title: Development of a standardized method for contouring the lumbosacral plexus: a preliminary dosimetric analysis of this organ at risk among 15 patients treated with intensity-modulated radiotherapy for lower gastrointestinal cancers and the incidence of radiation-induced lumbosacral plexopathy.
abstract: OBJECTIVE
  To generate a reproducible step-wise guideline for the delineation of the lumbosacral plexus (LSP) on axial computed tomography (CT) planning images and to provide a preliminary dosimetric analysis on 15 representative patients with rectal or anal cancers treated with an intensity-modulated radiotherapy (IMRT) technique.
  
  
  METHODS
  A standardized method for contouring the LSP on axial CT images was devised. The LSP was referenced to identifiable anatomic structures from the L4-5 interspace to the level of the sciatic nerve. It was then contoured retrospectively on 15 patients treated with IMRT for rectal or anal cancer. No dose limitations were placed on this organ at risk during initial treatment planning. Dosimetric parameters were evaluated. The incidence of radiation-induced lumbosacral plexopathy (RILSP) was calculated.
  
  
  RESULTS
  Total prescribed dose to 95% of the planned target volume ranged from 50.4 to 59.4 Gy (median 54 Gy). The mean (± standard deviation [SD]) LSP volume for the 15 patients was 100 ± 22 cm(3) (range, 71-138 cm(3)). The mean maximal dose to the LSP was 52.6 ± 3.9 Gy (range, 44.5-58.6 Gy). The mean irradiated volumes of the LSP were V40Gy = 58% ± 19%, V50Gy = 22% ± 23%, and V55Gy = 0.5% ± 0.9%. One patient (7%) was found to have developed RILSP at 13 months after treatment.
  
  
  CONCLUSIONS
  The true incidence of RILSP in the literature is likely underreported and is not a toxicity commonly assessed by radiation oncologists. In our analysis the LSP commonly received doses approaching the prescribed target dose, and 1 patient developed RILSP. Identification of the LSP during IMRT planning may reduce RILSP. We have provided a reproducible method for delineation of the LSP on CT images and a preliminary dosimetric analysis for potential future dose constraints.
authors.affiliation: Department of Radiation Oncology, University of California, Davis Cancer Center, Sacramento, CA 95817, USA. sun.yi@ucdmc.ucdavis.edu
authors.forename: Sun K
authors.initials: SK
authors.lastname: Yi
authors.forename: Walter
authors.initials: W
authors.lastname: Mak
authors.forename: Claus C
authors.initials: CC
authors.lastname: Yang
authors.forename: Tianxiao
authors.initials: T
authors.lastname: Liu
authors.forename: Jing
authors.initials: J
authors.lastname: Cui
authors.forename: Allen M
authors.initials: AM
authors.lastname: Chen
authors.forename: James A
authors.initials: JA
authors.lastname: Purdy
authors.forename: Arta M
authors.initials: AM
authors.lastname: Monjazeb
authors.forename: Ly
authors.initials: L
authors.lastname: Do
mesh_terms: D059925:Anatomic Landmarks / Q000033:anatomy & histology; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use; D001005:Anus Neoplasms / Q000188:drug therapy / Q000532:radiotherapy*; D005472:Fluorouracil / Q000008:administration & dosage; D006801:Humans; D008160:Lumbosacral Plexus / Q000033:anatomy & histology* / Q000528:radiation effects; D008279:Magnetic Resonance Imaging / Q000592:standards; D008488:Medical Illustration; D016685:Mitomycin / Q000008:administration & dosage; D058958:Organs at Risk / Q000033:anatomy & histology* / Q000528:radiation effects; D010523:Peripheral Nervous System Diseases / Q000209:etiology*; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000150:complications*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012004:Rectal Neoplasms / Q000188:drug therapy / Q000532:radiotherapy*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed / Q000379:methods
chemical_list: D016685:Mitomycin; D005472:Fluorouracil