title: Evaluation of the dose calculation accuracy for small fields defined by jaw or MLC for AAA and Acuros XB algorithms.
abstract: OBJECTIVE
  Small field measurements are challenging, due to the physical characteristics coming from the lack of charged particle equilibrium, the partial occlusion of the finite radiation source, and to the detector response. These characteristics can be modeled in the dose calculations in the treatment planning systems. Aim of the present work is to evaluate the MU calculation accuracy for small fields, defined by jaw or MLC, for anisotropic analytical algorithm (AAA) and Acuros XB algorithms, relative to output measurements on the beam central axis.
  
  
  METHODS
  Single point output factor measurement was acquired with a PTW microDiamond detector for 6 MV, 6 and 10 MV unflattened beams generated by a Varian TrueBeam STx equipped with high definition-MLC. Fields defined by jaw or MLC apertures were set; jaw-defined: 0.6 × 0.6, 0.8 × 0.8, 1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5, and 10 × 10 cm2; MLC-defined: 0.5 × 0.5 cm2to the maximum field defined by the jaw, with 0.5 cm stepping, and jaws set to: 2 × 2, 3 × 3, 4 × 4, 5 × 5, and 10 × 10 cm2. MU calculation was obtained with 1 mm grid in a virtual water phantom for the same fields, for AAA and Acuros algorithms implemented in the Varian eclipse treatment planning system (version 13.6). Configuration parameters as the effective spot size (ESS) and the dosimetric leaf gap (DLG) were varied to find the best parameter setting. Differences between calculated and measured doses were analyzed.
  
  
  RESULTS
  Agreement better than 0.5% was found for field sizes equal to or larger than 2 × 2 cm2for both algorithms. A dose overestimation was present for smaller jaw-defined fields, with the best agreement, averaged over all the energies, of 1.6% and 4.6% for a 1 × 1 cm2field calculated by AAA and Acuros, respectively, for a configuration with ESS = 1 mm for both X and Y directions for AAA, and ESS = 1.5 and 0 mm for X and Y directions for Acuros. Conversely, a calculated dose underestimation was found for small MLC-defined fields, with the best agreement averaged over all the energies, of -3.9% and 0.2% for a 1 × 1 cm2field calculated by AAA and Acuros, respectively, for a configuration with ESS = 0 mm for both directions and both algorithms.
  
  
  CONCLUSIONS
  For optimal setting applied in the algorithm configuration phase, the agreement of Acuros calculations with measurements could achieve the 3% for MLC-defined fields as small as 0.5 × 0.5 cm2. Similar agreement was found for AAA for fields as small as 1 × 1 cm2.
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy.
authors: Antonella
authors: A
authors: Fogliata
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy.
authors: Francesca
authors: F
authors: Lobefalo
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy.
authors: Giacomo
authors: G
authors: Reggiori
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy.
authors: Antonella
authors: A
authors: Stravato
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy.
authors: Stefano
authors: S
authors: Tomatis
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy and Department of Biomedical Sciences, Humanitas University, via Manzoni 113, Milan-Rozzano 20089, Italy.
authors: Marta
authors: M
authors: Scorsetti
authors: Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, via Manzoni 56, Milan-Rozzano 20089, Italy and Department of Biomedical Sciences, Humanitas University, via Manzoni 113, Milan-Rozzano 20089, Italy.
authors: Luca
authors: L
authors: Cozzi
mesh_terms: D000465:Algorithms; D002138:Calibration; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*