title: 4π non-coplanar liver SBRT: a novel delivery technique.
abstract: OBJECTIVE
  To improve the quality of liver stereotactic body radiation therapy (SBRT) treatments, a novel 4π framework was developed with accompanying algorithms to optimize non-coplanar beam orientations and fluences. The dose optimization is performed on a patient-specific deliverable beam geometry solution space, parameterized with patient and linear accelerator gantry orientations.
  
  
  METHODS
  Beams causing collision between the gantry and the couch or patient were eliminated by simulating all beam orientations using a precise computer assisted design model of the linear accelerator and a human subject. Integrated beam orientation and fluence map optimizations were performed on remaining beams using a greedy column generation method. Testing of the new method was performed on 10 liver SBRT cases previously treated with 50 to 60 Gy in 5 fractions using volumetric modulated arc therapy (VMAT). For each patient, both 14 and 22 non-coplanar fields were selected and optimized to meet the objective of ≥95% of the planning target volume (PTV) covered by 100% of the prescription dose. Doses to organs at risk, normal liver volumes receiving <15 Gy, integral dose, and 50% dose spillage volumes were compared against the delivered clinical VMAT plans.
  
  
  RESULTS
  Compared with the VMAT plans, the 4π plans yielded reduced 50% dose spillage volume and integral dose by 22% (range 10%-40%) and 19% (range 13%-26%), respectively. The mean normal liver volume receiving <15 Gy was increased by 51 cc (range 21-107 cc) with a 31% reduction of the mean normal liver dose. Mean doses to the left kidney and right kidney and maximum doses to the stomach and spinal cord were on average reduced by 70%, 51%, 67%, and 64% (P≤.05).
  
  
  CONCLUSIONS
  This novel 4π non-coplanar radiation delivery technique significantly improved dose gradient, reduced high dose spillage, and improved organ at risk sparing compared with state of the art VMAT plans.
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California, USA.
authors.forename: Peng
authors.initials: P
authors.lastname: Dong
authors.forename: Percy
authors.initials: P
authors.lastname: Lee
authors.forename: Dan
authors.initials: D
authors.lastname: Ruan
authors.forename: Troy
authors.initials: T
authors.lastname: Long
authors.forename: Edwin
authors.initials: E
authors.lastname: Romeijn
authors.forename: Yingli
authors.initials: Y
authors.lastname: Yang
authors.forename: Daniel
authors.initials: D
authors.lastname: Low
authors.forename: Patrick
authors.initials: P
authors.lastname: Kupelian
authors.forename: Ke
authors.initials: K
authors.lastname: Sheng
mesh_terms: D000465:Algorithms; D006801:Humans; D007668:Kidney / Q000528:radiation effects; D008099:Liver / Q000000981:diagnostic imaging / Q000528:radiation effects; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D059351:Organ Sparing Treatments / Q000379:methods; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects; D010315:Particle Accelerators / Q000295:instrumentation; D058996:Quality Improvement; D011832:Radiation Injuries / Q000517:prevention & control; D011859:Radiography; D016634:Radiosurgery / Q000379:methods* / Q000592:standards; D011879:Radiotherapy Dosage / Q000592:standards; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D013116:Spinal Cord / Q000528:radiation effects; D013270:Stomach / Q000528:radiation effects