title: MRI-Based Evaluation of the Vaginal Cuff in Brachytherapy Planning: Are We Missing the Target?
abstract: Although recurrences and toxicity occur after vaginal cuff (VC) brachytherapy, little is known about dosimetry due to the inability to clearly visualize the VC on computed tomography (CT). T2-weighted (T2W) magnetic resonance imaging (MRI) is superior to CT in this setting, and we hypothesized that it could provide previously unascertainable dosimetric information.
  
  
  
  In a cohort of 32 patients who underwent cylinder-based brachytherapy for endometrial cancer with available MR simulation images, the VC was retrospectively contoured on T2W images, and cases were replanned to treat the upper VC to a dose of 7 Gy/fraction prescribed to 5 mm. Relevant dose-volume parameters for the VC were calculated.
  
  
  
  T2W MRI identified significant underdosing not observed on CT or T1-weighted imaging. Over two-thirds (69%) of patients had at least 1 cm(3) of VC that received less than 75% of the prescription dose and half (50%) of patients had a least 1 cm(3) of VC that received less than 50% of the prescription dose. The mean minimum point dose to the VC was 2.4 Gy, or 34% of the intended prescription dose (range: 0.53-6.4 Gy).
  
  
  
  We identified previously unreported VC underdosing in over two-thirds of our patients, with most of these patients having volumes of undistended VC that received less than half of the prescription dose. The maximum dimension was along the craniocaudal axis in some patients or left-right/anterior-posterior axis in others, suggesting that suture material may be restricting access to the vaginal apex and that alternative applicators may be needed when the diameter of the apex is larger than the introitus. Additional follow-up will be needed to determine whether underdosing is associated with isolated VC failure or whether low failure rates across the cohort suggest that some patients are being exposed to excessive dose and unnecessary risk of toxicity.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan.
authors.forename: Christina Hunter
authors.initials: CH
authors.lastname: Chapman
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan.
authors.forename: Joann I
authors.initials: JI
authors.lastname: Prisciandaro
authors.affiliation: Department of Radiology, University of Michigan, Ann Arbor, Michigan.
authors.forename: Katherine E
authors.initials: KE
authors.lastname: Maturen
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan; Department of Radiology, University of Michigan, Ann Arbor, Michigan; Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan.
authors.forename: Yue
authors.initials: Y
authors.lastname: Cao
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan; Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan.
authors.forename: James M
authors.initials: JM
authors.lastname: Balter
authors.affiliation: Division of Gynecologic Oncology, Department of Obstetrics and Gynecology, University of Michigan, Ann Arbor, Michigan.
authors.forename: Karen
authors.initials: K
authors.lastname: McLean
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan. Electronic address: shrutij@med.umich.edu.
authors.forename: Shruti
authors.initials: S
authors.lastname: Jolly
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D016889:Endometrial Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014621:Vagina / Q000000981:diagnostic imaging / Q000528:radiation effects*