Mitchell D. Schnall, MD, PhD •¿ Yutaka Imai, MD •¿ John Tomaszewski, Robert E. Lenkinski, PhD •¿ Herbert Y. Kressel, MD
MD •¿ Howard M. Pollack, MD .
Prostate Cancer: Local Staging with Endorectal Surface Coil MR Imaging' Endorectal surface coil magnetic resonance (MR) imaging was used to stage the local extent of prostate cancer in 2.2 patients. The endorectal coil images were acquired with a 10-12-cm field of view and a 4-mm section
thickness.
All pathologic
specimens were reviewed by one pathologist. Endorectal surface coil MR imaging was 82% accurate in the differentiation of stage B from stage C cancer. One case was overestimat ed, and three cases were underesti mated. The three underestimated cases consisted of two cases of mi croscopic capsular invasion and one case of minimal seminal vesicle in vasion. In a comparison of retro spective, blinded readings of endo rectal coil and body coil images, there was an average improvement in accuracy of 16% in staging pros tate cancer with endorectal coil im ages.
P
ROSTATEcancer is a significant health problem for the elderly man. It is the most common malig nancy
and the third
leading
cause of
cancer-related death in men (1). If detected when confined to the pros tate, prostate cancer can be cured by means of a radical prostatectomy. Unfortunately,
historically
this pro
cedune has been associated with sig nificant morbidity—most notably impotence and incontinence. Im pnovements in surgical technique that reduce morbidity have lead to an increase in the popularity of radical prostatectomy in the treatment of lo calized prostatic cancer (2,3). This has placed more emphasis on accurate preoperative staging to identify open ative candidates. An important aspect of staging prostate cancer is deter mining the local extent of the tumor. Magnetic resonance (MR) imaging with a body coil has been helpful in this regard,
but has limited
spatial
Radiology
1991; 178:797-802
ease. Abnormal in the seminal
low signal intensity with vesicles on the long TRITE
images was considered to be evidence of seminal vesicle invasion for both exami nations. The improved resolution of the endonectal coil images allowed two addi tional criteria to be used. The prostatic capsule, which is not reliably seen on the body coil images, is well seen on the en dorectal coil images. Thus, disruption of the capsule
on the endorectal
coil images
could be used as a sign of capsular inva sion. The high resolution of the endorec tal coil examination also provided im
Index terms: Prostate, MR studies, 844.1214 •¿ resolution in imaging small tumors in the pelvis (4,5). We have recently Prostate, neoplasms, 844.32
described
The body coil and surface coil long and short repetition time/echo time (TR/TE) axial images for 18 patients were later re read blindly by each of three readers. The surface coil and body coil images were read in separate sessions. The prospective and retrospective readings were corn pared with the pathologic findings. The criteria used to evaluate the extent of the prostatic tumor were similar for both the body coil and endorectal surface coil images. Infiltration of the penipros tatic fat, peniprostatic veins, or neurovas cular bundle was considered evidence of capsular invasion and, thus, stage C dis
proved images of the seminal vesicle
the use of an endonectal
architecture.
Prostate cancer, when invad
surface coil to obtain very high meso lution images of the prostate (6,7). Here, we report on the application of
cal thickening of the tubular walls on the
this technique
endorectal
in the local staging
of
prostate cancer and compare it with the body coil technique.
ing the seminal
vesicles,
tends
to infil
trate the penitubulan stroma (9). Thus, fo coil images
was considered
to
be evidence of early seminal vesicle inva sion.
MR Imaging MATERIALS
AND
METHODS
This study included 22 patients who underwent
endorectal
coil MR imaging
examination of the prostate and radical prostatectomy. On the basis of digital nec
tal examination and measurement of se 1 From
the
Departments
of
Radiology
(M.D.S., H.P., R.L., H.K.) and Pathology (J.T.), the University of Pennsylvania School of Medi cine, 3400 Spruce St, Philadelphia, PA 19104, and the Department of Radiology, Keio Un!versity School of Medicine, Tokyo (Y.I.). From the 1989 RSNA scientific assembly. Received April 26, 1990; revision requested June 22; nevi sion received October 25; accepted October 29. Address reprint requests to M.D.S. ©RSNA,1991
rum prostate specific antigen and acid phosphatase, all patients were believed to
All imaging was performed with a Signa (GE Medical Systems, Milwaukee) system at 1.5 T with a 256 X 128 matrix and two acquisitions. For the body coil images
the patient
was imaged
in the
prone position, and the rectum was in flated with air. The short TRITE (600/20
[TR msec/TE msec]) axial images were
have stage B lesions. The time interval be tween biopsy and MR imaging examina
obtained with a 10-mm section thickness, a 2.5-mm gap, and a 36-cm field of view.
tion varied from 1 week to 3 months. All patients were imaged with both body and surface coils as described later. All images
The long TRITE (2,500/40,80) axial im ages were obtained with a 5-mm section thickness, a 1-mm gap, and a 36-cm field
were read prospectively, with particular reference made to the local stage of the tumor determined according to the Jew ett-Whitmore staging classification (8).
Abbreviations: tion time.
TE
echo time, TR = nepeti
797
of view. Optional images were often ob tamed in the sagittal plane, but these were available in less than half of the pa tients.
The surface coil images were obtained with the endonectal surface coil described previously (Mednad, Pittsburgh) (7). The coil was a 3.5 X 6-cm oval surface coil mounted on the inner surface of a bal loon. The balloon was concave anteriorly
to assure reliable seating against the pros
tate (Fig la). The coils were tuned near the resonant frequency of 63.8 MHz with a capacitor placed across the coil within the balloon. Fine-tuning was accom plished by adding a variable capacitor at the end of the coil shaft, away from the patient.
It was necessary
to have tuning
adjustment to compensate for the varia tion in balloon inflation and loading. The coil was decoupled from the body coil during transmit as previously described
(7). The endorectal with
the patients
surface coil was placed in the lateral
position.
a.
b.
Figure 1. (a) Close-up photo of the inflated endorectal surface coil shows the concave ante nor surface of the balloon (arrows). (b) Sagittal localizing image (400/20) obtained with a body coil demonstrates proper positioning of the coil (large white arrows) within the rec tum. The coil is large enough to cover the entire length of the prostate gland (thin white an row) and the seminal vesicles (black arrow).
