DOI:10.2214/AJR.07.3188
AJR 2008; 190:1495-1504
© American Roentgen Ray Society
Perspective on the Role of Transrectal and Transvaginal Sonography of Tumors of the Rectum and Anal Canal
Francesca Berton1,2,
Giada Gola1,3 and
Stephanie R. Wilson1,4
1 Section of Ultrasound, Department of Medical Imaging, Toronto General
Hospital, University of Toronto, Toronto, ON, Canada.
2 Present address: Institute of Radiology, IRCCS San Matteo Hospital, University
of Pavia, Pavia Italy.
3 Present address: Serves di Radiologia, Ospedale Civile di Voghera, Voghera,
Pavia, Italy.
4 Department of Diagnostic Imaging, Foothills Medical Centre, 1403 29 St. NW,
Calgary, AB, Canada T2N 2T9.
Received January 29, 2007;
accepted after revision December 27, 2007.
Address correspondence to S. R. Wilson
(stephanie.wilson{at}calgaryhealthregion.ca).
S. R. Wilson acts as an advisor on sonography to Philips Medical
Systems.
Abstract
OBJECTIVE. Intestinal sonography is characterized by excellent
resolution of the multiple layers of the intestinal wall and sensitive
depiction of the degree of invasion of rectal tumors. Traditional transrectal
sonography has been enhanced by the addition of transvaginal scanning for
women and by advances in transducer technology. Our purpose is to describe the
current status of sonography in the evaluation of rectal and anal tumors and
in the staging of rectal cancer.
CONCLUSION. Endorectal and transanal sonography are fast, minimally
invasive techniques that can be performed with portable equipment and yield
rapidly interpreted images. They are considered the reference standard for the
preoperative staging of rectal and anal cancers and have relatively high
accuracy in categorization of tumors and nodes in TNM staging.
Keywords: anal cancer • Doppler imaging • rectal cancer • rectal tumors • staging • transrectal sonography • transvaginal sonography
Introduction
Sonography of the rectum can be performed with an echoendoscope,
which is a sonographic crystal array attached to an endoscopic device suitable
for study of the lower gastrointestinal tract, or with an endorectal
sonographic probe, which comprises a rigid probe and a fixed sonographic
crystal array. Examinations with the devices yield similar information related
to pathologic changes in the rectum. Endorectal sonography was introduced to
clinical practice in 1983 and has been successfully developed for evaluation
of both the prostate and the rectum. The prostate gland, which maintains a
constant orientation and relation to the rectum, is easily and consistently
evaluated with transrectal sonography. Scanning of the rectum is challenging
with a probe positioned within the rectum because retained fecal material may
be present, rectal lesions can be mobile or large, and general orientation is
difficult.
To facilitate study of the rectal wall, a rotating mechanical crystal was
introduced for transrectal sonography. Positioned within the rectal lumen and
surrounded by a fluid-filled condom or balloon, the device gives a 360°
axial view of the rectal wall and surrounding soft tissues. Reports of studies
performed with such a transducer compose most of the early literature on the
technique of transrectal sonography
[1,
2]. Although these rotating
crystals are durable and popular, the technique has potential mechanical
problems related to jamming of the mechanism, entanglement of the inflated
balloon, and poor resolution because of reduction in the effective aperture of
a rotating crystal.
A number of changes in our scanning techniques over the last decade have
transformed our ability to study diseases of the rectum and anal canal with
sonography. These changes include introduction of biplane capability on a
single transrectal sonographic probe of a high-end sonographic system, which
allows placement of two transducer arrays. The probes provide state-of-the-art
resolution and have sensitive Doppler capability, enabling acquisition of
useful information about tumor structure and vascularity (Figs.
1A,
1B and
2A,
2B), which enhances the
familiar gray-scale transrectal sonographic technology.
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Fig. 1A —71-year-old woman with biopsy-proven tubulovillous adenoma
who has undergone unsuccessful transrectal sonography. Images show advantages
of biplane transducer technology, use of sodium phosphate enema, addition of
color Doppler, and transvaginal scanning. Axial transrectal sonographic image
shows soft mass on left wall of rectum outlined by fluid in rectum from sodium
phosphate enema and evident on real-time examination.
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Fig. 1B —71-year-old woman with biopsy-proven tubulovillous adenoma
who has undergone unsuccessful transrectal sonography. Images show advantages
of biplane transducer technology, use of sodium phosphate enema, addition of
color Doppler, and transvaginal scanning. Color transrectal sonographic image
confirms presence of lesion.
