title: Grid therapy using high definition multileaf collimators: realizing benefits of the bystander effect.
abstract: BACKGROUND
  In microbeam radiotherapy (MRT), parallel arrays of high-intensity synchrotron x-ray beams achieve normal tissue sparing without compromising tumor control. Grid-therapy using clinical linacs has spatial modulation on a larger scale and achieves promising results for palliative treatments of bulky tumors. The availability of high definition multileaf collimators (HDMLCs) with 2.5 mm leaves provides an opportunity for grid-therapy to more closely approach MRT. However, challenges to the wider implementation of grid-therapy remain because spatial modulation of the target volume runs counter to current radiotherapy practice and mechanisms for the beneficial effects of MRT are not fully understood. Without more knowledge of cell dose responses, a quantitative basis for planning treatments is difficult. The aim of this study is to determine if therapeutic benefits of MRT can be achieved using a linac with HDMLCs and if so, to develop a predictive model to support treatment planning.
  
  
  METHODS
  HD120-MLCs of a Varian Novalis TXTMwere used to generate grid patterns of 2.5 and 5.0 mm spacing, which were characterized dosimetrically using GafchromicTMEBT3 film. Clonogenic survival of normal (HUVEC) and cancer (NCI-H460, HCC-1954) cell lines following irradiation under the grid and open fields using a 6 MV photon beam were compared in-vitro for the same average dose.
  
  
  CONCLUSIONS
  Relative to an open field, survival of normal cells in a 2.5 mm striped field was the same, while the survival of both cancer cell lines was significantly lower. A mathematical model was developed to incorporate dose gradients of the spatial modulation into the standard linear quadratic model. Our new bystander extended LQ model assumes spatial gradients drive the diffusion of soluble factors that influence survival through bystander effects, successfully predicting the experimental results that show an increased therapeutic ratio. Our results challenge conventional radiotherapy practice and propose that additional gain can be realized by prescribing spatially modulated treatments to harness the bystander effect.
authors.affiliation: a School of Physics , University of Sydney , Camperdown , NSW , Australia.
authors.forename: Valery
authors.initials: V
authors.lastname: Peng
authors.affiliation: a School of Physics , University of Sydney , Camperdown , NSW , Australia.|b Department of Radiation Oncology, Chris O'Brien Lifehouse , VectorLAB , Camperdown , NSW , Australia.
authors.forename: Natalka
authors.initials: N
authors.lastname: Suchowerska
authors.affiliation: a School of Physics , University of Sydney , Camperdown , NSW , Australia.|b Department of Radiation Oncology, Chris O'Brien Lifehouse , VectorLAB , Camperdown , NSW , Australia.
authors.forename: Linda
authors.initials: L
authors.lastname: Rogers
authors.affiliation: b Department of Radiation Oncology, Chris O'Brien Lifehouse , VectorLAB , Camperdown , NSW , Australia.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Claridge Mackonis
authors.affiliation: c The Garvan Institute of Medical Research , Darlinghurst , NSW , Australia.
authors.forename: Samantha
authors.initials: S
authors.lastname: Oakes
authors.affiliation: a School of Physics , University of Sydney , Camperdown , NSW , Australia.|b Department of Radiation Oncology, Chris O'Brien Lifehouse , VectorLAB , Camperdown , NSW , Australia.
authors.forename: David R
authors.initials: DR
authors.lastname: McKenzie
mesh_terms: D001943:Breast Neoplasms / Q000532:radiotherapy*; D024201:Bystander Effect; D002470:Cell Survival / Q000528:radiation effects; D002478:Cells, Cultured; D005260:Female; D061307:Human Umbilical Vein Endothelial Cells / Q000528:radiation effects*; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy*; D010315:Particle Accelerators / Q000295:instrumentation; D011879:Radiotherapy Dosage; D017356:Synchrotrons / Q000295:instrumentation