Keywords
anal - perianal - neoplasms - cancer - MRI
Introduction
The anal canal has a mixture of different coexisting types of cells and tissues, which
predispose to the development of a wide variety of neoplasms. Squamous cell carcinomas
(SCCs) are the most common anal canal carcinomas, accounting for 80% of all anal canal
carcinomas.[1] The less common anal canal malignancies include adenocarcinoma and melanoma, which
account for 15 and 4% cases, respectively. Lymphoma, neuroendocrine tumors (NETs),
and mesenchymal neoplasms are even rarer in the anal canal. Tumors originating from
the adjacent structures and organs can also secondarily involve the anal canal. These
include lesions arising in the ischioanal fossa, vagina, vulva, urethra, and pelvic
musculoskeletal system.
The perianal region refers to the skin surrounding a perimeter of 5 cm around the
anal verge.[2] Tumors of the perianal region are at least five times less common than tumors of
the anal canal.[1] Any cancer occurring in the skin elsewhere can occur in this region, with SCC being
the most common. Less common neoplasms of perianal skin include verrucous carcinoma
(VC), giant condyloma acuminatum (Buschke–Löwenstein tumor), basal cell carcinoma,
Bowen's disease, and Paget's disease.
Although anal and perianal neoplasms can be clinically evaluated and biopsied easily,
imaging with magnetic resonance imaging (MRI) is done to define the anatomical origin
and the internal characteristics of the mass, and for staging.[2] Through this pictorial review, we demonstrate the anal canal and perianal anatomy
on MRI and exhibit the imaging spectrum of tumors and tumorlike lesions in these regions.
We also review the radiologist's role in staging and treatment planning of common
neoplasms.
MR Imaging Technique
MR imaging performed with phased array coils provides excellent anatomical detail
of the anal sphincters and the anatomical boundaries of the pelvis. High-resolution
(HR) T2-weighted imaging (T2WI) in three orthogonal planes are the key sequences for
knowing the extent of the lesion and proper assessment of the anal sphincter complex.[3] These are obtained with a slice thickness of 3 to 3.5 mm, with a small field of
view (FOV) of 18 to 20 cm and high matrix (e.g., 320 × 256). Because of the forward
tilt of the anal canal, the axial and coronal planes for HR imaging should be aligned
orthogonal and parallel to the long axis of the anal canal, respectively ([Fig. 1]). Large FOV (covering the whole of the pelvis) T1 weighted imaging (T1WI), T1 fat-suppressed
imaging, short tau inversion recovery (STIR) imaging, and diffusion weighted imaging
(DWI) are other useful imaging sequences for disease characterization, determination
of the extent of disease, and nodal spread. Intravenous gadolinium contrast may not
be needed in most cases but can be used in selected cases to characterize uncommon
lesions.
Fig. 1 (A) Sagittal image of the anal canal demonstrating the planes to obtain axial (yellow dashed line) and coronal (red dashed line) T2 high-resolution images of the anal canal. (B,C) Magnetic resonance imaging (MRI) of the anatomy of the anal canal in coronal plane.
MRI Anatomy of Anal Region
The anal region includes the anal canal, anal verge (anus), and perianal skin ([Fig. 1]). The anal canal is the terminal part of the intestine that lies between the rectum
above and the anal verge below.[2] The anorectal junction is at the superior limit of the puborectalis muscle, which
forms a U-shaped sling around the superior portion of the anal canal. The anal verge is a band
of squamous epithelial tissue that lacks hair follicles.[2] The anal canal has the following layers from inside-out: the mucosa, submucosa,
and the sphincter muscles. An important landmark of the anal canal mucosa is the dentate
line, which marks the junction of the endoderm-derived upper two-thirds and ectoderm-derived
lower one-third of the anal canal.[2] The lining epithelium, the lymphatic and venous drainage, and nerve supply above
and below the dentate line are hence different. The lining epithelium in the lower
third of the anal canal is nonkeratinized squamous epithelium, that in the upper one-third
is the columnar epithelium, and the middle third is a mix of both and is hence called
the transitional zone.[2] The difference in lymphatic drainage above and below the dentate line will be seen
in nodal spread of anal canal tumors. Tumors above the dentate line drain into the
perirectal and internal iliac nodes, whereas tumors below the dentate line and the
anal margin drain into the inguinal and femoral lymph nodes.[4]
Deep to the anal canal mucosa is the submucosa followed by two muscular layers, namely,
the internal anal sphincter (IAS) and the external anal sphincter (EAS). The IAS is
an involuntary muscle that is the thickened extension of the circular smooth muscle
layer of the rectum. On MRI, IAS appears homogenous and moderately hyperintense on
T2WI.[5] The EAS, which is a voluntary muscle, along with the puborectalis sling (PS), forms
the outermost layer of the anal canal. The PS surrounds the upper third of the anal
canal, while the EAS surrounds the lower two-thirds. According to the traditional
description, EAS the is made of three muscle bundles, namely, deep, superficial, and
subcutaneous bundles.[2] The deep bundle surrounds the middle two-thirds anal canal. The superficial bundle
sweeps around and encircles the lower one-third of the anal canal. The subcutaneous
bundle lies inferolateral to the IAS and beneath the anal verge. On MRI, the EAS complex
has low signal intensity with “striated” appearance on T2WI. On HR T2WI, it may be
possible to distinguish the three components of EAS. The dentate line, although not
visible on MRI, is located at the level between the upper one-third and the lower
two-thirds anal canal.[5]
[6]
The perianal skin, also known as the anal margin, is the hair-bearing skin surrounding
the anal verge for a radius of 5 cm2. The ischioanal fossa is a pyramid-shaped space situated on both sides of the anal
sphincter complex with its base directed to the surface of the perineum and its apex
directed anteromedially toward the pubic symphysis.[2] It is bounded superiorly by the levator ani muscle, medially by the EAS muscles,
laterally by the obturator internus muscle and its fascia, and inferiorly by skin
of the perineum.