An introducer provided the torque con trol needed to reliably position the bal
loon. The balloon was inflated with ap proximately
100 cm3 of air. The patients
were imaged in the supine position and were intramuscularly administered 1 mg of glucagon
(Lilly,
Indianapolis)
prior
to
imaging. An initial sagittal localizing image was obtained to confirm coil positioning and
select locations for the axial images (Fig ib). The axial long and short
TR/TE
en
dorectal coil images were obtained with a 3-4-mm section thickness, a 1-mm gap, and a 10-12-cm field of view. Additional images were acquired in the sagittal on coronal plane for approximately 30% of
the patients. The images were processed with a spatial filter to remove some of the surface coil falloff and make the images easier to display. An oversampling meth od was used to reduce
aliasing
artifacts.
On approximately half of the long TR/TE images, gradient moment nulling was used to reduce motion artifacts. The fre quency encoding direction was anterior to posterior for all patients to prevent phase-related motion artifacts from ob scuring
b.
Figure 2. Advantages of endorectal MR imaging. Long TR/TE (2,500/80) surface (a) and body (b) coil images of a prostate with marked benign prostatic hyperplasia. The enlarged central gland is seen with both coils. The peripheral zone is compressed into a thin rim of tissue, which is seen only on the endorectal coil image (small arrows). Note the excellent vi sualization of the benign prostatic hyperplasia nodules (large arrows) and associated low signal-intensity rim on the endorectal coil image.
the prostate.
Pathologic
Examination
Resection specimens were fixed over night in 10% neutral buffered formalin. The margins were inked and the prostate glands
a.
serially
sectioned
at 3—5-mm inter
vals in a plane perpendicular to the ure thral axis. Histologic samples were taken from areas that were grossly suggestive of carcinoma (eg, areas of nodule formation and/on tissue with yellow-orange color ation). Samples were always taken so as to include the nearest prostatic capsular margin. Routine selected site sections
were taken from the urethral margins in each lobe and the seminal vesicles. Each resected lymph node was embedded in toto. An average of 20 samples were ex
amined per patient. All specimens were reviewed by one pathologist. Each gland 798 •¿ Radiology
was evaluated for pathologic stage and extent of capsular invasion. Microscopic capsular invasion was defined as micro scopic foci of tumor seen outside the prostatic capsule at histologic examina tion, while the prostate capsule appeared grossly normal.
RESULTS As reported previously (7), the en domectal surface coil images provided more detailed visualization of the pnostate and peniprostatic structures than the body coil images. This was
particularly true when the prostate was enlarged from coincidental be nign prostatic hyperplasia. In cases of marked benign pnostatic hyperplasia,
it was
often
difficult
zonal architecture
to identify
the
of the prostate (Fig
2). Seminal vesicle architecture and the prostatic capsule were well seen on all of the endorectal coil images.
In 20 of the 22 patients, prostatic ma lignancy was visualized as a low-sig nal-intensity peripheral zone lesion on long TR/TE images. In one pa tient, the tumor had high signal in tensity on long TRITE images. This lesion was shown to have mucinous
elements at pathologic examination (Fig 3). In another patient, no abnom mality This
was seen on the MR images. patient
foci of tumor tion.
only
had
microscopic
at histologic
examina
March 1991
cancer. With the earlier identification of more lesions, apparently enhanced by the growing use of tnansnectal ul trasound (US) (12), theme is an in creased
responsibility
for umologists
and radiologists to accurately stage prostate cancer to identify surgical candidates. In clinical practice, it is very important
a.
b.
Figure 3. (a) Endorectal surface coil long TR/TE image of a patient with stage B mucinous adenocarcinoma of the prostate. Note the high-signal-intensity tumor that lacks the smooth border and complete low-signal-intensity rim that are typical of benign prostatic hyperpla sia (arrows). (b) Transrectal US scan of the same patient demonstrates that this tumor has mixed echogenicity (arrows).
5) that was seen at pathologic
In general, images obtained with use of gradient moment nulling dis played less motion artifact than those obtained without it. However, no dif ferences in contrast were noted be tween these two sets of images. Fourteen patients had surgical
pathologic disease,
confirmation
and eight
of stage B
had stage C dis
ease. Two of the eight patients with stage C disease had seminal vesicle invasion and six had capsular inva sion. The results of the prospective readings are compared with the pathologic staging in Table 1. The prospective MR imaging stage agreed with the pathologic findings of the disease in 18 of the 22 patients (82% accuracy). One case was ovenstaged on the basis of the incorrect reading of capsular invasion. This was the first of the cases included in the se ries and, in retrospect, was probably misinterpreted.
Pathologically
proved stage C tumors in three pa tients were incorrectly staged as stage B tumors at endonectal MR im aging. In two of those patients, there was microscopic capsular invasion only (Fig 4). In the third, theme was subtle seminal vesicle invasion (Fig Volume 178 •¿ Number 3
exami
tient (Fig 6).
