Desmoid tumours, also known as aggressive fibromatoses, are soft tissue neoplasms arising from musculoaponeurotic tissues. They belong to a family of fibroblastic proliferations that includes a variety of fibromatoses, as well as low-grade fibrosarcomas. These tumours are rare, a series of 44 patients treated for abdominal wall tumours being the largest reported. More than 75 per cent of these tumours arise in extra-abdominal sites, primarily the shoulder girdle, the inguinal area, and the lower extremities. Abdominal wall desmoids occur most commonly in women of child-bearing age during or shortly following pregnancy; they may be associated with scars from prior operations or trauma. The tumours are also seen in patients with familial polyposis coli (Gardner's syndrome), who may present with unresectable intra-abdominal tumours, as well as desmoid tumours at other sites. Desmoid tumours are often painless masses located deep in the abdominal wall, distinguishable in most situations from hernias or intra-abdominal masses by physical examination. They arise most commonly in the rectus abdominis muscles or the linea alba.

On microscopic examination, the tumours are composed of uniform, normal appearing fibroblasts distributed through dense collagenous stroma. Nuclei are thin and elongated; mitotic figures are seen typically in less than one per 50 high-power microscopic fields, and the fibroblastic component has neither the cellularity nor pleomorphism required for classification as low-grade fibrosarcoma. The lesions are never encapsulated. They insinuate themselves along fascial planes and muscle bundles in a fashion identical to that of sarcomas. The natural history of abdominal wall desmoid tumours is one of local progression with potential invasion of intra-abdominal structures, extremely rare pulmonary metastases, and multiple rapid local recurrences after excision: 80 per cent of recurrences develop within 2 years of excision. Because of the low incidence of dissemination, controversy regarding the inclusion of these lesions as bona fide sarcomas has existed for the last 50 years.

The diagnosis of desmoid tumour is confirmed by examination of a biopsy specimen. If the tumour is large or if there is a question of intra-abdominal extension or fixation to adjacent bony structures on physical examination, preoperative CT examination is useful. No search for distant metastases is necessary. If the lesion is less than 4 cm in diameter, excisional biopsy is appropriate. For larger lesions, incisional biopsy is preferred; evaluation of permanent sections of histological specimens to rule out sarcoma is then possible, and potential seeding of neoplastic cells along tissue planes for large distances as a result of a more extensive dissection is minimized. Haemostasis should be absolute. The biopsy incision should be placed so that it may be excised by a second, definitive excision.

Wide local excision of the tumour, which may require excision of the full thickness of the abdominal wall, is the operation of choice. Local recurrence rates are lower than for extra-abdominal tumours, ranging from 10 to 40 per cent. The rate increases with each recurrence. Although the presence of a pseudocapsule may give the surgeon a false sense of security when excising a presumably benign mass, tumour cells invariably extend beyond the grossly apparent tumour, making simple excisional biopsy a seemingly inadequate operation. There is, however, no clear correlation between margins of excision and local recurrence of these tumours: Posner found a highly significant correlation between the likelihood of local recurrence and the presence of inadequate or marginal excisions; on the other hand Reitamo et al. and Easter and Halasz found no such correlation. The lack of careful systematic evaluation of margins in most studies further confuses the issue. There is no correlation between tumour size and local recurrence rate. For tumours less than 5 cm in diameter, excision with a margin of 2 to 3 cm of grossly normal adjacent soft tissue is adequate, and primary closure of the abdominal wall is generally possible. The occult extension of larger tumours may be greater, and, if anatomically possible, a margin of excision approaching 5 cm should be considered. Synthetic materials or rotational flaps may be required to effect closure of the defect. Margins of excision should be carefully evaluated by the pathologist, and consideration given to additional therapy if excision of all gross tumour is not feasible. Recurrent tumours should be re-excised when the lesion can be resected grossly.

Radiation therapy to doses of 5000 to 6500 cGy is useful in controlling many unresectable tumours, producing complete responses, with long-term tumour control in 60 to 70 per cent of patients. Response to radiation is often slow and desmoid tumours may require 2 years to shrink. Since not all patients undergoing tumour excision with histologically positive margins suffer recurrence, postoperative radiation should be considered only when there is grossly evident residual tumour following an attempt at resection. For the postoperative patient with a small area of microscopically positive margin, careful follow-up and prompt treatment of recurrence with another excision or with radiation therapy is an effective alternative to adding radiation empirically.

A variety of hormonal, anti-inflammatory, and chemotherapeutic agents with acceptable toxicity have been evaluated on a very limited basis for therapy of desmoid tumours when surgery and radiotherapy are no longer options. Prolonged responses to therapy with vinblastine and methotrexate occurred in six of eight patients so treated; this regimen is currently the most active reported. Anecdotal responses have been seen to tamoxifen alone or in combination with indomethacin.