MAINTENANCE THERAPY: Patients receive isotretinoin orally (PO) twice daily (BID) on day 1 and days 16-28 and cisplatin, vincristine sulfate, cyclophosphamide, and filgrastim as in Arm I maintenance therapy. Treatment repeats every 28 days for a total of 6 courses in the absence of disease progression or unacceptable toxicity. Patients then proceed to continuation therapy.