title: A patient immobilization device for prone breast radiotherapy: Dosimetric effects and inclusion in the treatment planning system.
abstract: OBJECTIVE
  To assess the dosimetric impact of a patient positioning device for prone breast radiotherapy and assess the accuracy of a treatment planning system (TPS) in predicting this impact.
  
  
  METHODS
  Beam attenuation and build-up dose perturbations, quantified by ionization chamber and radiochromic film dosimetry, were evaluated for 3 components of the patient positioning device: the carbon fiber baseplate, the support cushions and the support wedge for the contralateral breast. Dose calculations were performed using the XVMC dose engine implemented in the Monaco TPS. All components were included during planning CT acquisition.
  
  
  RESULTS
  Beam attenuation amounted to 7.57% (6MV) and 5.33% (15MV) for beams obliquely intersecting the couchtop-baseplate combination. Beams traversing large sections of the support wedge were attenuated by 12.28% (6MV) and 9.37% (15MV). For the support cushion foam, beam attenuation remained limited to 0.11% (6MV) and 0.08% (15MV) per centimeter thickness. A substantial loss of dose build-up was detected when irradiating through any of the investigated components. TPS dose calculations accurately predicted beam attenuation by the baseplate and support wedge. A manual density overwrite was needed to model attenuation by the support cushion foam. TPS dose calculations in build-up regions differed considerably from measurements for both open beams and beams traversing the device components.
  
  
  CONCLUSIONS
  Irradiating through the components of the positioning device resulted in a considerable degradation of skin sparing. Inclusion of the device components in the treatment planning CT allowed to accurately model the most important attenuation effect, but failed to accurately predict build-up doses.
authors.affiliation: Department of Radiation Oncology and Experimental Cancer Research, Ghent University, Ghent, Belgium. Electronic address: Annemieke.DePuysseleyr@UGent.be.
authors.forename: A
authors.initials: A
authors.lastname: De Puysseleyr
authors.affiliation: Department of Radiation Oncology and Experimental Cancer Research, Ghent University, Ghent, Belgium.
authors.forename: W
authors.initials: W
authors.lastname: De Neve
authors.affiliation: Department of Radiation Oncology and Experimental Cancer Research, Ghent University, Ghent, Belgium.
authors.forename: C
authors.initials: C
authors.lastname: De Wagter
mesh_terms: D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D016684:Prone Position; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D014057:Tomography, X-Ray Computed
keywords: Build-up dose; Patient positioning device; Radiotherapy; Treatment planning system