Hybrid PET-MR in evaluation of peripheral nerve sheath tumors in patients with neurofibromatosis type I (NF I): Initial experience with six patients

Objectives Clinical manifestations of patients with NF I include neurofibromas, which can undergo transformation into malignant peripheral nerve sheath tumors (MPNST), an aggressive lesion and a leading cause of morbidity in this population(2). Currently functional 18-F FDG PET combined with anatomic CT imaging is an accepted modality for detection of MPNSTs in these patients(3). Because this a radiosensitive population, simultaneous PET and MR acquisition maybe an alternative and safer imaging study for detection of MPNST(4).

Methods In this IRB-approved, HIPAA-compliant prospective study, 8 patients with NF1 (6F, 2M; mean age 31.8 years) undergoing clinically indicated FDG PET-CT were subsequently imaged with hybrid PET-MR, including diffusion weighted imaging, using residual FDG activity. 2 patients were excluded for technical reasons affecting PET-MR SUV quantification. FDG avid lesions on both PET-CT and PET-MR were assessed by a single reader for the following parameters: SUVmax from PET-CT, SUVmax from PET-MR, and ADCmean. For each FDG avid lesion, comparison was made between SUVmax PET-CT and SUVmax PET-MR values. SUVmax PET-CT and ADCmean values for FDG avid lesions were also compared.

Results 14 FDG avid lesions were detected on both PET-CT and PET-MR with an accuracy of 100%. SUVmax values measured on PET-MRI and PET-CT demonstrated excellent correlation which was statistically significant (r = 0.82, p = 0.0022). There was poor negative correlation between SUVmax of PET-CT and ADCmean values (r = -0.26, p = 0.503).

Conclusions PET-MRI is a feasible alternative for PET-CT in patients with NF1 when assessing for malignant peripheral nerve sheath tumors, with excellent correlation between SUVmax from PET-CT and PET-MR. Poor negative correlation between SUVmax and ADCmean may indicate that these parameters provide different information regarding tumor biology. Larger studies are needed to further validate the feasibility of PET-MR in NF I patients and to re-evaluate the significance of ADC values in these lesions.