title: Optimal set of grid size and angular increment for practical dose calculation using the dynamic conformal arc technique: a systematic evaluation of the dosimetric effects in lung stereotactic body radiation therapy.
abstract: OBJECTIVE
  To recommend the optimal plan parameter set of grid size and angular increment for dose calculations in treatment planning for lung stereotactic body radiation therapy (SBRT) using dynamic conformal arc therapy (DCAT) considering both accuracy and computational efficiency.
  
  
  METHODS
  Dose variations with varying grid sizes (2, 3, and 4 mm) and angular increments (2°, 4°, 6°, and 10°) were analyzed in a thorax phantom for 3 spherical target volumes and in 9 patient cases. A 2-mm grid size and 2° angular increment are assumed sufficient to serve as reference values. The dosimetric effect was evaluated using dose-volume histograms, monitor units (MUs), and dose to organs at risk (OARs) for a definite volume corresponding to the dose-volume constraint in lung SBRT. The times required for dose calculations using each parameter set were compared for clinical practicality.
  
  
  RESULTS
  Larger grid sizes caused a dose increase to the structures and required higher MUs to achieve the target coverage. The discrete beam arrangements at each angular increment led to over- and under-estimated OARs doses due to the undulating dose distribution. When a 2° angular increment was used in both studies, a 4-mm grid size changed the dose variation by up to 3-4% (50 cGy) for the heart and the spinal cord, while a 3-mm grid size produced a dose difference of <1% (12 cGy) in all tested OARs. When a 3-mm grid size was employed, angular increments of 6° and 10° caused maximum dose variations of 3% (23 cGy) and 10% (61 cGy) in the spinal cord, respectively, while a 4° increment resulted in a dose difference of <1% (8 cGy) in all cases except for that of one patient. The 3-mm grid size and 4° angular increment enabled a 78% savings in computation time without making any critical sacrifices to dose accuracy.
  
  
  CONCLUSIONS
  A parameter set with a 3-mm grid size and a 4° angular increment is found to be appropriate for predicting patient dose distributions with a dose difference below 1% while reducing the computation time by more than half for lung SBRT using DCAT.
authors.forename: Ji-Yeon
authors.initials: JY
authors.lastname: Park
authors.forename: Siyong
authors.initials: S
authors.lastname: Kim
authors.forename: Hae-Jin
authors.initials: HJ
authors.lastname: Park
authors.forename: Jeong-Woo
authors.initials: JW
authors.lastname: Lee
authors.forename: Yeon-Sil
authors.initials: YS
authors.lastname: Kim
authors.affiliation: Department of Biomedical Engineering, The Catholic University of Korea Songeui Campus, Banpo4-dong, Seocho-gu, Seoul 137-701, Korea. suhsanta@catholic.ac.kr.
authors.forename: Tae-Suk
authors.initials: TS
authors.lastname: Suh
mesh_terms: D000465:Algorithms; D006801:Humans; D008175:Lung Neoplasms / Q000473:pathology / Q000601:surgery*; D009929:Organ Size; D058958:Organs at Risk; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D016634:Radiosurgery / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D047368:Tumor Burden