Classification and Staging of Anal and Perianal Neoplasms
The World Health Organization (WHO) classifies tumors of the anal canal broadly into
two categories: (1) benign epithelial tumors and precursors and (2) malignant epithelial
tumors.[7] They are further divided based on the histologic type. Perianal tumors are classified
just like skin tumors elsewhere.
Staging of cancers arising from both the anal canal and perianal regions is based
on the 8th edition of the American Joint Committee on Cancer (AJCC)/Union for International
Cancer Control (UICC) TNM (tumor size, node involvement, and metastasis status) classification ([Table 1]).[8]
[9] The T stage is based on size of the lesion and not based on the depth of invasion.
Regional lymph nodes for N staging include inguinal, mesorectal, superior rectal,
internal iliac, and external iliac nodes.
Table 1
TNM staging of anal and perianal lesions according to the 8th edition of AJCC/UICC
TNM classification
|
T category
|
T criteria
|
|
T0
|
No evidence of primary tumor
|
|
Tis
|
High-grade squamous intraepithelial lesion (previously termed carcinoma in situ, Bowen's
disease, anal intraepithelial neoplasia II–III, high-grade anal intraepithelial neoplasia)
|
|
T1
|
Tumor ≤2 cm
|
|
T2
|
Tumor >2 cm but ≤5 cm
|
|
T3
|
Tumor >5 cm
|
|
T4
|
Tumor of any size invading adjacent organ(s), such as the vagina, urethra, or bladder
|
|
N category
|
N criteria
|
|
N0
|
No regional lymph node metastasis
|
|
N1
|
Metastasis in the inguinal, mesorectal, internal iliac, or external iliac nodes
|
|
N1a
|
Metastasis in the inguinal, mesorectal, or internal iliac lymph nodes
|
|
N1b
|
Metastasis in the external iliac lymph nodes
|
|
N1c
|
Metastasis in the external iliac with any N1a nodes
|
|
M category
|
M criteria
|
|
M0
|
No distant metastasis
|
|
M1
|
Distant metastasis
|
Abbreviations: AJCC, American Joint Committee on Cancer; TNM, tumor size, node involvement, and metastasis status; UICC, Union for International Cancer Control.
Tumors Arising in the Anal Canal
Tumors Arising in the Anal Canal
Squamous Cell Carcinoma of the Anal Canal
SCC is the most common neoplasm of the anal canal, accounting for nearly 80% of all
neoplasms.[4] It can affect adults of any age group but peaks in sixth decade of life.[4] No gender predilection is seen. Human papillomavirus (HPV) infection (mainly HPV-16
and HPV-18 subtypes) and human immunodeficiency virus (HIV) infection remain the most
common risk factors for anal SCC.[4] Some of the other known risk factors include cervical dysplasia, autoimmune disorders,
immune suppression in transplant recipients, cigarette smoking, and long-standing
perianal fistulizing Crohn's disease. Typical imaging appearance of SCC is as an infiltrative,
usually circumferentially spreading lesion in the anal canal and the lower rectum,
which is intermediate on T2WI and mildly hypointense on T1WI, and shows restricted
diffusion ([Fig. 2]).[6]
, Lymph node involvement, if present, is commonly seen at the inguinal, internal, and
external iliac and mesorectal stations.[4]
[8] Metastasis can occur to any organ, but the liver and lungs are most frequently involved.[4]
[8]
Fig. 2 Anal canal squamous cell carcinoma (SCC) before treatment (A,B) and posttreatment (C,D). (A) Axial T2-weighted image (T2WI) and (B) high b-value diffusion weighted imaging (DWI) show typical appearance of treatment naïve
anal canal SCC, which is Infiltrative, usually a circumferentially spreading lesion,
which is intermediate on T2 and shows restricted diffusion. Postchemoradiotherapy
(CRT) T2WIs in (C) axial and (D) high b-value DWI show complete response to CRT indicated by loss of bulk of lesion
with T2 hypointensity signal without restricted diffusion.
