title: Dosimetric and delivery efficiency investigation for treating hepatic lesions with a MLC-equipped robotic radiosurgery-radiotherapy combined system.
abstract: OBJECTIVE
  The CyberKnife M6 (CK-M6) Series introduced a multileaf collimator (MLC) for extending its capability from stereotactic radiosurgery/stereotactic radiotherapy (SBRT) to conventionally fractionated radiotherapy. This work is to investigate the dosimetric quality of plans that are generated using MLC-shaped beams on the CK-M6, as well as their delivery time, via comparisons with the intensity modulated radiotherapy plans that were clinically used on a Varian Linac for treating hepatic lesions.
  
  
  METHODS
  Nine patient cases were selected and divided into three groups with three patients in each group: (1) the group-one patients were treated conventionally (25 fractions); (2) the group-two patients were treated with SBRT-like hypofractionation (5 fractions); and (3) the group-three patients were treated similar to group-one patients, but with two planning target volumes (PTVs) and two different prescription dose levels correspondingly. The clinically used plans were generated on the eclipse treatment planning system (TPS) and delivered on a Varian Linac (E-V plans). The multiplan (MP) TPS was used to replan these clinical cases with the MLC as the beam device for the CK-M6 (C-M plans). After plans were normalized to the same PTV dose coverage, comparisons between the C-M and E-V plans were performed based on D(99%) (percentage of prescription dose received by 99% of the PTV), D(0.1cm(3)) (the percentage of prescription dose to 0.1 cm(3) of the PTV), and doses received by critical structures. Then, the delivery times for the C-M plans will be obtained, which are the MP TPS generated estimations assuming having an imaging interval of 60 s.
  
  
  RESULTS
  The difference in D(99%) between C-M and E-V plans is +0.6% on average (+ or - indicating a higher or lower dose from C-M plans than from E-V plans) with a range from -4.1% to +3.8%, and the difference in D(0.1cm(3)) was -1.0% on average with a range from -5.1% to +2.9%. The PTV conformity index (CI) for the C-M plans ranges from 1.07 to 1.29 with a mean of 1.19, slightly inferior to the E-V plans, in which the CI ranges from 1.00 to 1.15 with a mean of 1.07. Accounting for all nine patients in three groups, 45% of the critical structures received a lower mean dose for the C-M plans as compared with the E-V plans, and similarly, 48% received a lower maximum dose. Furthermore, the average difference of the mean critical structure dose between the C-M and E-V plans over all critical structures for all patients showed only +2.10% relative to the prescription dose and the similar comparison finds the average difference of the maximum critical structure dose of only +1.24%. The estimated delivery times for the C-M plans on the CK-M6 range from 18 to 24 minutes while they are from 7 to 13.7 min for the E-V plans on the Varian Linac.
  
  
  CONCLUSIONS
  For treating hepatic lesions, for the C-M plans that are comparable to E-V plans in quality, the times needed to deliver these C-M plans on the CK-M6 are longer than the delivery time for the E-V plans on the Varian Linac, but may be clinically acceptable.
authors: Department of Radiation Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, Pennsylvania 19111.
authors: Lihui
authors: L
authors: Jin
authors: Department of Radiation Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, Pennsylvania 19111.
authors: Robert A
authors: RA
authors: Price
authors: Department of Radiation Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, Pennsylvania 19111.
authors: Lu
authors: L
authors: Wang
authors: Department of Radiation Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, Pennsylvania 19111.
authors: Joshua
authors: J
authors: Meyer
authors: Department of Radiation Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, Pennsylvania 19111.
authors: James Jiajin
authors: JJ
authors: Fan
authors: Department of Radiation Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, Pennsylvania 19111.
authors: Chang Ming Charlie
authors: CM
authors: Ma
mesh_terms: D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy* / Q000601:surgery*; D010315:Particle Accelerators; D011874:Radiometry; D016634:Radiosurgery / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012371:Robotics