in significant
savings in health-care costs.
At the present time, the use of radiolabeled antibodies
(indium-111 capromab pendetide [ProstaScint]) has been shown to have an 80% PPV
for the detection of extraprostatic disease, which is superior to that of CT,
MRI, or ultrasound (each about 50%). It was also reported that the predictive
value of a capromab scan prior to staging lymphadenectomy was significantly
better than the predictive value of models based on PSA levels (66.7%) and
Gleason score (about 45%).
However, at present, the majority of patients diagnosed with
prostate cancer have a 5% to 10% probability of pelvic lymph node metastases,
and the capromab pendetide scan has not been thoroughly evaluated in a clinical
setting. Therefore, it should not be used in the routine evaluation of patients
with favorable or intermediate prostate cancer, and its value in patients with
high-risk tumors has not been documented.
Likewise, bone scans to detect bony metastases have a low
applicability in patients with localized carcinoma of the prostate with
pretreatment PSA levels of 10 ng/mL or less and a Gleason score of 6 or less.
Not obtaining these studies in this selected group of patients results in
significant financial savings.[4,5]
Delivery of Higher Doses
to Direct Targets
Practical applications of some imaging modalities lie in the
treatment plan of radiation therapy. Frequently, CT or MRI is used to outline
the prostatic volume as well as the bladder, rectum, and other pelvic organs
that need to be spared during external-beam radiation therapy. Application of
these modalities and recent computer developments with more sophisticated,
robust software have effected accurate three-dimensional (3D) dose-computation
models, which, in turn, have produced dose-escalation studies featuring delivery
of higher doses to the target volume with maintenance of normal tissue doses
below tolerance levels. This has translated into improved local tumor control
and chemical disease-free survival, lower treatment morbidity, and better
quality of life.
Recently, Ling et al described more complex treatment techniques
using functional and metabolic imaging to determine tumor subpopulations that
require higher doses. With this information, intensity-modulated radiation
therapy (IMRT) has been used to deliver doses as high as 81 Gy to limited
volumes of the prostate.
The Role of Ultrasound
Ultrasound has a definite application in preplanning
interstitial brachytherapy for prostate cancer, as well as a role during the
procedure itself. The images are used to determine the prostatic volume, patient
eligibility for the procedure, appropriate placement, and intensity of the
radioactive sources (iodine-125 or palladium-123 for low-dose-rate permanent
implants or iridium-192 for temporary high-dose-rate implants).
During the implant procedure, real-time ultrasound allows more
accurate placement of catheters or guides and insertion of the radioactive
sources in the low-dose-rate permanent implants. Several authors have reported
on the use of ultrasound to calculate real-time doses of irradiation with high-dose-rate
brachytherapy.[8,9] Dose optimization has become an achievable goal using this
modality. Currently, the CT or MRI is used following the low-dose-rate
permanent implant procedure in order to more accurately calculate the final dose
distribution. This is an important quality-assurance step that aids the
radiation oncologist in improving technical skills. It also helps him/her better
determine the dose administered to the tumor.
In conclusion, current imaging modalities have no significant
clinical application in the detection and diagnosis of carcinoma of the
prostate, compared with pretreatment PSA and multiple-needle biopsies. Imaging
modalities are of value in the evaluation of tumor extent and staging of
selected patients with carcinoma of the prostate who have intermediate- or
high-risk prognostic factors. Some imaging modalities, including CT scan and MRI
of the pelvis, have definite applications in the planning of radiation therapy
with 3D conformal or intensity-modulated techniques and ultrasound for
brachytherapy of localized prostate cancer. It remains to be demonstrated
whether higher levels of chemically determined disease-free survival will ensue
from the use of MRI spectroscopy (which can detect more aggressive segments of
prostate cancer within the gland) to direct the delivery of higher doses of
radiation therapyeither with IMRT or brachytherapy.
It is obvious that some of these imaging modalities and others
under development may play an important role in the management of patients with
localized prostate cancer, and additional clinical studies are necessary.
Biological imaging that yields genotype and phenotype information may be useful
in the diagnosis, staging, and treatment of patients with prostate cancer. A
great deal of investigation must be done before these new modalities can be
routinely used in clinical practice.
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