Curative intent chemoradiotherapy (CRT) is the treatment of choice in most cases as
it preserves the anal sphincter function.[6] Following CRT, response assessment is done with MRI or fluorine-18 fluorodeoxyglucose
positron emission tomography/computed tomography (FDG-PET/CT) 6 months after CRT.[10]
[11] Complete response to CRT on MRI is indicated by loss of the bulk of the lesion with
T2 hypointense signal without restricted diffusion replacing it, indicating posttreatment
fibrosis ([Fig. 2]). There may be increased T2 signal intensity if there is edema, inflammation, and
necrosis following treatment, which can mimic residual tumor. Kochhar et al have described
“tram track” sign on post-CRT MRI scans, which is defined as parallel linear low signal
at the inner and outer margins of the internal sphincter at the site of the original
tumor.[10] This sign was present in more than half the patients studied at 6 months post-CRT
and had a negative predictive value for early local relapse of 83%. Absence of residual
metabolic activity (complete metabolic response) at posttreatment FDG-PET/CT suggests
low risk of recurrence and longer progression-free survival.[11] Salvage surgery as abdominoperineal resection is reserved for incomplete responders
to CRT.[6]
Adenocarcinoma of Anal Canal
Anal adenocarcinoma accounts for around 15% of all anal canal cancers.[12] Like SCC, it affects an adult of any age group with no gender predilection.[4]
[12] However, it has a lower peak age than SCC, occurring at the fifth decade of life.[4] In the majority of the cases, they are rectal adenocarcinomas that extend to the
anal canal. Adenocarcinomas arising from the anal gland are thought to be rare.[2] The differentiation between them is challenging, especially in large tumors where
both anal and low rectal involvement is observed. The AJCC uses the anatomical location
to differentiate between them. They recommend that if the epicenter of the tumor is
located more than 2 cm proximal to the dentate line or proximal to the anorectal ring
on digital examination, the tumor should be classified as rectal cancer. If the epicenter
of the tumor is located ≤2 cm from the dentate line, the tumor should be classified
as anal cancer.[8] TNM staging of rectal adenocarcinoma extending into the anal canal is like rectal
carcinoma, while adenocarcinoma arising from the anal gland is similar to anal SCC.[8] The differentiation, however, has no relevant implication as their pathological
characteristics and natural history are similar and hence they are managed similarly.[13] The European Society for Medical Oncology (ESMO) therefore includes all anorectal
adenocarcinomas as low rectal cancer, if the distance from the anal verge to the lower
margin of the tumor is less than 5 cm.[14]
Adenocarcinomas can be mucinous and nonmucinous.[15] Nonmucinous adenocarcinomas are the common subtype. They cannot be distinguished
from SCCs of the anal canal and appear as an infiltrative, circumferentially spreading
T2 intermediate lesion in the rectum and anal canal, and show restricted diffusion.[15] Mucinous adenocarcinomas are the less common subtype with worst prognosis and are
defined as tumors with extracellular mucin greater than 50% of tumor volume. On MRI,
mucinous tumors are characteristically T2 hyperintense lesions that show facilitated
diffusion. Following contrast, it will show a lacelike peripheral contrast enhancement
([Fig. 3]).[15] Mucin has low attenuation at CT and usually will be false negative on PET examinations
([Fig. 3]). Mesorectal lymph node involvement is very common for adenocarcinomas, while inguinal,
internal, and external iliac node involvements are less common.[4] Metastasis to the liver and lungs are most frequently observed.[4]
Fig. 3 Mucinous adenocarcinoma of anal canal. (A) T2-weighted image (T2WI) and (B) short tau inversion recovery (STIR) image in axial plane show T2/STIR markedly hyperintense
lobulated mass in the anal canal. (C) Axial computed tomography (CT) image through the anal canal done for planning radiotherapy
(RT) shows the low attenuation of mucin. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows mucin lakes within the specimen.
Imaging in restaging of adenocarcinomas with MRI is error prone because of difficulties
in distinguishing residual cellular from acellular mucin and persistent tumor bulk
despite potential treatment response.[16] Nonmucinous tumors can develop acellular mucin following neoadjuvant CRT, which
is a good prognostic sign and indicates response.[17]
The standard of care for nonmetastatic anorectal adenocarcinoma is neoadjuvant long-course
CRT followed by surgical resection.[17] Various abdominoperineal excision (APE) techniques are used for resection of low
rectal/anal adenocarcinoma. They include standard APE, intersphincteric APE, and extralevator
APE.[18]
[19] The technique is chosen, depending on the accurate assessment of the anal sphincter
involvement using an MRI-based low rectal cancer staging system ([Table 2]).[2]
[5]
Table 2
MRI low rectal cancer staging system (mrLR)
|
MRI stage
|
Anatomical definition
|
|
mrLR1
|
Tumor confined to the bowel wall and does not extend through full thickness
|
|
mrLR2
|
Tumor replaces the muscle coat but do not extend to the intersphincteric plane
|
|
mrLR3
|
Tumor invades the intersphincteric space
|
|
mrLR4
|
Tumor invades the external anal sphincter
|
Abbreviations: MRI, magnetic resonance imaging.