The 18 patients whose images were read retrospectively by the three readers
included
five pa
tients with pathologically proved stage C lesions and 13 with patholog ically proved stage B lesions. The me sults of the retrospective
readings
are
shown in Table 2. In each case, theme was an improvement in the staging accuracy with use of the endorectal surface coil images relative to the body coil images. The average accu racy for all three readers was 69%
with the body coil and 84% with the surface coil images. The average im provement in accuracy was 16% (mange, 11%-22%). DISCUSSION Prostate cancer will affect 30% of men over the age of 50 years (10). When
localized
to the prostate
(stage
A or B), it can be cured with radical prostatectomy (1 1). Recent advances in the surgical
technique
disease
issue in the staging evaluation of a patient with prostatic carcinoma,
nation only on careful sectioning of the specimen. If we exclude cases of microscopic capsular invasion, the prospective readings of the endomec tal surface coil images were 91% accu rate. The sensitivity and specificity of the prospective endomectal coil im ages for depicting stage B, or opera ble, lesions were 93% and 84%, me spectively. The endorectal coil im ages were critical in the establish ment of the correct stage in one pa
blinded
to distinguish
localized to the prostate (stage A or B) from tumor that has extended be yond the confines of the gland (stage C or 0), since this will influence therapy. In general, patients with stage A or B lesions are candidates for extirpative sungical therapy, while those with stage C and D lesions are treated with radiation and hormonal therapy. The local extent of the tu mom distinguishes stage B from C dis ease. This is perhaps the most critical
that reduce
the morbidity of this procedure (2) have increased the popularity of de finitive surgical therapy for prostate
since it divides patients into operable and nonopenable groups. The distinc tion between
patients
with stage C
and stage D disease is also important because these two categories carry different prognoses and require dif fenent therapeutic approaches. Stage D disease usually is seen as lymphad enopathy or bone metastases. These entities are not detected on endonec tal MR images,
and, therefore,
the
endonectal MR examination must be augmented with other modalities to exclude bone metastases and lymph adenopathy. We find it convenient to obtain short TR/TE axial body coil MR images to exclude lymphadenop athy and a bone scan to exclude
bone
metastases. For years, the mainstay of local staging of prostate cancer has been the digital rectal examination. Digital rectal examination, however, has been shown to underestimate the ex tent of the cancer in 25%-78% of pa tients (10,13). The sensitivity of com puted tomography for depicting stage C disease is 35%—55%(14,15), which gives it little or no advantage over the digital rectal examination in the local staging
of pnostatic
cancer.
While some advocate tnansnectal US as an improved method of staging carcinoma of the prostate (16), others are of the opposite opinion (17). Scar dino et al (16) found tmansnectal US to be 86% accurate for staging, while Andniole
nificant patients. known dent, it lished
et al (17) experienced
sig
understaging (62%) in their Since tmansmectal US is to be very operator depen is not surprising that pub
reports
of its accuracy
vary
greatly. This is particularly evident with regard to seminal vesicle inva Radiology •¿ 799
sion, where accuracy of preoperative detection manges from 29% to 82% (16). Because of the high soft-tissue con tnast and multiplanar
capability
of
MR imaging, it has been proposed as a method to improve local staging of prostate
cancer.
reported
Its accuracy
has been
to be from 69% to 89% (5,18-
20). A large multicenter
study
that
compared body coil MR imaging with tnansnectal US and digital rectal examination in staging prostate canci noma in 234 surgically confirmed cases found the accuracy of body coil MR imaging to be 69% and the accu racy of US to be 58% (18). In a previ ous series, we found body coil MR
imaging to be 78% accurate in the de termination of overall local staging of prostate cancer (5). Our accuracy for determining seminal vesicle inva sion was 83%; we were only 44% sen sitive in detecting direct capsular in
vasion. The unsatisfactory the detection
of capsular
Figure 5. Long TR/TE body coil (a) and endorectal surface coil (b) images of a patient with early seminal vesicle invasion. The low signal intensity (arrows in a) on the body coil image represents either partial volume averaging with the top of an enlarged prostate or invasion from prostate cancer. The detailed architecture of the seminal vesicles is seen only on the surface coil image. The area of focal low signal intensity (small arrows in b) represents inva sion of prostate cancer. The ampulla of vas deferens (large arrows in b) normally has thick ened walls.
results for invasion
were in large part due to the limited
(6,7,21). The endonectal surface coil images provide reliable identifica tion of the prostatic capsule and bet ten visualization of the periprostatic
better than that achieved in the same cohort of patients examined with the body coil. The prospective sensitivity
is often difficult to identify the pen pnostatic veins and neurovascular bundles, particularly if the prostate is
architecture
for the detection
enlarged (Fig 3). These deficiencies served as our stimulus to develop an improved technology for prostatic MR imaging. We and others have shown previ
tate cancer, we have found a signifi cant improvement over staging with
ously that the use of an endomectal
confirmed
surface coil to image the prostate sig
This was essentially
nificantly
of our multiple
spatial
resolution
of body coil MR
imaging. The capsule is not reliably seen on the body coil images, and it
improves
800 •¿ Radiology
resolution
and seminal vesicles. In
applying this technique to stage pa tients with clinically localized pros
the body coil in the same cohort of patients. The prospective readings for the 22 patients with surgically
lesions were 82% accurate. the same as that
reader retrospective
series,
in which
the
accuracy
was
16%
of stage C disease in
the patients with surgically con firmed lesions was 63%. Although this
seems
low,
it is important
to un
derstand the selection bias in this study. Unlike our original pilot study (5), in which all patients were accept
ed for examination,
the current co
homt consisted of patients with nela tively small lesions, and the diffenen tiation
between
stage
B and
C disease
March 1991
to be from biopsy-related hemom rhage. On occasion, abnormalities with high signal intensity were also seen in the pemiprostatic fat on the long TRITE images. We believe this represents biopsy-related edema. The criteria used for assessing the integmi ty of the prostatic capsule had to be modified during the study so that mild signal
abnormalities
within
the
capsule, adjacent to extensive biopsy related changes, were not considered evidence of capsular invasion. Fail ume to recognize biopsy-related changes may thus result in ovemesti a.
mating
b.
Figure 6. Long TRITE body coil (a) and endorectal coil (b) images of a patient with patho logically confirmed stage C prostate cancer. A low-signal-intensity lesion is seen on both im ages (arrows in a, large black arrows in b), representing the cancer. The capsule is seen only on the endorectal coil images (small black arrows in b). A disruption of the capsule by tumor infiltrating periprostatic fat is seen only on the surface coil image (white arrows in b). This patient was correctly determined to have stage C disease at prospective MR imaging evalua tion.
the disease.
Unfortunately,
with use of the criteria stated above, the disease in patients with early cap sulam invasion biopsy-related
in an area of extensive signal change might
be underestimated. However, this never occurred in our group of pa tients.
