An unusual ischiorectal fossa mass.

INTRODUCTION: Peri-anal fistulae commonly present with collections requiring surgical intervention. The most common cause of a peri-anal mass is abscess formation secondary to anal gland sepsis. In certain patient groups such as those over 65 or with atypical presenting symptoms there are other important considerations. PRESENTATION OF CASE: A 70-year old male was referred by his general practitioner with symptoms of obstructed defaecation and a palpable mass in the ischiorectal fossa. He had previously undergone three operations for complex peri-anal fistulae in another hospital. Due to the previous history of surgery, seroma formation was considered and computed tomography guided drainage of the lesion was performed unsuccessfully. Given symptoms of obstructed defecation and need for histological diagnosis excision was undertaken. The approach was through a right pararectal incision over the bulk of the mass. Skin and pararectal tissue were divided revealing a mucinous lesion with multiple lobules adherent to pararectal tissue. Following histopathological examination a diagnosis of low grade mucinous neoplasm was made. DISCUSSION: Primary mucinous neoplasm in the ischiorectal fossa is very rare. Diagnostic criteria for adenocarcinoma arising from perianal fistulae have previously been established by Rosser et al. but this lesion does not fall into this category. It is categorised as a mucinous cystic neoplasm of uncertain malignant potential. The differential diagnoses are discussed.
CONCLUSION: Consideration should be given to a range of pathologies in cases of atypical peri-anal masses.

Introduction

Complex peri-anal fistulae commonly present with peri-anal collections requiring surgical intervention. The most common cause of a peri-anal collection or mass is abscess formation secondary to anal gland sepsis. In certain patient groups such as those over 65 or with a long history of inflammatory bowel disease there are other important considerations. Among the differential diagnoses to be considered are primary tumours such as rectal or anal adenocarcinoma or secondary spread of intra-abdominal origin, developmental disorders including tailgut cysts and inflammatory or traumatic causes. Here we present an unusual case of a 70-year old male presenting with a palpable mass in the right ischiorectal fossa with a history of multiple previous operations for peri-anal fistulae. Due to worsening symptoms of obstructed defaecation, normal colonoscopy and failure of radiological drainage he underwent excision of a right sided ischiorectal mass revealing a complex mucinous lesion.

Case report

A 70-year old male was referred by his general practitioner with symptoms of obstructed defaecation and a palpable mass in the right ischiorectal fossa. He had previously undergone three operations for complex peri-anal fistulae in another hospital four years earlier. Unfortunately no histopathology reports were available. His medical history was significant for ischaemic heart disease and atrial fibrillation.

A check colonoscopy showed diverticular disease but no visible mass. On digital rectal examination he had a palpable swelling in the right ischiorectal fossa compressing the rectum. Magnetic resonance imaging (MRI) showed an 8 cm × 9 cm lobulated fluid collection in the right ischiorectal fossa displacing the rectum (Figs. 1 and 2). There was no evidence of fistulous communication to the collection and no fat stranding or enhancement suggestive of inflammation. Due to the previous history of surgery, seroma formation was considered and computed tomography (CT) guided drainage of the lesion was undertaken. A 17 gauge needle was inserted directly into the lesion in 4 different locations but no fluid was drained. Given symptoms of obstructed defecation and need for histological diagnosis the lesion was excised. The surgical approach was through a right para-rectal incision over the bulk of the mass. Skin and pararectal tissue were divided revealing a multi-lobulated mucin containing mass adherent to para-rectal tissue. Differential diagnosis following inspection included a chronic inflammatory process, mucin producing tumour or cyst secondary to previous fistulae and preceding anorectal surgery. The mucin was drained and the associated sac excised from surrounding tissues. No further resection was performed as the lesion was deemed to be fully excised.