Anal Melanoma
Anal melanoma is rare, comprising less than 1% of all melanomas and 4% of anal malignancies.[20] It commonly occurs in the elderly and has a slight female predominance. The lesion
can be sessile or polypoidal in nature. Patients present with nonspecific symptoms
of rectal bleeding, anal lump, and tenesmus.[21] On clinical examination, polypoidal melanoma of anal canal can be easily mistaken
as a thrombosed hemorrhoid and often diagnosed only after pathologic examination of
hemorrhoidectomy specimens. Around 70% of anal melanomas are pigmented and hence are
called melanocytic melanomas, and the remaining are amelanotic melanomas.[20]
The imaging features of anal melanomas depend on the melanin content and the presence
or absence of hemorrhage. Melanocytic melanomas demonstrate T1 and T2 shortening ([Fig. 4]), which is seen as T1 hyperintensity and T2 hypointensity, respectively, whereas
amelanotic melanomas do not show substantial T1 or T2 shortening.[20] Most lesions show a homogenous enhancement pattern. The management of nonmetastatic
disease is surgical, either with APE or wide local excision, followed by adjuvant
chemotherapy, immunotherapy, radiation therapy, or a combination of these.[22]
Fig. 4 Anal melanoma. T2-weighted image (T2WI) in (A) axial and (B) coronal planes and (C) T1WI in axial plane show a T2 intermediate and T1 hyperintense lesion in the anal
canal infiltrating the sphincter. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows melanin containing tumor cells.
Anal Neuroendocrine Neoplasms
Neuroendocrine neoplasms, formerly known as carcinoid tumors, are rare in the anal
canal, occurring in individuals older than 50 years.[23] The new WHO classification of 2022 ([Table 3]) classifies them into NETs, if they are well differentiated, and as neuroendocrine
carcinoma (NEC), if they are poorly differentiated.[24] They are further subclassified based on the number of mitoses, Ki-67 proliferation index, and cytomorphology. NETs, being well differentiated, have
low cellular atypia and express neuroendocrine markers including chromogranin, synaptophysin,
and hormones, while NECs with marked cellular atypia, frequent necrosis, and high
proliferative activity will have variable and usually low sensitivity and specificity
for the common neuroendocrine markers.[24] NECs can also have a coexisting conventional carcinoma (i.e., adenocarcinoma or
SCC) in them. NETs of the anorectum are usually clinically asymptomatic unless metastatic,
while NECs present with signs and symptoms of an aggressive neoplasm and often disseminated.[23]
Table 3
The WHO classification (2022) of neuroendocrine neoplasm
|
Type
|
Classification
|
Diagnostic criteria
|
|
Well-differentiated neuroendocrine tumor (NET)
|
NET, grade 1
|
<2 mitoses/2 mm2 and/or Ki67 < 3%
|
|
NET, grade 2
|
2–20 mitoses/2 mm2 and/or Ki67 3–20%
|
|
NET, grade 3
|
>20 mitoses/2 mm2 and/or Ki67 > 20%
|
|
Poorly differentiated neuroendocrine carcinoma (NEC)
|
Small cell NEC
|
>20 mitoses/2 mm2 and/or Ki67 > 20% (often >70%), and small cell cytomorphology
|
|
Large cell NEC
|
>20 mitoses/2 mm2 and/or Ki67 > 20% (often >70%), and large cell cytomorphology
|
On MRI, the lesions can be of varied morphology, from small submucosal nodule to large
polypoidal ulcerating mass.[25] No specific MRI characteristics are present to differentiate them from other common
cancers of the anal canal. Functional imaging with 68-Gallium DOTA peptide (DOTATOC,
DOTATATE, DOTANOC) PET scan is used to stage NETs as they show high avidity.[25] NECs are commonly nonavid on 68 - Gallium DOTA PET imaging and hence FDG-PET/CT
is the preferred modality for staging.
Surgery (endoscopic or open) remains the cornerstone of treatment of a localized neuroendocrine
neoplasm. However, many patients develop metastases and require a multidisciplinary
approach for optimal management.[23]
[26] Chemotherapy is also recommended along with surgery, especially in cases of metastatic
NET and in NEC.[23]
Lymphoma
Primary anorectal lymphomas account for only 3% of all gastrointestinal lymphomas
and only approximately 0.1% of all anal malignancies.[16] HIV infection is a well-known risk factor due to the interplay between poor immune
response and infection by oncologic viruses, especially Epstein–Barr virus (EBV) infection.
The most common histologic subtype is diffuse large B-cell non-Hodgkin's lymphoma.[16]
At MRI, anal lymphoma usually appears as isointense on T1WI and intermediate signal
on T2WI, solid mass causing focal and/or circumferential thickening of the anal canal
([Fig. 5]).[27] They also show restricted diffusion on DWI and mild postcontrast enhancement. FDG-PET/CT
will show avid uptake and is the preferred modality for staging.
Fig. 5 A human immunodeficiency virus (HIV) positive man with anal lymphoma. T2-weighted
image (T2WI) in (A) axial and (B) coronal planes through the anal canal show an infiltrative T2 mildly hyperintense
mass in the anal canal. (C) T2WI in axial plane through the inguinal level shows bilateral disproportionately
large homogenous inguinal nodes. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
100x, shows a tumor with plasmablastic cells suggestive of plasmablastic lymphoma.
The treatment for anal lymphoma is not without confusion due to its rarity.[28]
[29] Medical management with chemotherapy is considered the primary treatment for most
cases.
