Sir,

We report on a case of mucinous adenocarcinoma arising from a fistula-in-ano, which formed a huge subcutaneous mass in the buttock.

A 68-year-old diabetic man was referred to our hospital. Two years prior to the admission, the patient had first recognized a small subcutaneous nodule which gradually increased in size. On admission, he complained of a spontaneous pain. The mass measured 12×7 cm, and the covering skin had some ulcers with a foul smell.

A contrast-enhanced transverse T1-weighted MR image (Fig. 1) revealed a huge cystic mass. The wall showed marked contrast uptake. Some enhancing solid components were also disclosed. The content showed homogenous low signal intensity on T1-weighted images, whereas a T2-weighted image (not shown) revealed marked hyperintensity.

Fig. 1.
figure 1

A transverse T1-weighted MR image (TR/TE=500 ms/15 ms, with fat suppression) after an administration of gadopentetate dimeglumine (Gd-DTPA) shows a cystic mass with a thick wall, which shows marked contrast uptake. Note some enhancing solid components (arrows). The lesion measures 12 cm in the greatest dimension. The content appears homogenously hypointense

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On another transverse image (Fig. 2), which was obtained at the cranial aspect of the lesion, a tubular structure was disclosed. This finding implied a fistula between the mass and the anus, although no direct continuity was identified.

Fig. 2.
figure 2

Another transverse image, obtained at the cranial aspect of the lesion, reveals an enhancing tubular structure (arrow). This finding implies a fistula between the mass and the anus, although no direct continuity is identified

Full size image

These MR findings in conjunction with the clinical course strongly suggested a perianal abscess, which formed an extremely huge mass under a poorly controlled diabetic condition. A histopathological examination of the surgical specimen, however, revealed mucin-producing adenocarcinoma containing sticky mucous. In the fistula, located between the mass and the anus, no evidence of malignancy was noted.

These clinical and pathological findings were interpreted as mucinous adenocarcinoma arising from fistula-in-ano.

This rare neoplasm usually occurs in a patient with long-lasting fistulas persisting for more than 10 years [1]. It is often of low grade and grows slowly. Distant metastasis is infrequent and possible metastases are located mainly in the inguinal regions [1, 2]. There has been some debate as to whether the fistula is the source of the tumor, or whether the fistula is the presenting feature of a slow-growing, indolent carcinoma [1, 2].

The imaging appearance of this neoplasm has not been well documented. According to Hussain et al., who reported MR imaging findings of mucinous adenocarcinoma in other organs such as rectum or prostate, markedly hyperintense content on T2-weighted images is the hallmark of this neoplasm [3].

The present case showed some enhancing foci after administration of Gd-DTPA. These components corresponded to the solid lesions, which were proved to be mucin-producing adenocarcinoma extruding into the lumen.

A tiny tubular structure, which was proved to be a fistula between the mass and the anus, was also well identified on conventional fat-suppressed T1-weighted images after Gd-DTPA administration.

Perianal fistulas often accompany abscesses, which are easily depicted on MR imaging; however, an abscess does not usually contain enhancing internal solid components. This would be the major clue in differentiating an abscess from a mucinous carcinoma.

In conclusion, mucinous adenocarcinoma arising in fistula-in-ano is a rare entity but can be diagnosed by the following three characteristic MR findings: (a) markedly hyperintense fluid on T2-weighted images; (b) enhancing solid components; and (c) a fistula between the mass and the anus.