Optimizing the effective spot size and the dosimetric leaf gap of the AcurosXB algorithm for VMAT treatment planning

Abstract

Purpose The aim of this study is to provide and test a new methodology to adjust the AcurosXB beam model for VMAT treatment plans. Method The effective target spot size of the AcurosXB v15 algorithm was adjusted in order to minimize the difference between calculated and measured penumbras. The dosimetric leaf gap (DLG) was adjusted using the asynchronous oscillating sweeping gap tests defined in the literature and the MLC transmission was measured. The impact of the four parameters on the small field output factors was assessed using a design of experiment methodology. Patient quality controls were performed for the three beam models investigated including two energies and two MLC models. Results Effective target spot sizes differed from the manufacturer recommendations and strongly depended on the MLC model considered. DLG values ranged from 0.7 to 2.3 mm and were found to be larger than the ones based on the sweeping gap tests. All parameters were found to significantly influence the calculated output factors, especially for the 0.5 cm × 0.5 cm field size. Interactions were also identified for fields smaller than 2 cm × 2 cm, suggesting that adjusting the parameters on the small field output factors should be done with caution. All patient quality controls passed the universal action limit of 90%. Conclusion The methodology provided is simple to implement in clinical practice. It was validated for three beam models covering a large variety of treatment types and localizations.

Guía para el control de calidad y seguridad de los sistemas de planificación y planes de tratamiento de radioterapia externa

“…En caso de desviación sistemática, se puede modificar el parámetro más directamente relacionado con el posicionamiento de las láminas (DLG, leaf offset, offset o similar) para mejorar el acuerdo entre cálculos y medidas. 101,102 Esto se puede realizar variando empíricamente este parámetro (por ejemplo, en pasos de 0.1 mm o 0.2 mm) y evaluando la coincidencia entre las dosis previamente medidas y los nuevos cálculos.…”

“…Existe una amplia bibliografía acerca de la comprobación de los algoritmos de cálculo y el modelado de las unidades en el sistema, desde el cálculo de haces de radioterapia conformada, 3,57,58,60,64,[109][110][111] pasando por técnicas de intensidad modulada 74,80,83,87,96,112,113 y publicaciones recientes que actualizan las anteriores. 62,63,81,82,91,92,102,114 Los algoritmos de cálculo deben estar documentados por el fabricante por medio de un manual donde se describa como se calcula la dosis absorbida en el paciente. El usuario debe tener documentado cómo funcionan los algoritmos y cuáles son sus limitaciones en cuanto a exactitud, para poderlo tener en cuenta en aquellos casos clínicos en los que puedan ponerse en juego dichas limitaciones.…”

Virtual film-based testing of film quality assurance software accuracy.

“…The virtual films generated in this project were modelled on image characteristics from GafChromic's EBT-XD film. The generated VFIs were 48-bit triple-channel red, green and blue (RGB) uncompressed TIFF files with a resolution of 150 dpi [10]. To reverse engineer the pixel values (PV) for each VFI, triple-channel RGB dosimetric models were attained from a digitally scanned image of exposed GafChromic EBT-XD films with a dosimetric range of 0-35 Gy.…”

“…These files can be DICOM based as is the case with Eclipse™ v16.01(Varian Inc.) TPS or text files as is the case with the Monaco v6.1 (Elekta Solutions AB) TPS. These files were generated without the dosimetric complexities encountered by the TPS's algorithm to account for a non-singular point source, dosimetric leaf gaps, transmission factors and scatter factors [10]. These factors would generate complex fluence distributions too difficult to model and not necessary for this report.…”

Evaluating AAPM‐TG‐218 recommendations: Gamma index tolerance and action limits in IMRT and VMAT quality assurance using SunCHECK

“…The radiotherapy planning process is subject to uncertainty owing to a multitude of circumstances. From the perspective of treatment planning, uncertainties may arise from the leaf end effect, tongue‐and‐groove effect resulting from the unique form of the multi‐leaf collimator, collimator transmission factor, penumbra, field output factor, and dosimetry grid size 6–8 …”