Smooth Muscle Neoplasms
Common benign smooth muscle tumors include leiomyomas and myofibromas, while leiomyosarcomas
(LMS) are malignant tumors.[21]
[30] They originate from the muscularis mucosae or muscularis propria. These lesions,
being submucosal in origin, show a submucosal bulge with normal overlying mucosa,
unless the lesion is large and/or malignant.[31]
Both leiomyoma and myofibroma appear as smooth marginated and well-defined T2 mildly
hypointense and T1 isointense lesions, which usually grow exophytically ([Fig. 6]).[21]
[31] Myofibromas demonstrate intense postcontrast enhancement, whereas leiomyomas show
mild to moderate homogenous enhancement. LMS, like leiomyomas, are exophytically growing
lesions that are T1 isointense, but with irregular margins, heterogeneous on T2WI,
and demonstrate a heterogeneous enhancement ([Fig. 7]).[21]
Fig. 6 Anal leiomyoma. T2-weighted image (T2WI) in (A) axial and (B) coronal planes and (C) T1WI in axial plane show an exophytic growing smooth marginated T2 hypointense and
T1 isointense lesion extending into the right ischioanal fossa. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows well-differentiated smooth muscle cells with red fibrillar cytoplasm showing
no nuclear atypia.
Fig. 7 Anal leiomyosarcoma. (A) T2-weighted image (T2WI), (B) short tau inversion recovery (STIR), and (C) T1-wegihted image (T1WI) in axial planes show as lobulated exophytic growing lesion
with irregular margins with T2 heterogeneous signal due to necrosis. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows smooth muscle cells with a prominent nuclear atypia (arrow).
Surgical excision is the treatment of choice for benign lesions.[32] The mainstay treatment of LMS is surgery; however, neoadjuvant radiotherapy and
adjuvant chemotherapy are recommended as they decrease local and distant recurrence
and improve survival.[21]
Gastrointestinal Stromal Tumor
Anal gastrointestinal stromal tumors (GISTs) are rare (accounting for 5–10% of all
GISTs), but are one of the most common mesenchymal tumors of the anal canal.[33] These tumors arise from the interstitial cells of Cajal in the myenteric plexus.
The biologic behavior of these tumors is variable with the majority presenting with
local disease only and less than 20% of patients develop distant metastases.[33] Aggressive lesions are usually greater than 5 cm in size, while lesions of size
2 to 5 cm have an intermediate risk profile and lesions less than 2 cm seem to remain
consistently free of metastases.
On MRI, a GIST appears as a large smoothly marginated T1 iso- to hypointense and T2
hyperintense mass from the anal canal with a large exophytic component ([Fig. 8]).[33] Following contrast administration, larger tumors show central necrosis. Lymph node
metastases are generally absent.
Fig. 8 Anal gastrointestinal stromal tumor (GIST). T2-weighted image (T2WI) in (A) axial and (B) coronal planes and (C) T1WI in axial plane through the anal canal show exophytic growing smoothly marginated
T1 isointense and T2 heterogeneous mass from the anal canal. The heterogeneity on
T2 is due to multiple necrotic foci within the tumor (yellow arrowheads in A and B). (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows bland spindle cells with elongated nuclei typical of spindle type of GIST.
Surgery is the definitive curative treatment.[34] Imatinib mesylate, a tyrosine kinase inhibitor, is routinely used for unresectable
tumors as neoadjuvant or salvage therapy.[34]
Malignancy Associated with Fistula-in-Ano
Malignancies associated with fistula-in-ano are rare. It can be due to a malignancy
presenting as fistula-in-ano (malignancy first and fistula later) or malignancy developing
within a chronic fistula (fistula first and malignancy later).[35] The former type is thought to be more common and occurs when the tumor blocks the
opening of the anal gland resulting in formation of a perianal abscess and fistula.[35] The second type, where malignancy develops within a chronic fistula, is very rare
with only a few reports in the literature. In 1934, Rosser et al[35] proposed the following three criteria to diagnose this entity: (1) the fistula should
antedate the carcinoma by a minimum of 10 years, (2) the tumor should be the only
tumor in the anorectum, and (3) the internal opening of the fistula should not be
into the lesion. Tumors arising from such nonhealing fistulas are either adenocarcinomas
or SCCs.[36]
MRI can easily identify cases of malignancy presenting as fistula-in-ano ([Fig. 9]). However, diagnosing malignancy developing within a fistula is challenging by imaging
and has demonstrated low sensitivity unless in advanced stages.[36] The presence of focal wall thickening as T2 intermediate signal nodularity or enhancing
soft tissue within a chronic fistula is suspicious for malignant transformation of
a fistula ([Fig. 9]). The management guidelines for a malignancy developing in a fistula has not been
established, and many times radical resection combined with chemo radiation is generally
practiced.[36]
[37]
Fig. 9 Malignancy associated with fistula-in-ano. T2-weighted image (T2WI) in (A) sagittal plane and (B) short tau inversion recovery (STIR) axial image through the anal canal show a T2/STIR
markedly hyperintense lobulated mass in the anal canal with a high trans-sphincteric
fistula-in-ano (arrowheads in A and B). The presence of a lesion at the internal opening of the fistula favors this to
be a case of a fistula developing due to anal malignancy (malignancy presenting as
a fistula-in-ano). Companion images of a case of malignancy developing in chronic
fistula, which is seen on (C) T2 image and (D) STIR image as a complex trans-sphincteric fistula with multifocal areas of nodular
soft tissue within its wall (yellow arrowheads in C and D). A heterogeneous enlarged mesorectal node is also seen (yellow star in C).
