title: Technical Note: Multicenter study of TrueBeam FFF beams with a new stereotactic diode: Can a common small field signal ratio curve be defined?
abstract: OBJECTIVE
  Small radiation fields (<30 mm) are typically involved in stereotactic body radiation therapy procedures. Output factor measurements are subjected to large uncertainties. The signal ratio (SR) readings, defined as the ratio of central axis reading, respectively, in the actual field size and in the reference field size, were evaluated in several centers and a common mathematical description of the SR curve was investigated.
  
  
  METHODS
  A couple of new unshielded stereotactic diodes (Razor, IBA) was tested under eight different TrueBeams using 10 MV flattering filter free beams with high dose rate (2400 MU/min). Small fields, ranging from 6 to 50 mm, were analyzed in terms of profiles and central axis point measurements. SRs were normalized to 30 mm field and were calculated as a function of nominal field size (NFS) and effective field size (EFS). From SRs acquired using Razor1 (four centers), a theoretical equation was extrapolated. Three centers with Razor2 were used to test the mathematical relationship. Finally, the two diodes were directly compared in the last center.
  
  
  RESULTS
  The EFS was systematically smaller than NFS (p < 0.01) for all field size ranges, with mean difference of 0.9 ± 0.5 mm. The SR fits using the NFS and EFS had, respectively, R2= 0.989 and R2≫ 0.999. The Razor2 centers' mean deviation from the predicted SRs, using the NFS and EFS fits, was, respectively, 3.4% and 0.5%. The maximum deviations were 5.0% (6 mm field size) for NFS and 1.9% for EFS. Maximum deviation of 0.5% between the two Razors was observed.
  
  
  CONCLUSIONS
  EFS measurements were confirmed to be mandatory when comparing SRs over different centers. An equation establishing a functional relation between SRs and the EFS was obtained and tested for the new Razor diode.
authors: Medical Physics Unit, Arcispedale Santa Maria Nuova - IRCCS, Reggio Emilia 42123, Italy.
authors: Elisabetta
authors: E
authors: Cagni
authors: Medical Physics Unit of Radiation Oncology Department, Azienda USL Toscana Centro, Firenze 50012, Italy.
authors: Serenella
authors: S
authors: Russo
authors: Medical Physics Unit of Radiation Oncology Department, Humanitas Clinical and Research Center, Milano-Rozzano 20089, Italy.
authors: Giacomo
authors: G
authors: Reggiori
authors: Medical Physics Department, Istituto di Candiolo - IRCCS, Torino 10060, Italy.
authors: Sara
authors: S
authors: Bresciani
authors: Radiotherapy Department, Casa di cura San Rossore, Pisa 56122, Italy.
authors: David
authors: D
authors: Fedele
authors: Medical Physics Department, Arcispedale S.Maria Nuova - IRCCS, Reggio Emilia 42123, Italy.
authors: Mauro
authors: M
authors: Iori
authors: Medical Physics Department, Humanitas-Centro Catanese di Oncologia, Catania 95126, Italy.
authors: Carmelo
authors: C
authors: Marino
authors: Medical Physics Department, UPMC San Pietro FBF, Roma 00189, Italy.
authors: Barbara
authors: B
authors: Nardiello
authors: Radiotherapy Department, Ospedale sacro Cuore don Calabria, VR, Negrar 37024, Italy.
authors: Ruggero
authors: R
authors: Ruggieri
authors: Laboratory of Medical Physics and Expert Systems, Regina Elena National Cancer Institute, Rome 00144, Italy.
authors: Lidia
authors: L
authors: Strigari
authors: Medical Physics Unit of Radiation Oncology Department, Humanitas Clinical and Research Center, Milano-Rozzano, 20089, Italy.
authors: Pietro
authors: P
authors: Mancosu
mesh_terms: D004566:Electrodes; D011874:Radiometry; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D035501:Uncertainty