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Fig. 2A —62-year-old woman with very small T1 rectal lesion. Images
show advantages of biplane transducer technology, use of sodium phosphate
enema, addition of color Doppler, and transvaginal scan. After normal findings
on transrectal sonography, axial sonographic image of rectum obtained with
transvaginal probe shows subtle superficial hypoechoic mass on left
anterolateral wall. Submucosa and muscularis propria are intact.
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Fig. 2B —62-year-old woman with very small T1 rectal lesion. Images
show advantages of biplane transducer technology, use of sodium phosphate
enema, addition of color Doppler, and transvaginal scan. Color Doppler image
confirms vascularity of lesion. Small superficial tumor can be difficult to
find with transrectal sonography and other imaging techniques.
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Staging of rectal cancer before selection of a treatment regimen is
critical for identifying superficial tumors amenable to transanal excision and
invasive lesions better managed with neoadjuvant chemoradiation therapy before
surgery. The contrast and spatial resolution of state-of-the-art sonography
enable visualization of the multiple layers of the rectal wall, making
sonography an excellent choice for the staging of both superficial and
invasive rectal tumors. Our purpose is to describe our experience with
sonography in the evaluation of all tumors of the rectum and to highlight the
advantages of a variety of transducers and techniques for optimizing
results.
Anatomy of the Rectum and Anal Canal
The rectum is the terminal part of the large intestine. At the level of S3,
it is in continuity with the sigmoid colon, and it ends at the anocutaneous
line. The rectum can be divided in two parts: the pelvic rectum or rectal
ampulla, which is a contractile reservoir, and the perineal rectum, or anal
canal (Figs. 3A,
3B and
3C).
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Fig. 3A —Anatomy of rectum and anal canal. Coronal schematic shows
caudal extent of rectum in anal canal. Thin black line in rectal wall,
representing muscularis propria, becomes thicker black internal anal sphincter
at anorectal junction. Fibers of levator ani muscle contribute to external
sphincter.
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Rectal Ampulla
The rectum at the level of the rectosigmoid flexure is in continuity with
the sigmoid colon. Where it traverses the pelvic diaphragm, the rectum becomes
the anal canal. The rectum starts at the sacral promontory and ends beyond the
coccyx, having a length of 12-15 cm. The rectum occupies a presacral position
and is surrounded by perirectal fat.
Anal Canal
The anal canal is surrounded by a double sphincter. The first component
consists of a smooth-muscle layer called the internal anal sphincter and a
thickening of the circular muscle layer of the rectum that surrounds the
superior three fourths of the anal canal. The internal anal sphincter is
controlled by the autonomic nervous system. The second component is a striated
muscle layer called the external anal sphincter, which forms a ring around the
anal canal in continuity in the superior aspect with the puborectalis and
levator ani muscles. The external anal sphincter is supplied by the
cerebrospinal nervous system. The anal canal begins at the anorectal line and
ends at the anal verge, generally having a length of 3 or 4 cm. The anal lumen
is divided into three parts: the colorectal zone with columnar mucosa, the
transitional zone or pecten, and the cutaneous zone.
Blood Flow and Rectal Lymph Nodes
The blood supply of the rectum depends on many arteries that originate in
the aorta and iliac and pudendal or inferior gluteal arteries. Lymphatic
drainage is important to carcinomatous dissemination. The lymph vessels begin
in the intramural lymphatic plexuses and drain to the pararectal lymph nodes
in the perirectal fat. The collectors drain in three nodal sites: superior,
middle, and inferior rectal nodes. The drainage ends in the iliac and
periaortic lymph nodes. The inferior rectal nodes drain only the anal canal
below the mucocutaneous junction to the superficial inguinal lymph nodes
[3].
Sonographic Technique
Transrectal sonography is an accepted standard for the staging of known
rectal cancer. It is not, however, an acceptable technique for screening the
rectum for tumors. Therefore, actual acquisition of a scan is best directed at
known rectal masses, although with sufficient effort, time, and meticulous
technique, a survey of the rectum can be performed. The technique of
transrectal sonography requires a learning curve for orientation of biplane
probes and for identification of rectal tumors.
We recommend that before sonography is performed for assessment of a rectal
tumor, all relevant clinical information about the tumor be obtained,
including the method of detection (digital rectal examination or colonoscopy),
biopsy confirmation, and size and location. Whether excisional biopsy of
rectal polyps has been performed and a diagnosis of cancer made is critical
information because the tumor may have been largely or completely removed at
biopsy. Digital rectal examination is performed before transrectal sonography,
and palpable masses are documented. In clinical practice, differentiation of a
soft, compliant tubulovillous adenoma from a firmer and often fixed malignant
rectal tumor at digital rectal examination is highly contributory to a
successful transrectal sonographic examination.
