title: Dosimetric coverage of the external anal sphincter by 3-dimensional conformal fields in rectal cancer patients receiving neoadjuvant chemoradiation: implications for the concept of sphincter-preserving radiation therapy.
abstract: BACKGROUND
  We evaluated the anatomic location of the external anal sphincter (EAS) to pelvic bony landmarks related to 3-dimensional conformal radiotherapy (3DRT) and studied the dosimetric coverage of the EAS in patients undergoing neoadjuvant chemoradiation for rectal cancer.
  
  
  METHODS
  Sixty-four consecutive rectal cancer patients treated with neoadjuvant chemoradiation were included. All patients were treated in a prone position on a bellyboard by 3DRT. The inferior border of the RT fields was at least 3-5 cm inferior to the gross tumorous volume (GTV) or at the inferior border of the obturator foramen (IBOF), whichever was more inferior. The EAS was contoured and dose distributions were determined using dose-volume histograms.
  
  
  RESULTS
  In 53 out of 64 cases (82.8%), the EAS was completely inferior to the IBOF. In the remaining 11 cases, the EAS was either overlapping the IBOF (10 cases; 15.6%) or completely superior to the IBOF (1 case; 1.7%). The average mean dose delivered to the EAS was 2795 cGy. Lower mean doses were delivered to the EAS when the center of the EAS was located more distant from the GTV.
  
  
  CONCLUSIONS
  Meticulous planning to define the inferior border of the RT field is recommended to avoid irradiating the EAS.
authors.affiliation: Radiation Oncology, City of Hope Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA.
authors.forename: Yi-Jen
authors.identifier: 0000-0001-8022-3316
authors.initials: YJ
authors.lastname: Chen
authors.affiliation: Radiation Oncology, City of Hope Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA.
authors.forename: Michelle B
authors.initials: MB
authors.lastname: Chen
authors.affiliation: Radiation Oncology, City of Hope Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA.
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Liu
authors.affiliation: Colorectal Surgery, City of Hope Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA.
authors.forename: Julian
authors.identifier: 0000-0002-7198-839X
authors.initials: J
authors.lastname: Sanchez
authors.affiliation: Radiation Oncology, City of Hope Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Tsai
authors.affiliation: Radiation Oncology, City of Hope Medical Center, 1500 East Duarte Road, Duarte, CA 91010, USA.
authors.forename: An
authors.identifier: 0000-0002-8903-5028
authors.initials: A
authors.lastname: Liu
mesh_terms: D001003:Anal Canal / Q000000981:diagnostic imaging / Q000528:radiation effects*; D059186:Chemoradiotherapy, Adjuvant / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D059351:Organ Sparing Treatments / Q000009:adverse effects / Q000379:methods*; D011832:Radiation Injuries / Q000209:etiology / Q000517:prevention & control*; D011859:Radiography; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000009:adverse effects / Q000379:methods; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging / Q000628:therapy*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016896:Treatment Outcome
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