CONCLUSION MR imaging of the prostate with an endomectal surface coil results in images
of the pnostatic
and penipnos
tatic anatomy that are more detailed than those provided
by means
of
body coil MR imaging. Although we are in the early stages of its clinical application, we believe that the me suits
could not be readily accomplished with conventional means. This me my who had perplexing tumors at digital rectal examination and subtle findings on endomectal MR images.
logically proved seminal vesicle in vasion underwent radical prostatec tomy. This also reflects the selection bias in our patients with pathologi cally confirmed lesions. Although the seminal vesicle invasion was missed on the prospective reading in
Nonetheless,
one patient, in retrospect,
subtle focal
thickening of the seminal bulan wall was suggestive
vesicle tu of seminal
suited in inclusion tients undergoing
of only those pa radical prostatecto
the positive
predictive
value of endomectal MR imaging for the determination of stage C disease was 84%, and the negative predictive value was 82%. Despite the population bias, the 67% sensitivity for the detection of capsular invasion on the endorectal images represents a significant im
vesicle invasion (Fig 5). This case was diagnosed correctly by two of the three investigators in the netnospec tive readings. Although there are too few surgically confirmed cases to make a meaningful comment on the
provement oven the 44% sensitivity for detection of capsular invasion on body coil images that we reported
reliability of tubular wall thickening as an early marker of seminal vesicle
previously (5). If we cases of microscopic sion, the sensitivity sion is 100%. This is
will be quite useful. With more expe nience, even cases of minimal semi nal vesicle invasion should be readi ly identified.
exclude the two capsular inva for capsular inva believed to be a
direct result of the improved visual ization of the prostatic capsule on the endorectal coil images. Five of the nine discordant
readings
of the body
and surface coil images were due to better visualization of the prostatic capsule and neumovascular bundle on the surface coil images (Fig 6). Only the two patients
Volume 178 •¿ Number 3
with patho
invasion, we believe that this sign
Another
important
issue to be con
sidened is biopsy-related signal ab nommalities. Most of the patients in this study were imaged within a month of their pmostatic biopsy. Most commonly, an area of high signal in tensity was seen in the region of the
previous biopsy on both long and short TR/TE images.
This is believed
presented
indicate
a clinical
ad
vantage of endonectal surface coil MR imaging oven body coil imaging in the preoperative evaluation of pa tients with prostate cancer. The most important advantage of the endonec tal coil images is the improved visu
alization of the prostatic capsule and the neunovasculam bundle. This is particularly true in evaluating a large
prostate, where the peripheral zone and pemiprostatic anatomy can be dif ficult to identify
on the body coil im
ages. While endomectal MR imaging of the prostate was 100% sensitive to grossly detectable prostatic capsule,
invasion of the we were not able to
detect subtle microscopic capsular in vasion. However, in both cases of mi croscopic invasion presented in this report, the pathologist described the gross appearance gin to be normal.
of the capsular The improved
mar im
ages of the seminal vesicles should also improve the sensitivity to semi nal vesicle
invasion,
although
few cases of seminal vesicle were included in this study conclusive. Although more with radiologic-pathologic tion is needed
to improve
too
invasion to be work commela
our ability
to interpret the endonectal coil im ages, it appears that this modality is Radiology •¿ 801
destined to become the method of choice in the determination of the lo cal extent of prostate cancer. U References 1. Silvery E, Boring CC, Squires TS. Cancer statistics. CA 1990; 40:9-26. 2. Eggleston JC, Walsh PC. Radical pnosta tectomy with preservation of sexual func tion: pathologic findings in the first 100 cases. J Urol 1985;134:1146—1148. 3, Smith JA Jr. Middleton RG. Clinical man agement of prostatic cancer. Chicago: Year Book Medical, 1987; 55. 4. Tempany C, Rahmouni A, Zerhouni E, Ep stein J, Walsh P. Evaluation of the role of MRI in predicting invasion of the neuro vascular bundle by prostate cancer. In: Proceedings of the Eighth Annual Society of Magnetic Resonance in Medicine Meet ing. Amsterdam: Society of Magnetic Res onance in Medicine, 1989; 129. 5. Bezzi M, Kressel HY, Allen KS, et al. Prostatic carcinoma: staging with MR at 1.5 T. Radiology 1988; 169:339-346. 6. Schnall M, Kressel HY, Pollack HP, Len kinski RL. The development of an intra cavitary inflatable surface coil for high resolution proton imaging and spectrosco py (abstr). In: Book of abstracts: Society of
Magnetic Resonance in Medicine 1986.
802 •¿ Radiology
7.
8.
9.
10.
11.
12.
13.
14.
Vol 1. Berkeley, Calif: Society of Magnetic Resonance in Medicine, 1986; 197. Schnal! MD, Lenkinski RL, Pollack HM, Imai Y, Kressel HY. Prostate: MR imag ing with an endorectal surface coil. Radi ology 1989; 172:570-574. Sagani P, Whitmore W Jr. Update in stag ing systems. In: Bruce AW, Trachtenberg J, eds. New York: Springer-Verlag, 1987; 107. Wheeler TM. Anatomic considerations in carcinoma of the prostate. Urol Clin North Am 1989; 16:623—634. Scardino PT. Early detection of prostate cancer. Urol Clin North Am 1989; 16:635655. Walsh PC, Lepor H. The role of radical prostatectomy in the management of pros tate cancer. Cancer 1989; 60 (suppl 3):526537. Lee F, Littrup PJ, Torp-Pederson ST. et al. Prostate cancer: comparison of transnectal ultrasound and digital rectal examination for screening. Radiology 1988; 169:389394. Fair WR, Kadman D. Carcinoma of the prostate diagnosis and staging: current status and future prospects. World J Urol 1983; 1:3—18. Golambu M, Morales P. Al-Askani SA, Shulman Y. CAT scanning in staging of prostatic cancer. Urology 1981; 18:305— 308.
15.
16.
17.
18.
19.
20.
21.
