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).