Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed aggressive fibromatosis (desmoid tumor)

- Relapse or disease progression despite surgery, chemotherapy, radiotherapy, or any
other treatment

- Tumors must meet the following criteria:

- Ineligible for complete surgical resection by carcinological exeresis OR surgery
would cause severe mutilation

- Cannot be treated with curative radiotherapy

- Measurable disease by RECIST criteria

- No prior malignancy

PATIENT CHARACTERISTICS:

- Not pregnant or nursing

- Fertile patients must use effective contraception during and for 6 months after
completion of study treatment

- Absolute neutrophil count > 1,000/mm^3

- Platelet count > 100,000/mm^3

- Bilirubin < 1.5 times upper limit of normal (ULN)

- SGOT and SGPT < 2.5 times ULN

- Creatinine ≤ 2.5 times normal

- No severe liver failure

- No chronic somatic or psychiatric illness that would preclude study compliance

- No known hypersensitivity to imatinib mesylate or one of its components

- No geographical, social, or psychological reason that would inhibit follow-up

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- No concurrent immunomodulators*

- No concurrent hormonal treatments* if used for fibromatosis

- No concurrent cytotoxic drugs*

- No concurrent nonsteroidal anti-inflammatory drug* if used for fibromatosis

- Allowed if used as an analgesic 3 months prior to disease progression

- No concurrent participation in another therapeutic investigational trial NOTE: *If
disease progression has occurred during this treatment, then the treatment must have
ended ≥ 1 month prior to study entry