Platt JF, Bree RL, Schwab RE. The accura cy of CT in the staging of prostatic carci noma. AJR 1987;149:315—318. Scardino PT, Shinohara K, Wheeler TM, Carter S. Staging of prostate cancer: val ue of ultrasound. Urol Clin North Am 1989; 16:713-740. Andniole GL, Colpen DE, Mikkelson DJ, Catalona WJ. Sonographic and patholog ic staging of patients with clinically local ized prostate cancer. J Unol 1989; 142:1259-1261. Rifkin M, Zerhouni E, Gatsonis C, et a!. Comparison of magnetic resonance imag ing and ultrasonography in staging early prostate cancer. N Engl J Med 1990; 323:621—626. Hnicak H, Dooms GC, Jeffrey RB, et al. Prostatic carcinoma: staging by clinical as sessment, CT, MR imaging. Radiology 1987;162:331-336. Biondetti PR, Lee JKT, Ling D, Catalona WJ. Clinical stage B prostate carcinoma: staging with MR imaging. Radiology 1987; 162:235-239. Martin JF, Hajek P. Baker L, Gylys-Monin V, Fitzmorris-Glass R, Mattrey RR. Inflat able surface coil for MR imaging of the prostate. Radiology 1988; 167:268-270.
March 1991
MD •¿ Howard M. Pollack, MD .
Prostate Cancer: Local Staging with Endorectal Surface Coil MR Imaging' Endorectal surface coil magnetic resonance (MR) imaging was used to stage the local extent of prostate cancer in 2.2 patients. The endorectal coil images were acquired with a 10-12-cm field of view and a 4-mm section
thickness.
All pathologic
specimens were reviewed by one pathologist. Endorectal surface coil MR imaging was 82% accurate in the differentiation of stage B from stage C cancer. One case was overestimat ed, and three cases were underesti mated. The three underestimated cases consisted of two cases of mi croscopic capsular invasion and one case of minimal seminal vesicle in vasion. In a comparison of retro spective, blinded readings of endo rectal coil and body coil images, there was an average improvement in accuracy of 16% in staging pros tate cancer with endorectal coil im ages.
P
ROSTATEcancer is a significant health problem for the elderly man. It is the most common malig nancy
and the third
leading
cause of
cancer-related death in men (1). If detected when confined to the pros tate, prostate cancer can be cured by means of a radical prostatectomy. Unfortunately,
historically
this pro
cedune has been associated with sig nificant morbidity—most notably impotence and incontinence. Im pnovements in surgical technique that reduce morbidity have lead to an increase in the popularity of radical prostatectomy in the treatment of lo calized prostatic cancer (2,3). This has placed more emphasis on accurate preoperative staging to identify open ative candidates. An important aspect of staging prostate cancer is deter mining the local extent of the tumor. Magnetic resonance (MR) imaging with a body coil has been helpful in this regard,
but has limited
spatial
Radiology
1991; 178:797-802
ease. Abnormal in the seminal
low signal intensity with vesicles on the long TRITE
images was considered to be evidence of seminal vesicle invasion for both exami nations. The improved resolution of the endonectal coil images allowed two addi tional criteria to be used. The prostatic capsule, which is not reliably seen on the body coil images, is well seen on the en dorectal coil images. Thus, disruption of the capsule
on the endorectal
coil images
could be used as a sign of capsular inva sion. The high resolution of the endorec tal coil examination also provided im
Index terms: Prostate, MR studies, 844.1214 •¿ resolution in imaging small tumors in the pelvis (4,5). We have recently Prostate, neoplasms, 844.32
described
The body coil and surface coil long and short repetition time/echo time (TR/TE) axial images for 18 patients were later re read blindly by each of three readers. The surface coil and body coil images were read in separate sessions. The prospective and retrospective readings were corn pared with the pathologic findings. The criteria used to evaluate the extent of the prostatic tumor were similar for both the body coil and endorectal surface coil images. Infiltration of the penipros tatic fat, peniprostatic veins, or neurovas cular bundle was considered evidence of capsular invasion and, thus, stage C dis
proved images of the seminal vesicle
the use of an endonectal
architecture.
Prostate cancer, when invad
surface coil to obtain very high meso lution images of the prostate (6,7). Here, we report on the application of
cal thickening of the tubular walls on the
this technique
endorectal
in the local staging
of
prostate cancer and compare it with the body coil technique.
ing the seminal
vesicles,
tends
to infil
trate the penitubulan stroma (9). Thus, fo coil images
was considered
to
be evidence of early seminal vesicle inva sion.
MR Imaging MATERIALS
AND
METHODS
This study included 22 patients who underwent
endorectal
coil MR imaging
examination of the prostate and radical prostatectomy. On the basis of digital nec
tal examination and measurement of se 1 From
the
Departments
of
Radiology
(M.D.S., H.P., R.L., H.K.) and Pathology (J.T.), the University of Pennsylvania School of Medi cine, 3400 Spruce St, Philadelphia, PA 19104, and the Department of Radiology, Keio Un!versity School of Medicine, Tokyo (Y.I.). From the 1989 RSNA scientific assembly. Received April 26, 1990; revision requested June 22; nevi sion received October 25; accepted October 29. Address reprint requests to M.D.S. ©RSNA,1991
rum prostate specific antigen and acid phosphatase, all patients were believed to
All imaging was performed with a Signa (GE Medical Systems, Milwaukee) system at 1.5 T with a 256 X 128 matrix and two acquisitions. For the body coil images
the patient
was imaged
in the
prone position, and the rectum was in flated with air. The short TRITE (600/20
[TR msec/TE msec]) axial images were
have stage B lesions. The time interval be tween biopsy and MR imaging examina
obtained with a 10-mm section thickness, a 2.5-mm gap, and a 36-cm field of view.
tion varied from 1 week to 3 months. All patients were imaged with both body and surface coils as described later. All images
The long TRITE (2,500/40,80) axial im ages were obtained with a 5-mm section thickness, a 1-mm gap, and a 36-cm field
were read prospectively, with particular reference made to the local stage of the tumor determined according to the Jew ett-Whitmore staging classification (8).
Abbreviations: tion time.
TE
echo time, TR = nepeti
797
of view. Optional images were often ob tamed in the sagittal plane, but these were available in less than half of the pa tients.
The surface coil images were obtained with the endonectal surface coil described previously (Mednad, Pittsburgh) (7). The coil was a 3.5 X 6-cm oval surface coil mounted on the inner surface of a bal loon. The balloon was concave anteriorly
to assure reliable seating against the pros
tate (Fig la). The coils were tuned near the resonant frequency of 63.8 MHz with a capacitor placed across the coil within the balloon. Fine-tuning was accom plished by adding a variable capacitor at the end of the coil shaft, away from the patient.
