Technical Note: Multicenter study of TrueBeam FFF beams with a new stereotactic diode: Can a common small field signal ratio curve be defined?

Purpose: Small radiation felds (<30 mm) are typically involved in stereotactic body radiation therapy procedures. Output factor measurements are subjected to large uncertainties. The signal ratio (SR) readings, defned as the ratio of central axis reading, respectively, in the actual feld size and in the reference feld 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 di?erent TrueBeams using 10 MV ?attering flter free beams with high dose rate (2400 MU/min). Small felds, ranging from 6 to 50 mm, were analyzed in terms of profles and central axis point measurements. SRs were normalized to 30 mm feld and were calculated as a function of nominal feld size (NFS) and effective feld 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 feld size ranges, with mean difference of 0.9±0.5 mm. The SR fts 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 fts, was, respectively, 3.4% and 0.5%. The maximum deviations were 5.0% (6 mm feld size) for NFS and 1.9% for EFS. Maximum deviation of 0.5% between the two Razors was observed. Conclusions: EFS measurements were confrmed 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.