title: Investigation of the dosimetric accuracy of the isocenter shifting method in prostate cancer patients with and without hip prostheses.
abstract: OBJECTIVE
  The use of image guided radiation therapy (IGRT) enables compensation for prostate movement by shifting the treatment isocenter to track the prostate on a daily basis. Although shifting the isocenter can alter the source to skin distances (SSDs) and the effective depth of the target volume, it is commonly assumed that these changes have a negligible dosimetric effect, and therefore, the number of monitor units delivered is usually not adjusted. However, it is unknown whether or not this assumption is valid for patient with hip prostheses, which frequently contain high density materials.
  
  
  METHODS
  The authors conducted a retrospective study to investigate dosimetric effect of the isocenter shifting method for prostate patients with and without hip prostheses. For each patient, copies of the prostate volume were shifted by up to 1.5 cm from the original position to simulate prostate movement in 0.5 cm increments. Subsequently, 12 plans were created for each patient by creating a copy of the original plan for each prostate position with the isocenter shifted to track the position of the shifted prostate. The dose to the prostate was then recalculated for each plan. For patients with hip prostheses, plans were created both with and without lateral beam angles entering through the prostheses.
  
  
  RESULTS
  Without isocenter shifting to compensate for prostate motion of 1.5 cm, the dose to the 95% of the prostate (D-95%) changed by an average of 30% and by up to 64%. This was reduced to less than 3% with the isocenter shifting method. It was found that for patients with hip prostheses, this technique worked best for treatment plans that avoided beam angles passing through the prostheses.
  
  
  CONCLUSIONS
  The results demonstrated that the isocenter shifting method can accurately deliver dose to the prostate even in patients with hip prostheses.
authors.affiliation: Department of Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, San Francisco, California 94143, USA. hwangab@radonc.ucsf.edu
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Hwang
authors.forename: Erica
authors.initials: E
authors.lastname: Kinsey
authors.forename: Ping
authors.initials: P
authors.lastname: Xia
mesh_terms: D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D009038:Motion; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed / Q000379:methods