Validity of equivalent square field concept in small field dosimetry

• Published in: Medical physics (Lancaster) 2022-06, Vol.49 (6), p.4043-4055
• Main Authors: Das, Indra J., Dogan, Serpil K., Gopalakrishnan, Mahesh, Ding, George X., Longo, Mariaconcetta, Franscescon, Paolo
• Format: Article
• Language: English
• Subjects: 4A/P; equivalent square; exchange factor; field output factor; FWHM; Particle Accelerators; Radiometry - methods; Sclin; small field; Uncertainty
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1002/mp.15624

Purpose The equivalent square (ES) concept has been used for traditional radiation fields defined by the machine collimating system. For small fields, the concept Sclin was introduced based on measuring dosimetric field width (full‐width half maximum, FWHM) of the cardinal axis of the beam profiles. The pros and cons of this concept are evaluated in small fields and compared with the traditional ES using area and perimeter (4A/P) method based on geometric field size settings, for example, light field settings. Methods One hundred thirty‐seven square and rectangular fields from 5–50 mm with every possible permutation (keeping one jaw fixed and varying other jaw from 5 to 50 mm) were utilized to measure FWHM for the validation of Sclin. Using a microSilicon detector and a scanning water tank, measurements were performed on an Elekta (Versa) machine with Agility head and a Varian TrueBeam with different MLC/Jaw design to evaluate the Sclin concept and to understand the effect of exchange factor in small fields. Field output factors were also measured for all 137 fields. Results The data fitting for fields ranging from 5–50 mm between the traditional 4A/P method and Sclin shows differences and indicates a linear relationship with distinct separation of slope for Elekta and Varian machines. For Elekta Agility machine ES based on 4A/P  Sclin for square fields. Our measured data show that both methods are equally valid but does vary by the machine design. The field output factor is dependent on the elongation factor as well as machine design. For fields with sides ≥10 mm, the exchange factor is nearly identical in both machines with magnitude up to 4%, which is close to measurement uncertainty (±3%), but for small fields (