It was necessary
to have tuning
adjustment to compensate for the varia tion in balloon inflation and loading. The coil was decoupled from the body coil during transmit as previously described
(7). The endorectal with
the patients
surface coil was placed in the lateral
position.
a.
b.
Figure 1. (a) Close-up photo of the inflated endorectal surface coil shows the concave ante nor surface of the balloon (arrows). (b) Sagittal localizing image (400/20) obtained with a body coil demonstrates proper positioning of the coil (large white arrows) within the rec tum. The coil is large enough to cover the entire length of the prostate gland (thin white an row) and the seminal vesicles (black arrow).
An introducer provided the torque con trol needed to reliably position the bal
loon. The balloon was inflated with ap proximately
100 cm3 of air. The patients
were imaged in the supine position and were intramuscularly administered 1 mg of glucagon
(Lilly,
Indianapolis)
prior
to
imaging. An initial sagittal localizing image was obtained to confirm coil positioning and
select locations for the axial images (Fig ib). The axial long and short
TR/TE
en
dorectal coil images were obtained with a 3-4-mm section thickness, a 1-mm gap, and a 10-12-cm field of view. Additional images were acquired in the sagittal on coronal plane for approximately 30% of
the patients. The images were processed with a spatial filter to remove some of the surface coil falloff and make the images easier to display. An oversampling meth od was used to reduce
aliasing
artifacts.
On approximately half of the long TR/TE images, gradient moment nulling was used to reduce motion artifacts. The fre quency encoding direction was anterior to posterior for all patients to prevent phase-related motion artifacts from ob scuring
b.
Figure 2. Advantages of endorectal MR imaging. Long TR/TE (2,500/80) surface (a) and body (b) coil images of a prostate with marked benign prostatic hyperplasia. The enlarged central gland is seen with both coils. The peripheral zone is compressed into a thin rim of tissue, which is seen only on the endorectal coil image (small arrows). Note the excellent vi sualization of the benign prostatic hyperplasia nodules (large arrows) and associated low signal-intensity rim on the endorectal coil image.
the prostate.
Pathologic
Examination
Resection specimens were fixed over night in 10% neutral buffered formalin. The margins were inked and the prostate glands
a.
serially
sectioned
at 3—5-mm inter
vals in a plane perpendicular to the ure thral axis. Histologic samples were taken from areas that were grossly suggestive of carcinoma (eg, areas of nodule formation and/on tissue with yellow-orange color ation). Samples were always taken so as to include the nearest prostatic capsular margin. Routine selected site sections
were taken from the urethral margins in each lobe and the seminal vesicles. Each resected lymph node was embedded in toto. An average of 20 samples were ex
amined per patient. All specimens were reviewed by one pathologist. Each gland 798 •¿ Radiology
was evaluated for pathologic stage and extent of capsular invasion. Microscopic capsular invasion was defined as micro scopic foci of tumor seen outside the prostatic capsule at histologic examina tion, while the prostate capsule appeared grossly normal.
RESULTS As reported previously (7), the en domectal surface coil images provided more detailed visualization of the pnostate and peniprostatic structures than the body coil images. This was
particularly true when the prostate was enlarged from coincidental be nign prostatic hyperplasia. In cases of marked benign pnostatic hyperplasia,
it was
often
difficult
zonal architecture
to identify
the
of the prostate (Fig
2). Seminal vesicle architecture and the prostatic capsule were well seen on all of the endorectal coil images.
In 20 of the 22 patients, prostatic ma lignancy was visualized as a low-sig nal-intensity peripheral zone lesion on long TR/TE images. In one pa tient, the tumor had high signal in tensity on long TRITE images. This lesion was shown to have mucinous
elements at pathologic examination (Fig 3). In another patient, no abnom mality This
was seen on the MR images. patient
foci of tumor tion.
only
had
microscopic
at histologic
examina
March 1991
cancer. With the earlier identification of more lesions, apparently enhanced by the growing use of tnansnectal ul trasound (US) (12), theme is an in creased
responsibility
for umologists
and radiologists to accurately stage prostate cancer to identify surgical candidates. In clinical practice, it is very important
a.
b.
Figure 3. (a) Endorectal surface coil long TR/TE image of a patient with stage B mucinous adenocarcinoma of the prostate. Note the high-signal-intensity tumor that lacks the smooth border and complete low-signal-intensity rim that are typical of benign prostatic hyperpla sia (arrows). (b) Transrectal US scan of the same patient demonstrates that this tumor has mixed echogenicity (arrows).
5) that was seen at pathologic
In general, images obtained with use of gradient moment nulling dis played less motion artifact than those obtained without it. However, no dif ferences in contrast were noted be tween these two sets of images. Fourteen patients had surgical
pathologic disease,
confirmation
and eight
of stage B
had stage C dis
ease. Two of the eight patients with stage C disease had seminal vesicle invasion and six had capsular inva sion. The results of the prospective readings are compared with the pathologic staging in Table 1. The prospective MR imaging stage agreed with the pathologic findings of the disease in 18 of the 22 patients (82% accuracy). One case was ovenstaged on the basis of the incorrect reading of capsular invasion. This was the first of the cases included in the se ries and, in retrospect, was probably misinterpreted.
Pathologically
proved stage C tumors in three pa tients were incorrectly staged as stage B tumors at endonectal MR im aging. In two of those patients, there was microscopic capsular invasion only (Fig 4). In the third, theme was subtle seminal vesicle invasion (Fig Volume 178 •¿ Number 3
exami
tient (Fig 6).
