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Multicentre Comparison of Scatter Factors for Square and Rectangular Small Fields Defined for TrueBeam Linacs

C Venencia1*,A Alarcon1*, F. Bregains2*, N. Larragueta2*, R. Hill3*, J. Morales3*, A. Adrada4*, D. Gonzalez4*, D. Vaca5*,J. Paz5*,(1) Instituto Zunino - Fundacion Marie Curie, ARGENTINA, (2) CEMENER, ARGENTINA, (3) Chris O'Brien Lifehouse, AUSTRALIA, (4) Clinica de Occidente, COLOMBIA, (5) Clinica Valle del Lili, COLOMBIA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Some TPSs require square and rectangular of scatter factors (SF). Measuring SF for field sizes where one dimension is less than 4 cm can be challenging. In addition, the recommendations from the IAEA TRS483 COP for rectangular fields requires approximations. The objective of this research was to compare SF for small field sizes between institutions.

Five institutions with Varian TrueBeam Linacs (Varian) participated in this study. SFs were measured at SSD=100 cm and depth=10 cm for the 6 MV x-ray beam. Square and rectangular fields sizes ranging from 1x1cm to 4x4cm were measured. Six datasets corresponding to a fixed field size in the in-plane (A) or cross-plane (B) direction (1x, 2x and 3x cm) and size variation on the other axis (up to 40 cm) were studied. The detectors used were ion chamber (3D PinPoint PTW 31016), diodes (PTW 60012, PTW 60017, IBA Razor, IBA SFD and Sun Nuclear Edge) and microDiamond (PTW 60019). TRS483 formalism was applied and users’ strategies for equivalent field size determination were registered.

Equivalent square field sizes outside of range established by TRS483 were determined by different methods in each institution: Interpolation of A.2 Table in BJR25, geometric mean independent of A/B and no use of equivalent size (Kqqclin calculated by an empirical formula). For field sizes larger than 2 cm, the average data variations were less than 0.9%. For field sizes of 1 cm, maximum variations were less than 3.1% and the average variation was 1.5% in A direction while the maximum variations were less than 3.2% and average variation was 3.0% for the B direction.

This data set provides new users a valuable data set for comparison with their SF values. Further work will include more photon beam energies and institutions.


Dosimetry, Small Fields


TH- External Beam- Photons: Small field experimental dosimetry

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