Session Item

TPS plays an important part in the dosimetric accuracy of small fields radiotherapy. Improper beam modelling of small fields will affect the dose delivery. As Eclipse TPS only considers beam profile data above 3x3 cm², modelling of small field by the MLC (while keeping jaw at 3x3cm²) might produce a better dose accuracy as compared to jaw delineated field size. In this work, both convolution-based anisotropic analytical algorithm (AAA) and the Linear Boltzmann Transport Equation solvers AcurosXB calculation algorithm (AXB) are evaluated on their dose accuracy under various jaw and MLC-delimited field sizes. Different beam parameters such as dosimetric leaf gap (DLG) of the MLC and effective spot size (ESS) of the beam are also varied to evaluate the dosimetric accuracy.

Four different energies are used in this study: - 6X, 6FFF, 10X and 10FFF. Jaws delineated field size of 3x3cm², 2x2cm², 1x1cm², 1x0.5cm² are measured at 95cm SSD with a fixed 100MU using PTW 31006 PinPoint detector. For MLC delineated field size, in additional to the above field size, 1x0.4cm², 1x0.3cm², 1x0.2cm² and 1x0.1cm² were also added in to evaluate the dosimetric accuracy. The measured results are then compared again TPS calculated values with different parameters. DLG was first varied by changing from the measured values of 0.1350, 0.120, 0.149, 0.140 for 6X, 6FFF, 10X and 10FFF using sliding gap technique to 0.030 across all energy. 0.25cm calculation grid size and ESS was varied from 0mm in both X and Y direction (0-0) to 1mm in both directions (1-1) for AAA. The measurement was repeated using AXB algorithm except the use of 0.10cm for calculation grid size. The percentage discrepancy between measurement and TPS are reported for various TPS configurations.

There are missing lines in the figures due to overlapping results. Across all plots for AAA, jaws delineated field are found to have similar percentage errors regardless of the values of ESS with the smallest field size of 1x0.5cm² having the biggest error. For MLC delineated fields, DLG value of 0.030 with ESS of 1-1 is seen to have the biggest percentage error at 1 x 0.1cm² while other MLC delineated fields have similar percentage errors with the biggest error at 1x0.4cm² and 1x0.3cm² field size.

Across all plots for AXB, jaw delineated field size has a slight difference when the ESS is changed from 1-1 to 0-0 with the smallest field size having the largest error. MLC delineated field size has a smaller error when a smaller DLG value of 0.030 is used for calculation. ESS of 1-1 also provides a more accurate calculation. MLC delineated field with DLG of 0.030 and ESS of 1-1 is then the better option for AXB calculation as compared to others. 

Using MLC delineated field size provides a better dosimetric accuracy when modelling in the TPS for field sizes between 3x3 cm² and 1x0.5 cm². For fields smaller than 1x0.5 cm², dose calculation can be made more accurate using AXB calculation with ESS of 1-1 with a DLG of 0.030.