(P 149) LACK OF CORRELATION OF FDG-PET/CT SUV MAXIMUM AND MALIGNANT TRANSFORMATION IN NEUROFIBROMATOSIS TYPE 1-RELATED PLEXIFORM NEUROFIBROMAS

Objective: Optimal management of malignant peripheral nerve sheath tumor (MPNST) in patients with Neurofibromatosis Type 1 (NF-1) relies on early diagnosis or prevention of malignant transformation in predisposing plexiform neurofibromas (PN). Intratumor heterogeneity is a known occurrence in PNs where malignant transformation to MPNST can occur in part, but not all, of the underlying PN. Efforts have been made to standardize imaging guidelines for evaluations of lesions suspicious for MPNST and FDG-PET/CT is routinely performed with new clinical or radiographic concerns. Higher standardized uptake value (SUV) raises the possibility for malignancy with studies using a SUV maximum (SUVmax) cut-off of 3.2-4.5, however no ideal cut-off value has been found and substantiated. We assessed correlation between higher SUVmax and malignant pathology in NF1 patients with PN.

Methods: We evaluated patients with NF1 enrolled on the Tissue and Blood Acquisition for Genomic Analysis and Collection of Health Information for Patients with Bone and Soft Tissue Sarcoma: The Sarcoma Oncology Group at Washington University in St. Louis who underwent FDG-PET/CT imaging. Those patients who had subsequent biopsy or resection due to suspected malignant transformation based on FDG-PET/CT findings were reviewed.

Results: We identified eight patients with FDG-PET/CT imaging showing a lack of correlation between high SUVmax and malignant pathology. Median age of this group was 22 years (range 11-59 years) and detailed FDG-PET/CT findings and pathology results are reported in the below table. Lesions with SUVmax range of 6.1-13.3 were determined to be either neurofibroma or atypical neurofibromatous neoplasm with uncertain biologic potential (ANNUBP), but not MPNST. One patient had two lesions evaluated due to findings on FDG-PET/CT; one lesion with SUVmax of 2.6 was determined to be high-grade MPNST while a lesion with SUVmax of 6.5 was determined to be ANNUBP (see below figure). Furthermore, one patient’s initial biopsy of a mass with SUVmax of 7.7 revealed neurofibroma, but subsequent resection identified ANNUBP.

Conclusion: Sampling of suspicious lesions in NF1-related PN is critical as wide resection of non-metastatic MPNST can be curative. FDG-PET/CT is a non-invasive tool to assess for malignant transformation in patients; however, we caution that higher SUVmax can be seen in benign neurofibromas and pre-malignant ANNUBPs, the latter which does carry a risk of transformation to MPNST. Additionally, SUVmax did not correlate with malignant pathology in the same patient. We suggest using SUVmax to direct biopsy or resection while understanding the limitations of this modality. In the future, we hope other non-invasive testing such as detecting circulating tumor DNA can aid in diagnostic work-up for this high-risk population and studies are underway.