Tumors Secondarily Involving anal Canal
Tumors Secondarily Involving anal Canal
Mesenchymal Tumors from Ischioanal Fossa
Mesenchymal tumors rarely can develop in the ischioanal fossa, arising from vascular,
neural, muscular, fibrous, and fatty elements.[38]
[Table 4] shows a simple classification of the ischioanal fossa tumors based on tissue of
origin. Among them aggressive angiomyxoma is the most common primary tumor of ischioanal
fossa and is discussed below. Lipomas and well-differentiated liposarcomas have predominant
fatty components and can be diagnosed easily on imaging.[38] Most other tumors have nonspecific MRI features and may need biopsy for a definitive
diagnosis.[38]
Table 4
A simple classification of ischioanal fossa tumors based on tissue of origin
|
Benign
|
|
• Vascular:
-Aggressive angiomyxoma, hemangioma
• Neural:
- Plexiform neurofibroma, schwannoma
• Adipocytic:
- Lipoma
• Muscle and fibrous tumors:
- Solitary fibrous tumor
• Congenital and developmental cysts:
- Tailgut cyst
- Epidermal cyst
- Dermoid cyst
|
|
Malignant
|
|
• Vascular:
- Angiosarcoma
• Neural:
- Malignant granular cell tumor
- Malignant peripheral nerve sheath tumor
• Adipocytic:
- Liposarcoma
• Muscle and fibrous tumors:
- Leiomyosarcoma
- Rhabdomyosarcoma
- Malignant PEComa
- Undifferentiated pleomorphic sarcoma, etc.
• Secondary neoplasms:
- Metastatic
- Direct tumor extension from the anal canal, vagina, vulva, urethra, and pelvic
musculoskeletal system
|
Abbreviations: PEComa, perivascular epithelioid cell tumor.
Aggressive Angiomyxoma
An aggressive angiomyxoma, although rare, is the most common tumor of the ischioanal
fossa and has a predilection for females of reproductive age.[38] These tumors present as a painless mass that can sometimes cause local pressure
on adjacent structures. They spread in multiple compartments without invading adjacent
organs.[39]
MRI features are very typical ([Fig. 10]) and include iso- to hypointense signal intensity relative to the muscle on T1WI,
and a characteristic “laminated” pattern on T2WI due to high signal intensity background
myxoid stroma with low signal intensity parallel lines representing collagen fibrils.
The lesion shows a heterogeneous postcontrast enhancement.[9]
Fig. 10 Aggressive angiomyxoma. T2-weighted image (T2WI) in (A) sagittal plane, (B) short tau inversion recovery (STIR) image in coronal plane, and (C) T1 image in axial plane show T2/STIR markedly hyperintense lobulated mass in the
anal and perianal region with a characteristic laminated appearance. The lesion is
hypointense on T1WI. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows myxoid stroma with scattered stellate to spindle cells.
The treatment of choice is wide surgical resection; however, due to infiltrative growth
and poor circumscription, it is difficult to excise this tumor completely, resulting
in high risk of local recurrences.[39]
Developmental Cysts
Congenital cysts, which can occur in the ischioanal fossa, include tailgut cyst, epidermoid
cyst, and dermoid cysts. Differentiation among them may be possible based on imaging.[40]
Tailgut cysts are usually multicystic lesions with the locules having variable intensity
due to the presence of mucin and blood ([Fig. 11]).[40] The signal intensity of mucinous fluid is variable and depends on protein concentration.
As protein concentration increases, T1 signal intensity changes from hypointense to
hyperintense, while T2 signal intensity shows inverse trend from hyperintense to hypointense
signals. Tailgut cysts can be complicated by infection and on rare occasions by malignant
transformation. Malignant transformation is suspected when there is focal irregular
wall thickening with enhancement.[40]
Fig. 11 Developmental cysts. (A) Tailgut cysts are multiloculated cysts in the retro-anal location, best demonstrated
on T2 image. (B) The high protein content within the cyst can be seen as T1 fat-suppressed sequence
(T1FS) hyperintensity. (C) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, showing tailgut cyst lined by a columnar epithelium (arrowhead) and squamous epithelium (arrow). (D) Epidermoid cysts are unilocular on T2 images with the presence of T2 hypointense
spheres within it representing keratin debris.
Epidermoid and dermoid cysts are often unilocular cysts lined by a stratified squamous
epithelium.[40] Epidermoid cysts contain a mixture of desquamated debris, cholesterol, keratin,
and water, but it lacks the skin appendages such as sweat glands, hair follicles,
and sebaceous glands, which are seen in dermoid cysts. On MRI, epidermoid cysts are
unilocular thin-walled cysts with low signal intensity on T1WI and high signal intensity
on T2WI, and may contain multiple T2 hypointense debris representing aggregates of
keratin ([Fig. 11]).[38] Dermoid cysts are similar to epidermoid cysts except that they can have fat components
within. Loss of signal intensity in these cysts on out-phase images as opposed to
in-phase images confirms their fatty nature.[40]
These cysts, if symptomatic, may need surgical excision.