The 18 patients whose images were read retrospectively by the three readers
included
five pa
tients with pathologically proved stage C lesions and 13 with patholog ically proved stage B lesions. The me sults of the retrospective
readings
are
shown in Table 2. In each case, theme was an improvement in the staging accuracy with use of the endorectal surface coil images relative to the body coil images. The average accu racy for all three readers was 69%
with the body coil and 84% with the surface coil images. The average im provement in accuracy was 16% (mange, 11%-22%). DISCUSSION Prostate cancer will affect 30% of men over the age of 50 years (10). When
localized
to the prostate
(stage
A or B), it can be cured with radical prostatectomy (1 1). Recent advances in the surgical
technique
disease
issue in the staging evaluation of a patient with prostatic carcinoma,
nation only on careful sectioning of the specimen. If we exclude cases of microscopic capsular invasion, the prospective readings of the endomec tal surface coil images were 91% accu rate. The sensitivity and specificity of the prospective endomectal coil im ages for depicting stage B, or opera ble, lesions were 93% and 84%, me spectively. The endorectal coil im ages were critical in the establish ment of the correct stage in one pa
blinded
to distinguish
localized to the prostate (stage A or B) from tumor that has extended be yond the confines of the gland (stage C or 0), since this will influence therapy. In general, patients with stage A or B lesions are candidates for extirpative sungical therapy, while those with stage C and D lesions are treated with radiation and hormonal therapy. The local extent of the tu mom distinguishes stage B from C dis ease. This is perhaps the most critical
that reduce
the morbidity of this procedure (2) have increased the popularity of de finitive surgical therapy for prostate
since it divides patients into operable and nonopenable groups. The distinc tion between
patients
with stage C
and stage D disease is also important because these two categories carry different prognoses and require dif fenent therapeutic approaches. Stage D disease usually is seen as lymphad enopathy or bone metastases. These entities are not detected on endonec tal MR images,
and, therefore,
the
endonectal MR examination must be augmented with other modalities to exclude bone metastases and lymph adenopathy. We find it convenient to obtain short TR/TE axial body coil MR images to exclude lymphadenop athy and a bone scan to exclude
bone
metastases. For years, the mainstay of local staging of prostate cancer has been the digital rectal examination. Digital rectal examination, however, has been shown to underestimate the ex tent of the cancer in 25%-78% of pa tients (10,13). The sensitivity of com puted tomography for depicting stage C disease is 35%—55%(14,15), which gives it little or no advantage over the digital rectal examination in the local staging
of pnostatic
cancer.
While some advocate tnansnectal US as an improved method of staging carcinoma of the prostate (16), others are of the opposite opinion (17). Scar dino et al (16) found tmansnectal US to be 86% accurate for staging, while Andniole
nificant patients. known dent, it lished
et al (17) experienced
sig
understaging (62%) in their Since tmansmectal US is to be very operator depen is not surprising that pub
reports
of its accuracy
vary
greatly. This is particularly evident with regard to seminal vesicle inva Radiology •¿ 799
sion, where accuracy of preoperative detection manges from 29% to 82% (16). Because of the high soft-tissue con tnast and multiplanar
capability
of
MR imaging, it has been proposed as a method to improve local staging of prostate
cancer.
reported
Its accuracy
has been
to be from 69% to 89% (5,18-
20). A large multicenter
study
that
compared body coil MR imaging with tnansnectal US and digital rectal examination in staging prostate canci noma in 234 surgically confirmed cases found the accuracy of body coil MR imaging to be 69% and the accu racy of US to be 58% (18). In a previ ous series, we found body coil MR
imaging to be 78% accurate in the de termination of overall local staging of prostate cancer (5). Our accuracy for determining seminal vesicle inva sion was 83%; we were only 44% sen sitive in detecting direct capsular in
vasion. The unsatisfactory the detection
of capsular
Figure 5. Long TR/TE body coil (a) and endorectal surface coil (b) images of a patient with early seminal vesicle invasion. The low signal intensity (arrows in a) on the body coil image represents either partial volume averaging with the top of an enlarged prostate or invasion from prostate cancer. The detailed architecture of the seminal vesicles is seen only on the surface coil image. The area of focal low signal intensity (small arrows in b) represents inva sion of prostate cancer. The ampulla of vas deferens (large arrows in b) normally has thick ened walls.
results for invasion
were in large part due to the limited
(6,7,21). The endonectal surface coil images provide reliable identifica tion of the prostatic capsule and bet ten visualization of the periprostatic
better than that achieved in the same cohort of patients examined with the body coil. The prospective sensitivity
is often difficult to identify the pen pnostatic veins and neurovascular bundles, particularly if the prostate is
architecture
for the detection
enlarged (Fig 3). These deficiencies served as our stimulus to develop an improved technology for prostatic MR imaging. We and others have shown previ
tate cancer, we have found a signifi cant improvement over staging with
ously that the use of an endomectal
confirmed
surface coil to image the prostate sig
This was essentially
nificantly
of our multiple
spatial
resolution
of body coil MR
imaging. The capsule is not reliably seen on the body coil images, and it
improves
800 •¿ Radiology
resolution
and seminal vesicles. In
applying this technique to stage pa tients with clinically localized pros
the body coil in the same cohort of patients. The prospective readings for the 22 patients with surgically
lesions were 82% accurate. the same as that
reader retrospective
series,
in which
the
accuracy
was
16%
of stage C disease in
the patients with surgically con firmed lesions was 63%. Although this
seems
low,
it is important
to un
derstand the selection bias in this study. Unlike our original pilot study (5), in which all patients were accept
ed for examination,
the current co
homt consisted of patients with nela tively small lesions, and the diffenen tiation
between
stage
B and
C disease
March 1991
to be from biopsy-related hemom rhage. On occasion, abnormalities with high signal intensity were also seen in the pemiprostatic fat on the long TRITE images. We believe this represents biopsy-related edema. The criteria used for assessing the integmi ty of the prostatic capsule had to be modified during the study so that mild signal
abnormalities
within
the
capsule, adjacent to extensive biopsy related changes, were not considered evidence of capsular invasion. Fail ume to recognize biopsy-related changes may thus result in ovemesti a.
mating
b.
Figure 6. Long TRITE body coil (a) and endorectal coil (b) images of a patient with patho logically confirmed stage C prostate cancer. A low-signal-intensity lesion is seen on both im ages (arrows in a, large black arrows in b), representing the cancer. The capsule is seen only on the endorectal coil images (small black arrows in b). A disruption of the capsule by tumor infiltrating periprostatic fat is seen only on the surface coil image (white arrows in b). This patient was correctly determined to have stage C disease at prospective MR imaging evalua tion.
the disease.
