title: Alignment focus of daily image guidance for concurrent treatment of prostate and pelvic lymph nodes.
abstract: OBJECTIVE
  To determine the dosimetric impact of daily imaging alignment focus on the prostate soft tissue versus the pelvic bones for the concurrent treatment of the prostate and pelvic lymph nodes (PLN) and to assess whether multileaf collimator (MLC) tracking or adaptive planning (ART) is necessary with the current clinical planning margins of 8 mm/6 mm posterior to the prostate and 5 mm to the PLN.
  
  
  METHODS
  A total of 124 kilovoltage cone-beam computed tomography (kV-CBCT) images from 6 patients were studied. For each KV-CBCT, 4 plans were retrospectively created using an isocenter shifting method with 2 different alignment focuses (prostate, PLN), an MLC shifting method, and the ART method. The selected dosimetric endpoints were compared among these plans.
  
  
  RESULTS
  For the isoshift contour, isoshift bone, MLC shift, and ART plans, D99 of the prostate was ≥97% of the prescription dose in 97.6%, 73.4%, 98.4%, and 96.8% of 124 fractions, respectively. Accordingly, D99 of the PLN was ≥97% of the prescription dose in 98.4%, 98.4%, 98.4%, and 100% of 124 fractions, respectively. For the rectum, D5 exceeded 105% of the planned D5 (and D5 of ART plans) in 11% (4%), 10% (2%), and 13% (5%) of 124 fractions, respectively. For the bladder, D5 exceeded 105% of the planned D5 (and D5 of ART) plans in 0% (2%), 0% (2%), and 0% (1%) of 124 fractions, respectively.
  
  
  CONCLUSIONS
  For concurrent treatment of the prostate and PLN, with a planning margin to the prostate of 8 mm/6 mm posterior and a planning margin of 5 mm to the PLN, aligning to the prostate soft tissue can achieve adequate dose coverage to the both target volumes; aligning to the pelvic bone would result in underdosing to the prostate in one-third of fractions. With these planning margins, MLC tracking and ART methods have no dosimetric advantages.
authors.affiliation: Department of Radiation Oncology, Cleveland Clinic, Cleveland, Ohio 44195, USA.
authors.forename: Samah
authors.initials: S
authors.lastname: Ferjani
authors.forename: Guangshun
authors.initials: G
authors.lastname: Huang
authors.forename: Qingyang
authors.initials: Q
authors.lastname: Shang
authors.forename: Kevin L
authors.initials: KL
authors.lastname: Stephans
authors.forename: Yahua
authors.initials: Y
authors.lastname: Zhong
authors.forename: Peng
authors.initials: P
authors.lastname: Qi
authors.forename: Rahul D
authors.initials: RD
authors.lastname: Tendulkar
authors.forename: Ping
authors.initials: P
authors.lastname: Xia
mesh_terms: D059925:Anatomic Landmarks / Q000000981:diagnostic imaging; D054893:Cone-Beam Computed Tomography / Q000379:methods; D006801:Humans; D008198:Lymph Nodes / Q000000981:diagnostic imaging; D015182:Lymphatic Irradiation / Q000379:methods*; D008297:Male; D009068:Movement; D058958:Organs at Risk / Q000000981:diagnostic imaging; D010384:Pelvic Bones / Q000000981:diagnostic imaging*; D010388:Pelvis; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012007:Rectum / Q000000981:diagnostic imaging; D001743:Urinary Bladder / Q000000981:diagnostic imaging