title: Multi-axis dose accumulation of noninvasive image-guided breast brachytherapy through biomechanical modeling of tissue deformation using the finite element method.
abstract: OBJECTIVE
  Noninvasive image-guided breast brachytherapy delivers conformal HDR (192)Ir brachytherapy treatments with the breast compressed, and treated in the cranial-caudal and medial-lateral directions. This technique subjects breast tissue to extreme deformations not observed for other disease sites. Given that, commercially-available software for deformable image registration cannot accurately co-register image sets obtained in these two states, a finite element analysis based on a biomechanical model was developed to deform dose distributions for each compression circumstance for dose summation.
  
  
  METHODS
  The model assumed the breast was under planar stress with values of 30 kPa for Young's modulus and 0.3 for Poisson's ratio. Dose distributions from round and skin-dose optimized applicators in cranial-caudal and medial-lateral compressions were deformed using 0.1 cm planar resolution. Dose distributions, skin doses, and dose-volume histograms were generated. Results were examined as a function of breast thickness, applicator size, target size, and offset distance from the center.
  
  
  RESULTS
  Over the range of examined thicknesses, target size increased several millimeters as compression thickness decreased. This trend increased with increasing offset distances. Applicator size minimally affected target coverage, until applicator size was less than the compressed target size. In all cases, with an applicator larger or equal to the compressed target size, > 90% of the target covered by > 90% of the prescription dose. In all cases, dose coverage became less uniform as offset distance increased and average dose increased. This effect was more pronounced for smaller target-applicator combinations.
  
  
  CONCLUSIONS
  The model exhibited skin dose trends that matched MC-generated benchmarking results within 2% and clinical observations over a similar range of breast thicknesses and target sizes. The model provided quantitative insight on dosimetric treatment variables over a range of clinical circumstances. These findings highlight the need for careful target localization and accurate identification of compression thickness and target offset.
authors.affiliation: Department of Radiation Oncology, Tufts University School of Medicine, Boston, MA.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Rivard
authors.affiliation: Department of Engineering, Farmingdale State College SUNY, Farmingdale, NY.
authors.forename: Hamid R
authors.initials: HR
authors.lastname: Ghadyani
authors.affiliation: Department of Engineering, University Massachusetts Lowell, Lowell, MA.
authors.forename: Adam D
authors.initials: AD
authors.lastname: Bastien
authors.affiliation: Department of Engineering, University Massachusetts Lowell, Lowell, MA.
authors.forename: Nicholas N
authors.initials: NN
authors.lastname: Lutz
authors.affiliation: Department of Radiation Oncology, Rhode Island Hospital, Brown University, Providence, RI, USA.
authors.forename: Jaroslaw T
authors.initials: JT
authors.lastname: Hepel
keywords: deformable image registration; noninvasive image-guided breast brachytherapy
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