perianal mucinous adenocarcinoma is a rare disease often associated with a long - standing anal fistula , representing approximately 23% of large bowel cancers . the occurrence of a carcinoma in association with a fistula is probably due to chronic inflammation , although the rarity of the condition precludes any definite assumption in regard to the etiologic relationship of the fistula and carcinoma . in most patients , early diagnosis of this disease detection is usually late as the symptoms often initially mimic benign inflammatory conditions of the anorectal region and biopsies fail to reveal the infiltrating carcinoma . therefore a tumor is often found in progress . due to the rarity of this tumor and the lack of sufficient patients for controlled trials , abdominoperineal resection ( apr ) is advocated in the vast majority of patients to definitively remove the entire lesion and lessen the risk of local recurrence . local resection is sometimes performed , but only in cases where the tumor margins can be confidently excised without damage to the anal sphincters . in order to prevent local recurrence , the apr has to be performed with wide local excision of both perianal soft tissue and the overlying skin . recently locally advanced rectal cancer has been considered an indication for preoperative chemoradiotherapy ( crt ) . preoperative crt for rectal cancer has been shown to increase the probability of tumor resectability and to decrease the rate of local recurrence . radiotherapy alone or combined with chemotherapy can be a curative treatment for squamous cell carcinoma of the anal canal . on the other hand , the role of radiotherapy has not been established for perianal mucinous carcinoma . the aim of this article is to report a rare case of large perianal mucinous adenocarcinoma , arising from an anorectal fistula , successfully resected after preoperative radiotherapy . a 60-year - old man with a 45-year history of perianal fistula who had recently been suffering from mucinous discharge , pain and perianal induration was referred to hospital for treatment . two years before , he had undergone surgery at another institution for incision and drainage of recurrent perianal abscess . physical examination revealed an indurated , ulcerative lesion 7 cm in diameter with an external anal fistula opening ( fig . the internal opening of the fistula was indicated at the right side of dentate line . magnetic resonance imaging ( mri ) demonstrated a large demarcated tumor at the level of the anorectal junction with extension to the right side of the ischiorectal fossa and presacral space ( fig . no enlarged pelvic lymph node was observed , nor was there evidence of distant metastasis . neither colonoscopy nor barium enema showed any evidence of mucosal lesion in the colon or rectum . laboratory data showed renal dysfunction caused by pyelonephritis , and the carcinoembryonic antigen serum level was extremely elevated to 40.8 ng / ml ( normal < 5 ng / ml ) . to increase resectability and decrease the rate of local recurrence , preoperative radiotherapy was proposed without chemotherapy because of the renal dysfunction . preoperative radiotherapy was given with 40 gy in 25 fractions for 5 weeks through posterior and bilateral portals . 1c , d ) and carcinoembryonic antigen serum level gradually decreased to 13.0 ng / ml . the resected specimen revealed that the tumor had developed in the ischiorectal fossa , but there was no evidence of cancer extension to the mucosal surface of the anal canal and rectum ( fig . histological examination of the excised specimen showed foci of mucinous adenocarcinoma involving the lower internal and external anal sphincter muscles , and the resected margins did not show any residual tumor ( fig . 3 ) . according to the uicc tnm staging criteria , the treatment outcome with preoperative radiotherapy revealed minimal response ( grade 2 ) . perianal mucinous adenocarcinomas arising from anorectal fistulae are rare tumors resulting in severe problems concerning diagnosis and treatment . it is suggested that adenocarcinoma developing in the perianal region is often associated with a long - standing anal fistula . early diagnosis of a perianal tumor is difficult because of the absence of a tumor within the lumen of the bowel and the slow growth of a lesion hidden within the ischioanal fossa . as local excision is inadequate in most cases , apr with wide excision of both ischiorectal fossae and the overlying skin is recommended . on the other hand , radiotherapy alone or combined with chemotherapy can be a curative treatment for squamous cell carcinoma of the anal canal . in the present case , total removal of the tumor without any residual tumor was quite difficult because of the size of the tumor and the fact that it had infiltrated the surrounding tissue . recently , preoperative crt has become common practice for stage ii and iii rectal cancers . it has been well documented that neoadjuvant crt induces tumor regression and downstaging , and therefore increases tumor resectability and r0 resection rate . therefore we performed preoperative radiotherapy to improve the resection rate and decrease the local recurrence rate . after treatment the tumor was markedly shrunk and could be completely resected without skin grafting . this suggests that preoperative radiotherapy with or without chemotherapy is appropriate for treatment of perianal mucinous carcinoma arising from an anorectal fistula if adequate surgical margins can not be ensured . the role of crt in the treatment of perianal mucinous adenocarcinoma has not yet been established . unfortunately there are an insufficient number of reported cases undergoing this therapy to determine the prognosis for individual patients [ 7 , 8 ] . further tests are required to standardize the treatment of preoperative crt for locally advanced perianal mucinous adenocarcinoma arising from an anorectal fistula . in conclusion , if surgical treatment for anorectal fistula is unsuccessful in the long term , mucinous adenocarcinoma should be suspected . the result of this case suggests that preoperative radiotherapy appears to be a valuable alternative treatment for patients with perianal mucinous adenocarcinoma arising from an anorectal fistula . the advantage of this approach can result in downstaging and r0 resection , which prevents local recurrence .