Histopathological examination was undertaken and expert confirmatory opinion from an international centre was sought. The lesion showed focal residual lining of mucin producing neoplastic epithelium with low grade dysplasia (Fig. 3). The luminal mucin dissected into the adjacent peri-rectal fibrous tissue (Fig. 4). Overt invasive adenocarcinoma was not seen. These features are similar to those seen in low grade mucinous neoplasm (of uncertain malignant potential) more frequenly seen in sites such as the appendix. Subsequent CT abdomen showed no evidence of an appendicular neoplasm, intra-abdominal mucinous tumour or pseudomyxoma peritonei. The patient was referred to the pseudomyxoma institute at Basingstoke, UK and is recurrence free at 7 months post operatively.

Fig. 3

Mucin producing neoplastic epithelium with low grade dysplasia.

Fig. 4

Luminal mucin dissecting into adjacent peri-rectal fibrous tissue.

Discussion

Primary mucin producing epithelial neoplasm arising in the ischiorectal fossa is rare. Fistula associated peri-anal mucinous adenocarcinoma is an uncommon but recognised occurrence due to malignant transformation of chronic fistula-in-ano. Diagnostic criteria for adenocarcinoma arising from perianal fistulae have previously been established by Rosser. These state that (1) the fistula should precede the carcinoma by a minimum of 10 years, (2) tumour present in the rectum or anal canal should be due to direct extension from carcinoma within the fistula and (3) the internal opening of the fistula should be into the anal canal and not the tumour. In cases fulfilling these criteria the appropriate treatment is abdominoperineal resection with excision of both ischiorectal fossa and overlying skin as local recurrence is common. While initially considered in the differential diagnosis the lesion described in our case does not fit any of these criteria.

Expert histopathological opinion initially raised the possibility of origin from an intra-abdominal lesion such as a low grade appendiceal mucinous tumour. Intra-abdominal dissemination of mucin and low grade low grade mucinous epithelium is associated with pseudomyxoma peritonei. This diagnosis was excluded clinically and with radiological imaging.

Other possibilities include developmental abnormalities such as tail gut cysts (retrorectal cystic hamartoma) which occur in the retrorectal space but may extend laterally into the ischiorectal fossa. These may present with peri-anal pain and frequent peri-anal fistulae or abscess formation. These are unilocular or multicystic masses that adhere to the rectum but do not communicate with the lumen. Malignant degeneration into mucinous carcinoma is a rare complication. Primary neoplasms occurring in the ischiorectal fossa have previously been described but are uncommon. Angiomyxoma is a rare neoplasm which can also present with a well defined lobulated mass in the ischiorectal fossa but with a different histology to our case.

Detailed imaging excluded primary origin within the abdominal cavity. The histology is neoplastic but not with overt features of adenocarcinoma. It does, histologically show features of a low grade cystic mucinous neoplasm similar to lesions recognised at other sites (e.g. appendix) where they are of uncertain malignant potential. The epithelium of origin of the neoplasm at this site is unclear but could be lining glandular epithelium from a long standing fistula tract or from peri-anal mucin producing glands. The latter, however, is unlikely given the absence of communication/continuity with the anal canal. Due to the rarity and uncertain malignant potential of this lesion, the management is not defined. It has been excised but will require close follow up. Perianal mucinous carcinomas have previously been successfully treated with combinations of external beam radiotherapy and brachytherapy and this is a possible treatment option if the lesion recurs.

Overall this case emphasises that consideration should be given to a wide range of diagnoses in cases of atypical peri-anal masses. It also reinforces that in patients presenting with obstructed defecation there must be a high index of suspicion for a pelvic lesion. There are no similar reported cases of primary low grade mucinous tumours presenting in the ischiorectal fossa and this case represents a challenge in both diagnosis and long term management.

Conflicts of interest statement

None.

Funding

None.

Ethical approval

Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Author contributions

Cillian Clancy – writing of manuscript, acquisition of data; Zafar Iqbal – acquisition of data; Margaret Sheehan – acquisition of data, writing of manuscript; Myles Joyce – concept.