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Fig. 4B —Normal layers of bowel wall. Bowel wall in cross section
(B) and long axis (C).On sonography wall consists of alternating
layers of echogenicity and hypoechogenicity beginning on inside with echogenic
layer that represents mucosal interface. Dominant echogenic layer is
submucosa, and each hypoechoic layer contains muscle fibers.
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Fig. 4C —Normal layers of bowel wall. Bowel wall in cross section
(B) and long axis (C).On sonography wall consists of alternating
layers of echogenicity and hypoechogenicity beginning on inside with echogenic
layer that represents mucosal interface. Dominant echogenic layer is
submucosa, and each hypoechoic layer contains muscle fibers.
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Transrectal sonography is well tolerated by most patients, is inexpensive,
and is easily performed. Preparation for the procedure is minimal, requiring
only a sodium phosphate (Fleet®, Fleet Laboratories) enema immediately
before the examination to clean the rectum and to add fluid to the rectal
lumen for outlining intraluminal masses (Figs.
1A and
1B). Endorectal sonography is
performed with a hand-sheld probe, which is inserted into the rectum through
the anal canal. We do not use probes that require an inflated fluid-filled
balloon and instead use biplane probes inserted with only a clean condom
covering the transducer head. We have used probes with a curved array coupled
with a linear array, both on the same side of the transducer (HDI 5000,
Philips Medical Systems), and with two curved arrays (Aplio, Toshiba), one on
the end of the probe and the other on the transducer side for scanning in
perpendicular planes. The minor disadvantages of the biplane probes, related
to the lack of a complete axial view of the rectum and perirectal soft
tissues, are more than alleviated by the superior imaging afforded by these
transducers.
In examinations of women, we use transvaginal probes to study rectal and
anal lesions when transrectal sonography is difficult or fails to stage the
rectal tumor. The accuracy of this technique is equivalent to that of
conventional transrectal sonography for staging of rectal cancer. Use of the
transvaginal technique overcomes problems related to the presence of stenotic,
large, very small, high, or very low tumors and enables more thorough
evaluation of the rectovaginal septum
[4]. Transvaginal scanning is
performed with high-frequency (9- to 12-MHz) transducer arrays. Modification
of the routine technique includes elevation of the examining hand to allow the
end-fired probe to be directed posteriorly at the region of interest. The
rectum is visualized in cross section or in the long axis depending on the
orientation of the probe. Alternatively, a biplane probe with a convex and
linear side-firing transducer can show the rectum well in the transvaginal
approach. Still experimental are improved methods of studying the rectum and
anal canal with 3D endorectal (endoanal) sonography
[5].
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Fig. 5 —42-year-old man who has undergone successful excision of
rectal carcinoid. Follow-up sagittal transrectal sonographic image obtained
with biplane linear rectal probe shows five layers of normal rectal wall.
Muscle layers are hypoechoic, and submucosa is dominant echogenic layer. Fluid
is present within rectal lumen, which appears black. Perirectal fat appears
echogenic or white.
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Fig. 6 —Schematic shows T component of TNM staging of rectal cancer.
Tumors are red and exhibit progressively deeper invasion beginning at
10-o'clock position, where T superficial noninvasive lesion involves only
superficial layers of intestinal wall. At 7-o'clock position, T1 lesion
invades submucosa (yellow). At 5-o'clock position, T2 lesion invades
muscularis propria (blue). At 2-o'clock position, T3 lesion exhibits
full-thickness invasion through layers of rectal wall with invasion of
surrounding perirectal fat. In directly anterior aspect (12-o'clock position),
T4 lesion exhibits invasion of prostate gland.
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Sonography of the Rectum
The sonographic appearance of the normal rectal wall is five layers of
alternating hyperechogenicity and hypoechogenicity with the inner and
outermost layers appearing echogenic. The sonographic layers correspond to the
histologic layers of the intestinal wall (Figs.
4A,
4B and
4C) and are constant
throughout the intestinal tract (Fig.
5). The muscular components of the wall appear hypoechoic, and the
other tissues are echogenic.
