title: Constituent components of out-of-field scatter dose for 18-MV intensity modulated radiation therapy versus 3-dimensional conformal radiation therapy: a comparison with 6-MV and implications for carcinogenesis.
abstract: OBJECTIVE
  To characterize and compare the components of out-of-field dose for 18-MV intensity modulated radiation therapy (IMRT) versus 3-dimensional conformal radiation therapy (3D-CRT) and their 6-MV counterparts and consider implications for second cancer induction.
  
  
  METHODS
  Comparable plans for each technique/energy were delivered to a water phantom with a sloping wall; under full scatter conditions; with field edge abutting but outside the bath to prevent internal/phantom scatter; and with shielding below the linear accelerator head to attenuate head leakage. Neutron measurements were obtained from published studies.
  
  
  RESULTS
  Eighteen-megavolt IMRT produces 1.7 times more out-of-field scatter than 18-MV 3D-CRT. In absolute terms, however, differences are just approximately 0.1% of central axis dose. Eighteen-megavolt IMRT reduces internal/patient scatter by 13%, but collimator scatter (C) is 2.6 times greater than 18-MV 3D-CRT. Head leakage (L) is minimal. Increased out-of-field photon scatter from 18-MV IMRT carries out-of-field second cancer risks of approximately 0.2% over and above the 0.4% from 18-MV 3D-CRT. Greater photoneutron dose from 18-MV IMRT may result in further maximal, absolute increased risk to peripheral tissue of approximately 1.2% over 18-MV 3D-CRT. Out-of-field photon scatter remains comparable for the same modality irrespective of beam energy. Machine scatter (C+L) from 18 versus 6 MV is 1.2 times higher for IMRT and 1.8 times for 3D-CRT. It is 4 times higher for 6-MV IMRT versus 3D-CRT. Reduction in internal scatter with 18 MV versus 6 MV is 27% for 3D-CRT and 29% for IMRT. Compared with 6-MV 3D-CRT, 18-MV IMRT increases out-of-field second cancer risk by 0.2% from photons and adds 0.28-2.2% from neutrons.
  
  
  CONCLUSIONS
  Out-of-field photon dose seems to be independent of beam energy for both techniques. Eighteen-megavolt IMRT increases out-of-field scatter 1.7-fold over 3D-CRT because of greater collimator scatter despite reducing internal/patient scatter. Out-of-field carcinogenic risk is thus increased (but improved in-field dose conformity may offset this). Potentially increased carcinogenic risk should be weighed against any benefit 18-MV IMRT may provide.
authors.affiliation: William Buckland Radiotherapy Centre, The Alfred, Melbourne, Australia; Department of Surgery, Monash University, Melbourne, Australia. Electronic address: jeremy.ruben@wbrc.org.au.
authors.forename: Jeremy D
authors.initials: JD
authors.lastname: Ruben
authors.affiliation: William Buckland Radiotherapy Centre, The Alfred, Melbourne, Australia; Royal Melbourne Institute of Technology, Melbourne, Australia.
authors.forename: Ryan
authors.initials: R
authors.lastname: Smith
authors.affiliation: Royal Brisbane and Women's Hospital, Brisbane, Australia.
authors.forename: Craig M
authors.initials: CM
authors.lastname: Lancaster
authors.affiliation: William Buckland Radiotherapy Centre, The Alfred, Melbourne, Australia.
authors.forename: Matthew
authors.initials: M
authors.lastname: Haynes
authors.affiliation: William Buckland Radiotherapy Centre, The Alfred, Melbourne, Australia.
authors.forename: Phillip
authors.initials: P
authors.lastname: Jones
authors.affiliation: William Buckland Radiotherapy Centre, The Alfred, Melbourne, Australia.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Panettieri
mesh_terms: D009381:Neoplasms, Radiation-Induced / Q000209:etiology*; D016609:Neoplasms, Second Primary / Q000209:etiology*; D009502:Neutrons; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000295:instrumentation / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D012542:Scattering, Radiation