This study is for slow growing tumors called plexiform neurofibromas (PNF) which are a relatively common problem in people with neurofibromatosis type 1 (NF1). These tumors are benign but as they grow, they can become disfiguring as well as disabling or even life threatening. They often cause pain, difficulty using arms or legs because of spinal cord compression, and/or nerve damage. At present, the only available therapy for plexiform neurofibromas is to try to surgically remove as much of the tumor as is possible. Because these tumors grow into the surrounding areas, total surgical resection is often impossible. Most tumors will re-grow after surgery if the entire tumor cannot be removed. To date, other treatments including chemotherapy and radiotherapy have not been able to shrink these tumors.

Interferon is a drug that is used for different types of tumors as well as for hepatitis. It has been used in the treatment of plexiform neurofibromas (PNF) with some subjects showing improvement in symptoms and/or a decrease in the size of the tumor. Most subjects had no further growth of their tumor while on the PEG-Intron. The drug used in this study is PEG (pegylated)-Intron. PEG-Intron is a long acting form of interferon which keeps the drug from being broken down in the body for a longer period of time and potentially could be more effective than the short-acting interferon. PEG-Intron has been approved by the Food and Drug Administration (FDA) for the treatment of Hepatitis C.

The goals of this study are:

1. To determine how your child's plexiform neurofibroma responds to PEG-Intron when given weekly.
2. To determine the side effects of PEG-Intron when given weekly to participants with plexiform neurofibromas.
3. To evaluate a new method of measuring changes in the size of tumors called volume analysis. This method measures the entire volume of a tumor in three dimensions. The standard method of measuring tumors uses only the length and/or width of the tumor. By studying the different ways of measuring tumors the investigators hope to be able to determine which method is the most accurate and useful.

PEG-Intron will be given every week through a small needle under the skin, in the same way that insulin is given to people with diabetes. Subjects will be taught to do this at home. As long as the tumor isn't growing and the side effects are tolerable, the injections will be given once a week for 2 years.

Children will be enrolled on the study into one of three strata (patient groups): strata 1 includes children who do not have any symptoms associated with their PNF and MRI scans over the past year may or may not show tumor growth, strata 2 includes children who have symptoms associated with the PNF but MRI scans over the past year have not shown growth and strata 3 includes children who have shown an increase in the size of the PNF on MRI scans over the past year, with or without any symptoms.

Inclusion Criteria:

• Age > or equal to 18 months to 21 years
• Diagnosis Patients with NF1 and progressive, symptomatic or life-threatening plexiform neurofibromas which are not surgically resectable. Patients without biopsy-proof of plexiform must have at least one other diagnostic criteria for NF1: 6 or more cafe-au-lait spots, freckling in the axilla or groin, optic glioma, 2 or more Lisch nodules, distinctive bony lesion, first degree relative with NF1.
• Only eligible if complete tumor resection is not feasible, or if a patient with a surgical option refuses surgery.
• Patients may be treated on this trial without having received prior therapy. If patients have received prior therapy, they must have recovered from all toxic effects prior to entering this study.
• Must have life expectancy of at least 12 months and a performance score (Karnofsky or Lansky) of > or equal to 50.
• Must have adequate liver, kidney and bone marrow function

Exclusion Criteria:

• Clinically significant unrelated systemic illness.
• Received an investigational agent within the past 30 days.
• Evidence of active visual pathway glioma
• History of malignant peripheral nerve sheath tumor or ther cancer other than surgically cured non-melanoma skin cancer or cervical carcinoma in situ
• Ongoing radiation therapy, chemotherapy, hormonal therapy directed at the tumor, or immunotherapy.
• Inability to return for follow-up visits or obtain follow-up studies
• Severe cardiovascular disease
• Pre-existing sever psychiatric condition or history of psychiatric disorder requiring hospitalization or a history of suicidal ideation or attempt.
• Thyroid dysfunction not responsive to therapy.
• Uncontrolled diabetes mellitus
• History of seropositivity for HIV
• Subjects who are pregnant, lactating, or of reproductive potential and not practicing an effective means of contraception.
• Any medical condition requiring chronic systemic corticosteroids.
• Subjects who are known to be actively abusing alcohol or drugs.

Adverse event data were collected during clinic visits at baseline, week 6, months 4, 8, 12, 18, and 24 while on treatment and every 6 months thereafter until off study

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