Sonography of the Anal Canal
Variation of the wall layering of the rectum occurs in the anal canal in
that the circular muscle layer of the muscularis propria becomes the internal
anal sphincter at the anorectal junction. In the upper two thirds of the anal
canal, the internal anal sphincter can be seen as a complete hypoechoic wide
band. The lower anal canal can be recognized by the circumferential
hyperechoic band of striated muscle, the external anal sphincter (Figs.
3A,
3B and
3C). All axial images are
oriented by convention as if they are viewed from below with the anterior
abdominal wall at the top of the image and the right side on the left side of
the image. Therefore, the anterior rectal wall appears at the top of the image
and the posterior wall appears at the bottom of the image. Sagittal images are
viewed as if the patient's head is on the left side of the image.
Rectal Carcinoma
Clinical Features
Colorectal carcinoma is an extremely common disease with great social
impact. It is one of the most common malignant tumors in the United States,
Europe, and Japan, after lung and prostate cancer among men and lung and
breast cancer among women. It is equally prevalent among men and women and
usually occurs in later life (60-70 years). Fifty to sixty percent of the
tumors are localized to the rectosigmoid region. The prognosis among patients
with malignant tumors of the rectum is negatively affected by advanced stage
at diagnosis; extramural spread of the tumor into the mesorectum; tumor
involvement of the anal sphincter, levator ani muscle, or adjacent organs; and
advanced nodal involvement. Optimal management of colorectal carcinoma is
predicated on curative resection and thus the ability to achieve surgical
clearance at the circumferential margins of resection. A 1-mm tumor-free
margin is the minimum requirement
[6,
7].
Transrectal Sonography and Rectal Cancer
Although there are both institutional and geographic preferences,
transrectal sonography is a well-established method and is considered by many
the reference standard for the preoperative assessment of rectal cancer.
Transrectal sonography depicts rectal wall stratification and tumor extension
within and through the rectal wall, especially for T1 and T2 lesions, for
which it is more accurate than MRI and CT
[8-11].
Used with MRI for identification of the mesorectal fascia in patients with
invasive disease [12],
transrectal sonography is an invaluable component of local staging of rectal
cancer
[13-15].
Accurate preoperative regional staging, based on the TNM system
(Fig. 6), is essential for
planning optimal therapy. Whereas a small and superficial tumor can be managed
with transanal excision, more extensive resection is necessary for larger
tumors and those involving the deeper layers of the rectal wall. With
transmural wall invasion and tumor involvement of the perirectal fat,
neoadjuvant chemoradiation before surgery is the accepted standard of care.
The addition of MRI to evaluation of patients with this lesion enables precise
identification of the mesorectal fascia because tumors that traverse this
plane are unlikely to allow a satisfactory circumferential margin of
resection. Trans rectal sonography can precisely depict sphincter
infiltration. Although CT and MRI show neoplastic wall thickening, these
examinations do not allow discrimination of involvement of individual layers
for accurate staging of tumors that do not extend beyond the rectal wall
[9].
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Fig. 7A —42-year-old woman with small palpable mass on left anterior
rectal wall found at digital rectal examination that proved to be T1
adenocarcinoma of rectum. Curved axial (A) and linear sagittal
(B) transrectal sonographic images show superficial hypoechoic tumor
with subtle involvement of echogenic submucosa.
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Fig. 7B —42-year-old woman with small palpable mass on left anterior
rectal wall found at digital rectal examination that proved to be T1
adenocarcinoma of rectum. Curved axial (A) and linear sagittal
(B) transrectal sonographic images show superficial hypoechoic tumor
with subtle involvement of echogenic submucosa.
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Fig. 8 —56-year-old man with early T3 rectal carcinoma imaged with
biplane rectal probe. Axial transrectal sonographic image shows hypoechoic
tumor that has destroyed submucosa and muscularis propria. Gross extension of
tumor into perirectal fat is evident. Echogenic margins (arrows) of
remaining submucosa are present on each side of invasive tumor.
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Fig. 9 —64-year-old man with extensive T3 mucinous rectal
adenocarcinoma. Axial transrectal sonographic image shows destruction of
submucosa, echogenic edge (arrow) of which is evident on right side
of image. Enlarged round perirectal node shows a hypoechoic tumor deposit.
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Sonographic evaluation of rectal tumors entails identification of a rectal
mass or focal mural thickening and determination of its size, extent, and
layer of origin. Superficial tumors are often isoechoic with the adjacent wall
layers and may produce only a small bump on the mucosal surface. Invasive
rectal carcinoma, by comparison, appears progressively more hypoechoic with
increasing invasion, so a T3 lesion exhibits clear disruption of the
underlying submucosa and muscularis propria with spikes of black or hypoechoic
tumor extending into the perirectal fat. The T category in the locoregional
spread of rectal cancer indicates the presence of infiltration of the mucosal
and submucosal layers (Tis-T1) (Figs.
