Malignant tumours
Malignant
lesions of the anus and anal canal
Squamous
cell carcinoma
Basaloid
carcinoma
Mucoepidermal carcinoma
Basal cell carcinoma
Malignant
melanoma
Anal
intraepithelial neoplasia (AIN)
Carcinoma of the anus differs from carcinoma
of the rectum in histological structure, behaviour and types of treatment. This
is mainly because of its accessibility, its sensitivity and its abundant lymph
drainage, both superficial and deep. Seventy per cent of anal tumours arise in
the anal canal: 30 per cent are squamous cell carcinomas of the anal verge.
Squamous cell carcinoma
Because of its superficial situation, the
presence of this lesion is frequently recognised by the patient, who often
presents early. However, there are exceptions.
•
Radiation carcinoma sometimes develops in the anal and perianal skin of a
patient unwisely treated with lightly filtered radiographs for pruritus ani. The
chronic radiation dermatitis becomes so familiar to the patient that too often
he or she does not perceive the superimposition of carcinoma.
•
Anal warts sometimes take on a carcinomatous change (Fig. 61.45).
This is particularly likely in HIV-positive individuals.
• Occasionally, a squamous cell carcinoma develops in the track of a
long-standing fistula in ano.
The
following malignant tumours of the anal canal are also found, but they are rare.
Basaloid
carcinoma
This is also known as cloacogenic carcinoma and is a form of nonkeratinising squamous carcinoma. It can metastasise to lymph nodes and can be highly malignant. It is not very sensitive to irradiation.
Mucoepidermoid
carcinoma
This tumour arises near the squamocolumnar
cell junction and is of average malignancy. It is not well keratinised and is
radiosensitive.
Basal cell
carcinoma
Like the true squamous cell carcinomas of the
anal verge and lower anal canal these are ‘skin tumours’ and behave
accordingly.
Melanoma
Melanoma of the anus presents as a
bluish-black soft mass that is apt to be confused with a thrombotic pile, and
therefore unfortunately incised. Such trauma, followed -by the trauma of
defecation, incites the tumour to rapid metastasis. Left undisturbed, it
ulcerates and the colour of the tumour changes from blue to black. The inguinal
lymph nodes are soon involved. Unless a melanoma is excised at an early stage,
it disseminates by the bloodstream. The tumour is radioresistant and has a very
poor prognosis (Fig. 61.46).
Lymphoma
This may rarely affect the anal region and may
be part of a more widespread lymphomatous condition.
Clinical
features
Anal cancer can occur at almost any age, but
is usually found in the 6th and 7th decades. It is a rare condition, accounting
for approximately 2 per cent of all colorectal cancers. Symptoms include
rectal bleeding, mucus discharge, tenesmus, the sensation of a lump in the anus
and a change in bowel habit. Occasionally, a patient may present with a mass in
the inguinal region due to metastatic lymph nodes.
Rectal examination may reveal an ulcerating, hard, tender, bleeding mass in the anal canal or at the anal verge. The lesion may fungate through the anal canal and appear on the pen-anal skin, or present through a chronic draining anal fistula.
Predisposing
conditions
There appears to be a relationship between
anal condylomata caused by the human papilloma virus, particularly type 16, and
anal cancer. Similarly, the disease is more prevalent in patients infected with
the human immunodeficiency virus and those with anal intraepithelial neoplasia (AIN)
(see later). Several reports suggest a significantly higher incidence of anal
cancer in patients with Crohn’s disease.
Treatment of
squamous carcinoma of the anus and anal canal
Tumours of the
anal verge
For small squamous cell lesions of the anal
verge, wide local excision leaving a margin of at least 2.5 cm of tissue all
round is often sufficient to effect a cure. Lymphatic dissemination will be to
the inguinal nodes, which should be watched carefully. If they become
involved, block dissection removal of the glands of one or both groins will be
necessary and carries a fair prognosis for cure.
Tumours of the
anal canal
The traditional treatment for carcinoma of the
anal canal has been abdominopenineal excision, removing the growth and perianal
area widely. If and when the inguinal lymph nodes became involved, a radical
dissection of the groins was carried out. Although this operation is based on sound pathological
principles, the need for a permanent colostomy and the morbidity associated with
it have encouraged surgeons to try a more conservative approach first.
Radiotherapy
alone was used for selected small tumours for a long time. This was applied by
external beam, interstitial and intracavitary
techniques (Pack). Approximately 50 per cent
of tumours treated in this way were said to be eradicated, making subsequent
abdominoperineal excision unnecessary (Quan). In patients who presented with
inguinal lymph node metastases, block dissection of the groin was indicated in
addition to the radiotherapy.
A
combination of chemotherapy and radiotherapy, so-called chemo radiation (Nigro)
has now become the preferred initial therapy for all anal canal tumours. The
patient is given a combination of 5-fluomouracil (5-FU)
and mitomyein for approximately 1 week. Some authors have used a
combination of bleomycin, cisplatinum and adriamycin. The chemotherapy is
followed by radiotherapy given over 3—7 weeks. The patient is then examined
4—6 weeks after cessation of treatment. If there is obvious tumour remaining,
an abdominoperineal excision is performed. If there has been a good response
to therapy, the scar is excised; and if no microscopic carcinoma is present, the
patient is followed up carefully. The therapy does have unpleasant side effects:
most patients suffer proctitis and perineal dermatitis from the radiotherapy,
and leucopenia and thrombocytopenia are frequent with the chemotherapy; all
patients must be warned about the possibility of alopecia. Nevertheless,
approximately two-thirds of patients respond to chemoradiation and avoid a major
surgical procedure, and 90 per cent of patients are alive and well 2 years after
treatment. This therapy is applicable for all tumours, but those with a diameter
of 5 cm or more have a higher failure rate. Although many of these are
likely to result in eventual abdominoperineal excision, it is probably best to
treat them initially by chemoradiation, as this may make subsequent surgery
easier.
In
the frail patient with an advanced lesion, a defunctioning colostomy may be the
only therapy available to relieve the patient of distressing symptoms such as
incontinence.
Anal
intraepithelial neoplasm
AIN appears to be a premalignant lesion of the anal region. The term describes a dysplastic change which is histologically characterised by a loss of epithelial cellular maturation with associated nuclear hyperchromasia, pleomorphism, cellular crowding and abnormal mitoses within the anal epithelium. These features are identical to those of similar cervical and vulval lesions, and AIN is classified according to nomenclature used for genital intraepithelial neoplasia. Thus:
•
AIN I — cellular and nuclear abnormalities are restricted to the lower
third of the epithelium;
•
AIN II — the lower two-thirds of the epithelium are affected;
•
AIN III — the full thickness of the epithelium is affected. This is
synonymous with carcinoma in situ of the anus (sometimes termed Bowen’s disease).
AIN
seems to be aetiologically linked with human papilloma virus (HPV), type 16,
and is more prevalent in women with genital intraepithelial neoplasia and those
individuals who are systemically immunosuppressed (Scholefield). The natural
history is unknown, but there is circumstantial evidence that in certain cases
AIN may lead to invasive squamous cell carcinoma.
AIN
I and II are considered low grade and probably have minimal malignant potential
and therefore do not require treatment. In contrast, AIN III is considered high
grade, but treatment is difficult because the surgeon is often dealing with a
wide field change. Options include simple excision, excision with grafting,
laser ablation, cryoablation or the application of cytotoxic creams (such as 5
FU).