Background Signs and symptoms of neurofibromatosis type 1 (NF1) often begin during early childhood. Plexiform neurofibromas (PNs) affect approximately 20% to 50% of patients with NF1 and can lead to pain, disfigurement, malignancy, and compression of vital structures (Nguyen et al., 2011; Tchernev et al., 2016; Miller et al., 2019). The aim of this study was to describe characteristics and diagnostic journey of pediatric patients in the US diagnosed with both NF1 and symptomatic, inoperable PNs. Methods This was a retrospective observational study of pediatric NF1 patients with at least one symptomatic inoperable PN from 3 NF treatment centers in the US. Patient records were abstracted if the patient (1) received NF1 diagnosis from 1 Jan 2000 to 31 Dec 2018, (2) was 2 to 18 years old at the time of diagnosis, (3) was diagnosed with ≥1 symptomatic inoperable PN, (4) received care on-site for ≥12 months, and (5) had ≥3 site visits prior to the end of the study period on 31 Dec 2019. Records were abstracted at the patient’s first visit, last visit, and visit nearest the midpoint of observation. All study measures were summarized with standard descriptive statistics using SAS Studio statistical software (version 3.8) (Cary, NC; SAS Institute, Inc.; 2011). Results The sample included 102 patients with an average of 3.1 PNs treated over 8.25 years. Most patients were diagnosed with NF1 before the age 3 (57.8%) and with their first symptomatic, inoperable PN by the age 6 (51.0%). Nearly 85% of patients had their first symptomatic, inoperable PN diagnosed by age 10, which typically involved a geneticist (68.6%). Nearly all patients had at least 1 PN diagnosed using MRI (92.2%) while 41.2% reported at least 1 PN through clinical assessment without imaging. Patient PNs were most often managed by a geneticist (85.3%) and/or a pediatric oncologist (30.4%), pediatric neurologist (22.5%), or pediatric neuro-oncologist (22.5%). The most common symptoms at diagnosis of first PN were pain (55.9%) and disfigurement (42.2%). Throughout the review period, pain (76.5%) and disfigurement (45.1%) remained most common. Most patients experienced ≥1 instance of PN progression (73.5%) with a median of 2 episodes per patient. Conclusion The burden of PN among pediatric NF1 patients is substantial with most patients having PN symptoms, such as pain and disfigurement, by age 6 and experiencing progression of their PN during childhood, highlighting the importance of early diagnosis and access to care and efficacious treatment. References Miller DT, Freedenberg D, Schorry E, Ullrich NJ, Viskochil D, Korf BR; Council on Genetics and American College of Medical Genetics and Genomics. Health supervision for children with neurofibromatosis type 1. Pediatrics. 2019;143(5). Nguyen R, Kluwe L, Fuensterer C, Kentsch M, Friedrich RE, Mautner VF. Plexiform neurofibromas in children with neurofibromatosis type 1: frequency and associated clinical deficits. J Pediatr. 2011; 159(4):652-5.e2. Tchernev G, Chokoeva AA, Patterson JW, Bakardzhiev I, Wollina U, Tana C. Plexiform neurofibroma: a case report. Medicine (Baltimore). 2016 Feb;95(6):e2663.