7A and
7B), of the muscularis propria
(T2), of the perivisceral fat tissue with obliteration of the interface
between muscle and perirectal fat (T3) (Figs.
8 and
9), and of the contiguous
organs (T4) (Figs. 10 and
11). This categorization is
fundamental for the surgeon because some lesions are managed with local
excision or endoscopic ablation (Tis-T1), and other lesions are managed with
more extensive surgery, such as anterior resection, abdominoperineal resection
or pelvic exenteration, and partial sacral resection.
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Fig. 10 —57-year-old man with invasive rectal cancer. Axial
transrectal sonographic image shows large anterior T4 invasive malignant tumor
of rectum with destruction of all wall layers and extension into perirectal
fat. Hypoechoic tumor is in intimate contact with posterior wall of prostate,
seemingly pushing it forward. Prostate involvement suspected on transrectal
sonography was confirmed on MRI.
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Fig. 11 —56-year-old woman with invasive rectal cancer who had
undergone undocumented incomplete transanal excision of polyp 10 years
previously. Transvaginal axial sonographic image shows hypoechoic solid
lobulated mass in anovaginal septum appearing almost independent of rectal
wall that represents incompletely excised polyp with malignant change and
regrowth over long interval.
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The presence of adenopathy (N category) in the perirectal space can
influence prognosis and management because it is widely accepted that positive
lymph node results justify the choice of radiation therapy and preoperative
chemotherapy in the management of T2 and early T3 lesions. Lymph node size is
a difficult consideration because small nodes can have microscopic tumor foci,
and large nodes can be reactive. Nonetheless, we report all lymph nodes
visualized on sonography, including their maximal dimension, because small
normal nodes are infrequently seen. The likelihood of malignancy increases for
nodes larger than 5 mm in diameter.
Gray-scale visualization of actual tumor deposits within lymph nodes is
uncommon, and Doppler criteria for tumor differentiation have not proved
reliable for us. Transrectal sonography, CT, and MRI all have been used for
different roles in staging of primary rectal tumors. Findings on color Doppler
sonography are useful because most tumors are highly vascular. Identification
of a hypervascular focus assists in determination of tumor margins (Figs.
12A and
12B) and in identification of
small and superficial tumors to confirm that the region of interest does
represent a neoplasm
[16-18]
(Figs. 1A,
1B,
13A and
13B). Furthermore, it is
helpful that fecal material never has color signal.
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Fig. 12A —58-year-old man with T2 rectal cancer. This case illustrates
contribution of color Doppler imaging to tumor differentiation. Axial
transrectal sonographic image shows hypoechoic tumor. Destruction of submucosa
is evident with involvement of muscularis propria on right side of image.
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Fig. 12B —58-year-old man with T2 rectal cancer. This case illustrates
contribution of color Doppler imaging to tumor differentiation. Color Doppler
transrectal sonographic image at default setting shows typical
hypervascularity. Color demarcates tumor from normal rectal wall on left side
of image. Technique is helpful for tumor staging.
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Fig. 13A — 55-year-old man with small rectal adenocarcinoma originating
in adenomatous polyp of posterior rectal wall. This case illustrates
contribution of color Doppler imaging to tumor differentiation. Axial
transrectal sonographic image shows isoechoic polypoid mass with broad base
surrounded by fluid within rectal lumen. Mass involves submucosal layer
only.
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Fig. 13B — 55-year-old man with small rectal adenocarcinoma originating
in adenomatous polyp of posterior rectal wall. This case illustrates
contribution of color Doppler imaging to tumor differentiation. Color Doppler
image shows profuse vascularity and vascular stalk of polypoid mass.
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Histology of Rectal Tumors
More than 90% of rectal carcinomas are adenocarcinomas. By definition, on
sonography all of these tumors appear as masses that involve the superficial
mucosal layer of the intestinal wall. Rectal carcinoma has many risk factors
and a known premalignant lesion, rectal adenoma. There is a relatively long
time course for malignant transformation from adenoma to carcinoma, and
outcome is markedly improved by early detection of adenoma and early cancer
[19] (Figs.
7A,
7B and
13A,
13B). The risk of cancer
increases from almost zero for polyps smaller than 5-6 mm to 40-50% for polyps
larger than 2 cm [20]. In a
meta-analysis, Worrell et al.
