Simplified sigmoidal curve fitting for 6 MV FFF photon beam of Halcyon to determine field size for beam commissioning and quality assurance

Abstract

Background Delivery of a single energy 6 MV flattening filter free (FFF) photon beam is a main characteristic of an O-ring gantry type linear accelerator (linac) Halcyon. The purpose of this study is to determine the field size of the beam through an application of the simplified sigmoidal curve fitting (SCF) to the beam profiles obtained from the preconfigured reference data of Halcyon, and then to compare its parametrization with the measured beam data from Halcyon. Methods After a mathematical definition of the SCF using four coefficients, the defined curves were fitted to both the reference and measured data. When a high agreement between the fitting curve and the profiles in each data, the field sizes were determined by identifying the maximum point along the third derivative of the fitting curve. The curve fitting included the field sizes for beam profiles as 2 × 2, 4 × 4, 6 × 6, 8 × 8, 10 × 10, 20 × 20 and 28 × 28 cm 2 as a function of depths (at 1.3, 5, 10, 20 cm). The results of the field size from the reference data were compared with the results in the measured data using same condition. Results All fitting curves show an average agreement ratio higher than 97% and the values of goodness of fit, R 2 , as better than 0.99. The differences of the field size between the reference data and the measured data were within the range of 0 to 0.2 cm. The least difference of the field sizes at depth 10 cm which is a surface to axis distance was reported. Conclusion The application of the SCF has been proved to accurately obtain the field size of the preconfigured reference and of measured FFF photon beam data for Halcyon. The current work can be useful to the beam commissioning as a countercheck methodology to the field size from the reference data in the treatment planning system of a newly installed Halcyon and to the routine quality assurance to ascertain the correctness of field sizes clinically used with the Halcyon.

Validación de métodos de control de calidad empleando imagen portal

“…En su lugar se adoptó la distancia entre los puntos de inflexión en las zonas de penumbra del haz 23,24 como indicador del tamaño de campo, dada su menor incertidumbre. 17,25,26 Para la definición de otras características del haz [27][28][29] Para los haces FFF también se calculó la homogeneidad solo a efectos de comparación, aunque sea una medida de interpretación diferente a los modos con filtro y sujeto a una gran variación.…”

Independent monitor unit verification for dynamic flattened beam plans on the Halcyon linac

“…A new radiotherapy delivery system, the Halcyon (Varian Medical Systems, Palo Alto), was introduced into clinical practice 13 . Halcyon is designed for faster delivery and higher throughput using a single 6‐MV flattening‐filter‐free (FFF) beam equipped with a dual‐layer MLC and a maximum field size of 28 × 28 cm 2 compared with a conventional treatment unit 14,15 . In Halcyon version 2.0, dynamic beam flattening (DBF) is used to mimic flattened beams for conventional radiation therapy treatments 16 .…”