Secondary Neoplasms from Adjacent Structures
Secondary Neoplasms from Adjacent Structures
Secondary tumors arising in the vagina, vulva, urethra, and pelvic musculoskeletal
system can involve the anal canal. Cancer of the vagina, if it involves the mucosa
of the anorectum, is considered T4 according to the eighth edition of the TNM (2018)
classification and stage IVa on the International Federation of Gynecology and Obstetrics
(FIGO) staging.[8] Vulval cancer with extension to the anal canal is T2 according to the eighth edition
of the TNM (2018) staging and stage II according to FIGO stage.[8] Urethral cancer (in both males and females) if fixed or invading the sphincter of
the anal canal is T4 according to the eighth edition of the TNM (2018) staging.[8]
Perianal Neoplasms
Bowen's Disease
Bowen's disease is an intraepithelial SCC that occurs most frequently on the face,
hands, and trunk, and less commonly in the perianal region.[41] In the perianal region, it probably represents severe anal intraepithelial neoplasia
(AIN) and is associated with HPV-16 and HPV-18 infections.[40] Like anal canal SCCs, Bowen's disease is much more common in patients with HIV.[41] In 2 to 6% of cases, the disease progress to an invasive SCC. Bowen's disease has
the highest incidence during the sixth and seventh decades of life.[41] Common symptoms include itching, burning, and occasionally bleeding, and on examination,
the lesion appears as a well-defined erythematous, scaly plaque.[41] Diagnosis is made by obtaining several full-thickness biopsy from the central portion
and edges of the lesion. Imaging is generally not done. The treatment of choice is
wide local excision.[41]
Perianal Paget's Disease
Perianal Paget's disease is quite an uncommon intraepithelial adenocarcinoma arising
from the dermal apocrine sweat glands and can be associated with underlying apocrine
or eccrine carcinoma, colorectal adenocarcinoma, and anal carcinoma in 38 to 69% of
patients.[41] It is most commonly found in older patients, averaging 66 years, and shows a preponderance
for women. The lesions are erythematous and crusty, eczematoid, or scaly appearing,
and present with nonspecific symptoms like pain, bleeding, and itching.[41] Diagnosis is made with full-thickness biopsies of the affected skin. FDG-PET/CT
or MRI is done before treatment to exclude associated underlying malignancy.[41] If the disease appears to be locally confined on preoperative workup and is noninvasive
on biopsy, wide local excision is the treatment of choice. When the tumor is invasive
or located close to the anal canal, and sufficient surgical margin cannot be achieved,
a combined modality treatment with chemoradiation may be needed.[41] If underlying malignancy is identified, it will also require appropriate treatment.
Squamous Cell Carcinoma of Perianal Skin
Perianal SCCs are at least five times less common than cancers of the anal canal and
have a more favorable prognosis as they are typically well differentiated, slow growing,
and without distant metastases.[41] The risk factors for perianal SCC are the same as for SCCs of the anal canal.[41] Most patients present after the fifth decade with an approximately equal male-to-female
predominance.[41] Symptoms of perianal SCC are nonspecific and include bleeding, pain, discharge,
and pruritus. On examination, they are usually hard, raised, and ulcerated skin lesions
with rolled, everted edges.[41] The anal canal can become involved late in the disease, although the sphincter complex
is rarely invaded. A VC is a rare histologic subtype of SCC characterized by highly
differentiated squamous cells and appears as a large locally aggressive tumor with
a cauliflowerlike appearance.[42] It is often associated and confused with a giant condyloma acuminatum (GCA), also
called Buschke–Löwenstein tumor due to similar clinical and imaging appearance. A
GCA has been linked to HPV, whereas a VC is negative for HPV.[42] On histological evaluation, VCs have no koilocytic atypia, whereas GCAs have koilocytic
atypia.[42] On the other hand, distinguishing VCs from conventional SCCs is also important because,
although large and in some cases locally aggressive, VCs are often amenable to wide
local resection since nodal metastases are extremely rare. However, there are published
reports showing that VCs and conventional SCCs can coexist in the same lesion.[41]
Any patient with a suspicious lesion needs biopsy for definitive diagnosis. Pretreatment
evaluation should include a full staging workup with imaging.[41] MRI of the pelvis can demonstrate the extent of lesion, anal canal involvement,
and regional nodes ([Fig. 12]). VCs on MRI appear as T2 hyperintense cauliflowerlike lesion from the perianal
skin ([Fig. 13]).[43]
Fig. 12 Perianal squamous cell carcinoma (SCC). T2- weighted image (T2WI) in (A) sagittal, (B) axial, and (C) coronal planes show T2 intermediate signal thickening in the posterior perianal
skin, which reaches up to the anal verge (arrowhead in A). The diagnosis was done by biopsy and magnetic resonance imaging (MRI) was done
as part of staging workup to evaluate anal canal involvement and to look for regional
nodes. (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows marked keratinization and minimal nuclear pleomorphism in a well-differentiated
SCC.