[21] found that among 258
rectal adenomas with negative findings at biopsy, 50 (81%) of 62 found to be
focal invasive cancer at surgical histopathologic examination had been
correctly predicted at transrectal sonography. Conversely, the stage of 12% of
benign adenomas was increased on the basis of findings at transrectal
sonography.
Tubulovillous adenoma is a distinct premalignant benign tumor that has a
high likelihood of malignant change. With malignant conversion, this tumor
forms a subset of rectal adenocarcinoma but has a more favorable prognosis. At
sonography, these soft and compliant tumors are often sizable and manifest as
dominant intraluminal masses. They are T0 lesions with an intact underlying
submucosa and muscularis propria (Figs.
14A and
14B). Differentiation of
tubulovillous adenoma from a T1 lesion is often difficult with sonography. In
the past, we and others [22,
23] noticed a tendency toward
overstaging these tumors at endosonography. We believe this tendency was
related to compression of the soft tumor into the rectal wall by the inflated
balloon covering the examining transducer. With the use of high-resolution
biplane transducers with no inflated balloon for real-time evaluation,
however, transrectal sonography can show an intraluminal soft and compressible
mass with no invasion. We also have found that color Doppler sonography
consistently shows tubulovillous adenoma with a vascular stalk to the tumor
and vessels with a characteristic stellate configuration (Fig.
14A,
14B), which differs from the
more chaotic vascular pattern of many invasive rectal cancers.
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Fig. 14A —58-year-old woman with tubulovillous adenoma. This case
illustrates contribution of color Doppler imaging to tumor differentiation.
Axial transvaginal sonographic image shows mixed echogenic mass that seems to
fill lumen of rectum. There is no evidence of invasive cancer.
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Fig. 14B —58-year-old woman with tubulovillous adenoma. This case
illustrates contribution of color Doppler imaging to tumor differentiation.
Color Doppler image shows characteristic stellate vascularity frequently
encountered with this pathologic condition.
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In addition to the common adenocarcinoma of the rectum, histologic types of
malignancy include mucinous varieties with a tendency to intralesional
calcification, mucinous deposits within metastatic nodes
(Fig. 9), and local
involvement of adjacent organs
[24]; carcinoids, which are
endocrine neoplasms with relatively low levels of malignancy
[25] that usually manifest as
small, mobile, submucosal nodules or focal areas of submucosal thickening
[26] with an incidence of
metastasis that correlates with tumor size
[27]; and rare primary
squamous cell carcinoma [28,
29], which seems to be
frequently locally invasive and involve regional lymphatic vessels. Lymphoma
[30], which is rare, can occur
as a primary lesion of the large intestine
[31] or as part of a
generalized malignant process involving the gastrointestinal tract with
infiltration of the perirectal soft tissues. Involvement of the deeper layers
of the intestinal wall is characteristic of lymphoma (Figs.
15A and
15B). Anorectal melanoma
[32] is an extremely rare
variety of rectal tumor but constitutes the third most common site of origin
of this tumor after the skin and eye (Figs.
16A and
16B).
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Fig. 15A —64-year-old man with rectal lymphoma and known disseminated
lymphoma with gastrointestinal involvement. Axial transrectal sonographic
image of posterior rectal wall shows mucosa and submucosal layers are intact,
excluding possibility of adenocarcinoma. Hypoechoic tumor is extensive and
involves deep layers of rectal wall with diffuse extension into perirectal fat
evident as many hypoechoic bands.
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Fig. 15B —64-year-old man with rectal lymphoma and known disseminated
lymphoma with gastrointestinal involvement. Axial CT image through rectum
shows diffuse wall thickening and infiltration of perirectal fat. Layers of
involvement are not defined.
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Fig. 16A —47-year-old woman with melanoma of anal canal who presented
with tumor prolapsing from anal canal. Axial transrectal sonographic image of
rectum immediately above anorectal junctions shows very black tumor.
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Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor
that originates in the alimentary tract. The most common source is the
stomach, followed by small bowel. In rare instances, the tumor originates in
the anorectal region, esophagus, colon, or appendix
[33-36].
