title: Minibeam therapy with protons and light ions: physical feasibility and potential to reduce radiation side effects and to facilitate hypofractionation.
abstract: OBJECTIVE
  Despite several advantages of proton therapy over megavoltage x-ray therapy, its lack of proximal tissue sparing is a concern. The method presented here adds proximal tissue sparing to protons and light ions by turning their uniform incident beams into arrays of parallel, small, or thin (0.3-mm) pencil or planar minibeams, which are known to spare tissues. As these minibeams penetrate the tissues, they gradually broaden and merge with each other to produce a solid beam.
  
  
  METHODS
  Broadening of 0.3-mm-diameter, 109-MeV proton pencil minibeams was measured using a stack of radiochromic films with plastic spacers. Monte Carlo simulations were used to evaluate the broadening in water of minibeams of protons and several light ions and the dose from neutron generated by collimator.
  
  
  RESULTS
  A central parameter was tissue depth, where the beam full width at half maximum (FWHM) reached 0.7 mm, beyond which tissue sparing decreases. This depth was 22 mm for 109-MeV protons in a film stack. It was also found by simulations in water to be 23.5 mm for 109 MeV proton pencil minibeams and 26 mm for 116 MeV proton planar minibeams. For light ions, all with 10 cm range in water, that depth increased with particle size; specifically it was 51 mm for Li-7 ions. The ∼2.7% photon equivalent neutron skin dose from the collimator was reduced 7-fold by introducing a gap between the collimator and the skin.
  
  
  CONCLUSIONS
  Proton minibeams can be implemented at existing particle therapy centers. Because they spare the shallow tissues, they could augment the efficacy of proton therapy and light particle therapy, particularly in treating tumors that benefit from sparing of proximal tissues such as pediatric brain tumors. They should also allow hypofractionated treatment of all tumors by allowing the use of higher incident doses with less concern about proximal tissue damage.
authors.affiliation: Departments of Radiation Oncology, Neurology, and Radiology, Stony Brook University Medical Center, Stony Brook, New York. Electronic address: avraham.dilmanian@stonybrook.edu.
authors.forename: F Avraham
authors.initials: FA
authors.lastname: Dilmanian
authors.affiliation: Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas.
authors.forename: John G
authors.initials: JG
authors.lastname: Eley
authors.affiliation: Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, Texas.
authors.forename: Sunil
authors.initials: S
authors.lastname: Krishnan
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy; D002648:Child; D019583:Dose Fractionation, Radiation; D005240:Feasibility Studies; D006371:Helium / Q000627:therapeutic use; D006801:Humans; D007554:Isotopes / Q000627:therapeutic use; D008094:Lithium / Q000627:therapeutic use; D009010:Monte Carlo Method; D059351:Organ Sparing Treatments / Q000379:methods*; D061766:Proton Therapy / Q000295:instrumentation / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control*
chemical_list: D007554:Isotopes; D006371:Helium; D008094:Lithium