title: Dosimetric investigation of accelerated partial breast irradiation (APBI) using CyberKnife.
abstract: OBJECTIVE
  To investigate the dosimetric feasibility of accelerated partial breast irradiation (APBI) using CyberKnife.
  
  
  METHODS
  Fourteen previously treated patients with early-stage breast cancer were selected for a retrospective study. Six of these patients had been treated to 38.5 Gy in 10 fractions in a phase III accelerated partial breast trial and the rest of the patients were treated to 50.4 Gy in 28 fractions. In this planning study, the guidelines in the protocol for the phase III partial breast trial were followed for organ delineation and CyberKnife planning. The achievable dosimetric parameters from all CyberKnife plans were compared to Intensity-modulated radiation therapy (IMRT) and 3D-CRT methods. The reproducibility of the dose delivery with and without respiratory motion was assessed through delivering a patient plan to a breast phantom. Different dose calculation algorithms were also compared between ray tracing and Monte Carlo.
  
  
  RESULTS
  For all the patients in the study, the dosimetric parameters met the guidelines from the NSABP B39∕RTOG 0413 protocol strictly. The mean PTV volume covered by 100% of the prescription dose was 95.7 ± 0.7% (94.7%-97.1%). The mean maximal dose was 104 ± 2% of the prescription dose. The mean V(50%) and mean V(100%) to the ipsilateral normal breast were 23.1 ± 11.6% and 9.0 ± 5.8%, respectively. The conformity index of all plans was 1.14 ± 0.04. The maximum dose to the contralateral breast varied from 1.3 cGy to 111 cGy. The mean V(5%) and mean V(30%) to the contralateral and ipsilateral lungs were 1.0 ± 1.6% and 1.3 ± 1.2%, respectively. In our study, the mean V(5%) to the heart was 0.2 ± 0.5% for right-sided tumors and 9.4 ± 10.1% for left-sided tumors. Compared with IMRT and 3D-CRT planning, the PTV coverage from CyberKnife planning was the highest, and the ratio of V(20%) to V(100%) of the breast from CyberKnife planning was the smallest. The heart and lung doses were similar in all the techniques except that the V(5%) for the lung and heart in CyberKnife planning was slightly higher.
  
  
  CONCLUSIONS
  The dosimetric feasibility of APBI using CyberKnife was investigated in this retrospective study. All the dosimetric parameters strictly met the guidelines from the NSABP B39∕RTOG 0413 protocol. With advanced real-time tracking capability, CyberKnife should provide better target coverage and spare nearby critical organs for APBI treatment.
authors.affiliation: Department of Radiation Oncology, Cooper University Hospital, Camden, NJ 08103, USA. xuqianyi@gmail.com
authors.forename: Qianyi
authors.initials: Q
authors.lastname: Xu
authors.forename: Yan
authors.initials: Y
authors.lastname: Chen
authors.forename: Jimm
authors.initials: J
authors.lastname: Grimm
authors.forename: Jiajin
authors.initials: J
authors.lastname: Fan
authors.forename: Lili
authors.initials: L
authors.lastname: An
authors.forename: Jinyu
authors.initials: J
authors.lastname: Xue
authors.forename: Niraj
authors.initials: N
authors.lastname: Pahlajani
authors.forename: Tamara
authors.initials: T
authors.lastname: Lacouture
mesh_terms: D001940:Breast / Q000601:surgery*; D001943:Breast Neoplasms / Q000601:surgery; D005260:Female; D006801:Humans; D015412:Mastectomy, Segmental / Q000379:methods*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011874:Radiometry; D016634:Radiosurgery / Q000379:methods*; D012189:Retrospective Studies