GIST accounts for 0.1-3.0% of all gastrointestinal neoplasms
[37]. The tissue of origin is
commonly the muscularis propria of the bowel wall
(Fig. 17), and the size can be
tiny to huge [38]. Seventy to
eighty percent of GISTs are benign, but there is a continuum from benign to
malignant. On sonography, a GIST appears as a hypoechoic, well-circumscribed,
round mass with areas of necrosis, hemorrhage, and cystic change. Large tumors
frequently extend into the ischiorectal fossa or vagina with juxtaposition to
the genital organs of women or the prostate. Transrectal sonography is
accurate for defining the submucosal origin of rectal GIST with an intact
mucosa, depicting the rectal wall layers, and can be used to differentiate
extramural lesions, extrinsic compression, vascular lesions, and solid
tumor.
The limitations of transrectal sonography include those of studying
stenosing and high lesions (Figs.
18A and
18B) in the proximal rectum
(greater than 15 cm from the anal verge) and rectosigmoid junction. We have
found the addition of transvaginal sonography a valuable adjunct for defining
the local extension of stenosing tumors in women and have made the technique a
complementary examination to transrectal sonography
[4,
36-39].
Placement of high-frequency sonographic probes in the vagina often enables
high-resolution assessment of all of the pelvic viscera, including the rectum,
anal canal
[40-42],
peritoneum of the pelvic pouch, and the rectovaginal septum, without
interference from bowel gas and adipose tissue.
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Fig. 18A —60-year-old man with stenotic high T3 rectal carcinoma.
Endorectal probe could not pass through stenotic lumen. Biplane capability of
probe allowed accurate measurement and staging of tumor in all planes.
Long-axis transrectal sonographic image obtained with end-fired component
shows large circumferential rectal tumor.
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Fig. 18B —60-year-old man with stenotic high T3 rectal carcinoma.
Endorectal probe could not pass through stenotic lumen. Biplane capability of
probe allowed accurate measurement and staging of tumor in all planes. Axial
transrectal sonographic image shows residual strictured lumen represented by
small amount of echogenic air. Wall is circumferentially thickened and
hypoechoicwith no residual normal wall layers. Invasion of perirectal fat is
evident.
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A further limitation of transrectal sonography, related to limited
soft-tissue resolution, is lack of depiction of the mesorectal fascia, the
fine linear envelope surrounding the mesorectum (a distinct anatomic unit that
comprises the rectum, perirectal fat, and superior hemorrhoidal vessels,
nerves, and lymphatic vessels). Furthermore, identification of normal-sized
lymph nodes with microscopic tumor invasion and differentiation of lymph nodes
with tumor from those with reactive change is a problem on sonographic images
[42], as it is on CT scans and
MR images. It may not be possible to identify lymph nodes outside the range of
the sonographic transducer because of the limited field of view and limited
penetration with high-frequency probes
[43]. Moreover, when the tumor
is extremely small or the rectum is collapsed and compressed by a rigid
transducer, it may be difficult to detect and assess the tumor completely. For
acquisition of an optimal sonic window for rectal sonography, an adequate
sonographic medium must be used. A water-filled balloon can be attached to the
tip of the transducer at endoscopic sonography, or the rectal lumen can be
directly filled with water
[44]. In the first technique,
the pressure of the balloon on the rectal wall can make it difficult to
identify individual layers, so the second technique is preferred. It is also
helpful in the evaluation of stenosing malignant tumors. We have found a
simple sodium phosphate enema with retention of some of the enema fluid
provides good contrast between the tumor and the luminal fluid in almost all
cases.
The accuracy of transrectal sonography for tumor staging ranges between 67%
and 97% with a median accuracy of 89%
[45-48].
Akasu et al. [46] found
similar accuracy in a study of 164 consecutively detected cases of rectal
cancer and reported on 12 cases in which examinations were incomplete because
of the presence of annular constricting tumors. The most important staging
error is overstaging of T2 lesions as T3 lesions owing to the presence of
spontaneous (desmoplastic) or iatrogenic inflammation, which appears
hypoechoic at sonographic examination and mimics invasion where there is none
[10,
11]. Overstaging results in
overtreatment, usually because the patient receives radiation therapy when it
was not warranted. Tubulovillous adenoma can be easily classified T1 rather
than T0. Understaging can be caused by failure to detect microscopic cancerous
infiltration because of the limits of resolution of the equipment.
In our experience with transrectal sonography and newer probe designs, we
have had continued success with staging. Our success rate with transrectal
sonography has improved because large, bulky, and low tumors and annular
stenotic tumors can usually be accurately staged despite inability to pass the
probe beyond the stenotic segment (Figs.