Unfortunately,
with use of the criteria stated above, the disease in patients with early cap sulam invasion biopsy-related
in an area of extensive signal change might
be underestimated. However, this never occurred in our group of pa tients.
CONCLUSION MR imaging of the prostate with an endomectal surface coil results in images
of the pnostatic
and penipnos
tatic anatomy that are more detailed than those provided
by means
of
body coil MR imaging. Although we are in the early stages of its clinical application, we believe that the me suits
could not be readily accomplished with conventional means. This me my who had perplexing tumors at digital rectal examination and subtle findings on endomectal MR images.
logically proved seminal vesicle in vasion underwent radical prostatec tomy. This also reflects the selection bias in our patients with pathologi cally confirmed lesions. Although the seminal vesicle invasion was missed on the prospective reading in
Nonetheless,
one patient, in retrospect,
subtle focal
thickening of the seminal bulan wall was suggestive
vesicle tu of seminal
suited in inclusion tients undergoing
of only those pa radical prostatecto
the positive
predictive
value of endomectal MR imaging for the determination of stage C disease was 84%, and the negative predictive value was 82%. Despite the population bias, the 67% sensitivity for the detection of capsular invasion on the endorectal images represents a significant im
vesicle invasion (Fig 5). This case was diagnosed correctly by two of the three investigators in the netnospec tive readings. Although there are too few surgically confirmed cases to make a meaningful comment on the
provement oven the 44% sensitivity for detection of capsular invasion on body coil images that we reported
reliability of tubular wall thickening as an early marker of seminal vesicle
previously (5). If we cases of microscopic sion, the sensitivity sion is 100%. This is
will be quite useful. With more expe nience, even cases of minimal semi nal vesicle invasion should be readi ly identified.
exclude the two capsular inva for capsular inva believed to be a
direct result of the improved visual ization of the prostatic capsule on the endorectal coil images. Five of the nine discordant
readings
of the body
and surface coil images were due to better visualization of the prostatic capsule and neumovascular bundle on the surface coil images (Fig 6). Only the two patients
Volume 178 •¿ Number 3
with patho
invasion, we believe that this sign
Another
important
issue to be con
sidened is biopsy-related signal ab nommalities. Most of the patients in this study were imaged within a month of their pmostatic biopsy. Most commonly, an area of high signal in tensity was seen in the region of the
previous biopsy on both long and short TR/TE images.
This is believed
presented
indicate
a clinical
ad
vantage of endonectal surface coil MR imaging oven body coil imaging in the preoperative evaluation of pa tients with prostate cancer. The most important advantage of the endonec tal coil images is the improved visu
alization of the prostatic capsule and the neunovasculam bundle. This is particularly true in evaluating a large
prostate, where the peripheral zone and pemiprostatic anatomy can be dif ficult to identify
on the body coil im
ages. While endomectal MR imaging of the prostate was 100% sensitive to grossly detectable prostatic capsule,
invasion of the we were not able to
detect subtle microscopic capsular in vasion. However, in both cases of mi croscopic invasion presented in this report, the pathologist described the gross appearance gin to be normal.
of the capsular The improved
mar im
ages of the seminal vesicles should also improve the sensitivity to semi nal vesicle
invasion,
although
few cases of seminal vesicle were included in this study conclusive. Although more with radiologic-pathologic tion is needed
to improve
too
invasion to be work commela
our ability
to interpret the endonectal coil im ages, it appears that this modality is Radiology •¿ 801
destined to become the method of choice in the determination of the lo cal extent of prostate cancer. U References 1. Silvery E, Boring CC, Squires TS. Cancer statistics. CA 1990; 40:9-26. 2. Eggleston JC, Walsh PC. Radical pnosta tectomy with preservation of sexual func tion: pathologic findings in the first 100 cases. J Urol 1985;134:1146—1148. 3, Smith JA Jr. Middleton RG. Clinical man agement of prostatic cancer. Chicago: Year Book Medical, 1987; 55. 4. Tempany C, Rahmouni A, Zerhouni E, Ep stein J, Walsh P. Evaluation of the role of MRI in predicting invasion of the neuro vascular bundle by prostate cancer. In: Proceedings of the Eighth Annual Society of Magnetic Resonance in Medicine Meet ing. Amsterdam: Society of Magnetic Res onance in Medicine, 1989; 129. 5. Bezzi M, Kressel HY, Allen KS, et al. Prostatic carcinoma: staging with MR at 1.5 T. Radiology 1988; 169:339-346. 6. Schnall M, Kressel HY, Pollack HP, Len kinski RL. The development of an intra cavitary inflatable surface coil for high resolution proton imaging and spectrosco py (abstr). In: Book of abstracts: Society of
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Platt JF, Bree RL, Schwab RE. The accura cy of CT in the staging of prostatic carci noma. AJR 1987;149:315—318. Scardino PT, Shinohara K, Wheeler TM, Carter S. Staging of prostate cancer: val ue of ultrasound. Urol Clin North Am 1989; 16:713-740. Andniole GL, Colpen DE, Mikkelson DJ, Catalona WJ. Sonographic and patholog ic staging of patients with clinically local ized prostate cancer. J Unol 1989; 142:1259-1261. Rifkin M, Zerhouni E, Gatsonis C, et a!. Comparison of magnetic resonance imag ing and ultrasonography in staging early prostate cancer. N Engl J Med 1990; 323:621—626. Hnicak H, Dooms GC, Jeffrey RB, et al. Prostatic carcinoma: staging by clinical as sessment, CT, MR imaging. Radiology 1987;162:331-336. Biondetti PR, Lee JKT, Ling D, Catalona WJ. Clinical stage B prostate carcinoma: staging with MR imaging. Radiology 1987; 162:235-239. Martin JF, Hajek P. Baker L, Gylys-Monin V, Fitzmorris-Glass R, Mattrey RR. Inflat able surface coil for MR imaging of the prostate. Radiology 1988; 167:268-270.
March 1991