Fig. 13 Verrucous carcinoma at perianal region. T2-weighted image (T2WI) in (A) sagittal and (B) coronal planes and (C) short tau inversion recovery (STIR) image in axial plane shows a cauliflower like
T2/STIR hyperintense mass in the perianal region with infiltration of the superficial
and subcutaneous bundles of external anal sphincter (arrowhead). (D) Hematoxylin and eosin–stained histology microphotograph, of original magnification
40x, shows massive hyperkeratosis and parakeratosis.
Perianal SCCs are generally treated as skin cancers with wide local excision.[41] Lesions involving the anal canal needs combined modality of chemoradiation as initial
treatment approach followed by salvage surgery if recurrent or residual disease is
present.[41] VCs are also treated with wide local excision; however, high recurrence rates (as
high as 66%) after excision have prompted the use of neoadjuvant or adjuvant chemoradiation
therapy.[41]
Tumorlike Lesions
Prolapsed Hemorrhoids
Hemorrhoids are vascular cushions surrounding the anastomoses between the superior
rectal artery and the superior, middle, and inferior rectal veins.[44] Internal hemorrhoids arise above the dentate line and external hemorrhoids arise
below the dentate line. External hemorrhoids are innervated by somatic nerves and
cause pain, whereas internal hemorrhoids are innervated by visceral nerve fibers and
are painless. Most hemorrhoids are a combined type of internal and external hemorrhoids.[44] Hemorrhoidal disease is commonly diagnosed on clinical examination but can also
be seen on MRI. On T2WI, hemorrhoids are seen as moderately hyperintense polypoidal
lesion. They may also show increased signal on T1WI ([Fig. 14]).[45]
Fig. 14 Tumorlike lesions.
Vascular Malformations
Rectal bleeding is the most common presentation in vascular malformation (VM) and
can mimic neoplasms. Colonoscopy in such cases may reveal nodular submucosal tumorlike
lesion in the anorectum with normal or edematous overlying mucosa.[46] They can also be divided into high- and low-flow lesions.[46] High-flow lesions should have an arterial component. MRI can help in diagnosing
VM and help in defining its extent.[46] Low-flow malformations have cluster of cystic spaces with or without fluid debris
layer ([Fig. 14]), whereas high-flow malformations contain signal voids. On dynamic postcontrast
MRI, high-flow lesions show rapid filling, whereas low-flow lesions show late filling.
Calcification and phleboliths can be mistaken as flow void; however, they appear as
focal signal void and are not tortuous as flow void. In case of doubt in differentiating
them on MRI, a noncontrast CT can be done to identify calcification.
Endometriosis
Perianal endometriosis is extremely rare. It usually occurs in childbearing women
following obstetric or gynecological procedures most commonly following delivery at
episiotomy scar.[47] A patient typically presents with cyclical pain. Many times, the diagnosis may not
be made on physical examination, and MRI may be needed.[48] On MRI ([Fig. 14]), endometriosis manifests as a nodular or plaquelike lesion with intermediate signal
intensity on T1WI and low signal intensity on T2WI. Small T2 and T1 hyperintense foci
corresponding to endometrial glands and hemorrhagic foci are almost always recognized
within the endometriotic lesions.[48]
Infections
A typical fistula-in-ano is mostly an idiopathic process thought to be caused by obstruction
of anal glands and its diagnosis via an MRI is straightforward due to the presence
of well-defined fluid contained tracts. Perianal infection in immunosuppressed patients
with hematological malignancy is atypical on MRI as they have extensive anal and perianal
edema and rarely have defined fistula tracts, creating confusing imaging appearance
([Fig. 14]).[32]
Common infections of the anorectum includes nonsexually transmitted infections due
to Escherichia coli, Shigella spp., Campylobacter spp., and Clostridium difficile, and sexually transmitted infections due to Neisseria gonorrhoeae, Chlamydia trachomatis, Treponema pallidum, and herpes simplex virus (HSV).[49] Most of these infections do no need imaging for diagnosis and if done may show nonspecific
anorectal inflammation.
Inflammatory Phlegmon
A fistula-in-ano is commonly complicated by abscesses and ramifications. Atypical
findings of solid enhancing masslike lesions with no fluid component can represent
inflammatory phlegmon in partly treated fistulas and can mimic a malignancy developing
in chronic fistulas ([Fig. 14]).[50] If differentiating them is not possible on imaging, tissue sampling needs to be
done to confirm the difference.
Clues to Differential Diagnosis and Algorithmic Approach
Clues to Differential Diagnosis and Algorithmic Approach
A stepwise algorithmic approach to diagnose anal and perianal lesions based on imaging
is highlighted in [Fig. 15].
Fig. 15 Diagnostic algorithm for an imaging-based diagnosis of lesions in and around the
anal canal.
Conclusion
In this article, we showed a plethora of pathological conditions in the anal and perianal
region. MRI is the modality of choice for assessing the extent of anal and perianal
lesions. Some tumors have characteristic imaging findings that may permit accurate
diagnosis; however, tissue sampling is often needed to confirm the diagnosis. Accurate
diagnosis needs good knowledge of anal and perianal anatomy as well as knowledge of
the spectrum of imaging findings of common and uncommon neoplasms.