18A and
18B). Furthermore, in a study
in which the subjects were women with histologically confirmed rectal cancer,
transvaginal sonographic findings were accurate in 25 (83%) of 30 cases and
led to understaging in two and overstaging in three of 30 cases
[4]. In the same study, a
blinded assessment of 45 cases of cancers with both transvaginal and
transrectal sonography showed good agreement between readers and comparable
accuracy for staging in adequate examinations with each technique
[4].
Lymph node classification (N) with transrectal sonography is less precise
than is tumor classification (T). It is more difficult to differentiate tumor
within a node than to identify the depth of wall invasion. The accuracy of N
categorization with transrectal sonography ranges between 65% and 83%
[47,
48]. In comparison with normal
oval lymph nodes, which may have a linear echogenic hilar streak, malignant
perirectal lymph nodes tend to be closer to round, to be more hypoechoic, and
to have a mixed echo pattern and therefore at transrectal sonography can be
easily visualized within the echogenic perirectal fat. Actual tumor deposits
within the perirectal nodes infrequently may be visualized as necrosis or
deposits of mucin (Figs. 8 and
9).
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Fig. 17 —59-year-old woman with asymptomatic gastrointestinal stromal
tumor found at routine physical examination. Transrectal sonographic image
shows solid, well-defined, round mass arising from muscularis propria. Tumor
is growing with submucosal pattern, and mucosal surface bulges into
fluid-filled lumen.
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Metastatic involvement of the rectal wall in ovarian cancer is common,
although metastasis can come from a variety of tumors, including those in the
upper gastrointestinal tract and pancreas, which are likely to produce drop
metastasis in the pelvis. On sonography, these tumors do not exhibit a mucosal
component and appear instead as a mass or thickening of the deep layers of the
rectal wall.
Postoperative transrectal sonography may show (accuracy, 80%) anastomotic
recurrence as a hypoechoic, irregularly shaped area in the anastomotic region
that infiltrates the perirectal fat of the rectal wall next to the
anastomosis. The addition of color and spectral Doppler techniques improves
detection of recurrent rectal cancer. Our experience suggests that recurrence,
like a primary tumor, is almost always hypervascular on color Doppler images
and is characterized by chaotic vessels. Higher-velocity blood flow in the
hemorrhoidal vessels of patients with recurrence than in those without
recurrence has been reported
[49]. The endosonographic
appearance of a normal anastomosis is characterized by mixed-echotexture
symmetric interruption of the typical structure of the wall. In the case of
stapled anastomosis, small, localized, bright echoes appear at the anastomosis
without producing a shadow behind them
[50].
Anal and Perianal neoplasms
Perianal and anal neoplasms are uncommon, accounting for approximately 1-5%
of all colorectal carcinomas. The peak incidence is in the seventh decade of
life with a marked predominance among women. Most (80-90%) of the tumors are
of squamous cell origin [51].
These tumors usually appear as a hard nodular mass or ulcerating lesion and
less frequently as an exophytic mass. Sonography reveals a hypoechoic central
mass that can involve the sphincters and invade deeply (Figs.
19A,
19B and
19C). The profuse vascularity
is similar to that of rectal cancer
[52]. The tumors spread to the
locoregional lymph nodes and rarely have distant metastasis
[53]. The TNM staging differs
from that of rectal cancer in that tumor size rather than depth of invasion
differentiates the T category (Table
1). Treatment is conservative: chemotherapy and radiation
therapy.
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Fig. 19A —62-year-old woman with cancer of anal canal. (Reprinted from
[4]) Sagittal sonographic image
of anal canal obtained with transvaginal probe shows focal hypoechoic mass in
outer third of anterior wall of anal canal.
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Summary
Endorectal and transanal sonography are fast, minimally invasive techniques
performed with portable equipment that yield rapidly interpretable images.
These techniques are considered the reference standard for preoperative
staging of rectal and anal cancers and have relatively high accuracy for T and
N categorization. The images depict rectal wall stratification and tumor
extension within and through the rectal wall, especially for T1 and T2
lesions, for which sonography is more accurate than MRI and CT, and can be
used for precise evaluation of sphincter infiltration. Other tumors, such as
GIST, lymphoma, and metastatic lesions, have characteristic appearances that
differ from those of adenocarcinoma. Advances in transducer technology also
have improved the transrectal sonographic depiction of tubulovillous adenoma.
Use of these techniques avoids the overstaging that previously occurred and
depicts the typical hypervascularity of rectal cancer. Transvaginal scanning
is an excellent adjunct to transrectal sonography in the care of women.
Acknowledgments
We thank Gordana Popovich for the schematics.
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Abstract
Introduction
