DISEASE CHARACTERISTICS:

* Histologically confirmed newly diagnosed supratentorial glioblastoma multiforme by surgical biopsy or excision

* No gliomas graded less than glioblastoma multiforme
* No recurrent malignant gliomas
* No tumor foci below the tentorium or beyond the cranial vault
* Study therapy must begin ≤ 5 weeks after surgery

PATIENT CHARACTERISTICS:

Age

* 18 and over

Performance status

* Zubrod 0-1

Life expectancy

* At least 8 weeks

Hematopoietic

* Absolute neutrophil count ≥ 1,500/mm\^3
* Platelet count ≥ 100,000/mm\^3
* Hemoglobin ≥ 10 g/dL\*
* Hematocrit ≥ 30%\* NOTE: \*Transfusion allowed

Hepatic

* Bilirubin ≤ 0.5 mg/dL
* ALT or AST ≤ 2 times upper limit of normal

Renal

* Creatinine ≤ 1.5 mg/dL
* BUN ≤ 25 mg/dL

Other

* Not pregnant or nursing
* Negative pregnancy test
* Fertile patients must use effective double-barrier contraception during and for 2 months after study participation
* No AIDS
* No other malignancy within the past 5 years except nonmelanoma skin cancer or carcinoma in situ of the cervix or bladder
* No other major medical illness or psychiatric impairment that would preclude study participation

PRIOR CONCURRENT THERAPY:

Biologic therapy

* No concurrent sargramostim (GM-CSF)

Chemotherapy

* No other concurrent chemotherapy

Endocrine therapy

* Concurrent steroid therapy allowed provided patient is on a stable or decreasing dose for at least 2 weeks before study entry

Radiotherapy

* No prior radiotherapy to the head or neck resulting in overlap of radiotherapy fields

* Prior radiotherapy for stage T1 glottic cancer allowed

Surgery

* See Disease Characteristics
* Recovered from prior surgery

Other

* No enzyme-inducing antiepileptic drugs within 14 days before the initiation of irinotecan

* Concurrent non-enzyme-inducing antiepileptic drugs allowed