ESTRO 2023 - Abstract Book

S1512

Digital Posters

ESTRO 2023

Materials and Methods Our Elekta Synergy linacs are equipped with 5 energies and 4 applicators (6). Literature states (7) that LSE is lost for field s smaller than 6x6cm ² . If LSE is not achieved the R85 value will shift towards the surface and this shift increases with energy. Five symmetrical cut-outs (6x6; 4x4; 2.5x2.5; 2x2; 2x1.5cm ² ) for the 6x6cm ² applicator were evaluated. PDD curves in a MP3 water tank were measured at a source skin distance (SSD) of 95 cm using an electron diode detector (60012), a diamond detector (60003), and a liquid filled ion chamber (31018) (PTW, Germany). Normalisation was performed by adjusting the profile using the same chamber in reference conditions. Lieteratur reported an underestimation of the virtual source distance (VSD) for small fields at extended SSD (6). Therefore, PDD data at an SSD of 115 cm were measured using the diode and liquid ion detector and compared with the RayStation 9B (RS) data (RaySearch Laboratories, Sweden). Results Measured PDD curves are in good mutual agreement (1D gamma; 3%, 2 mm). As expected, the R85 value shift towards the surface was observed (17 mm-12MeV; 12mm-10MeV; 8mm-8MeV; 4.4mm-6MeV; 1.3mm-4MeV). For the liquid ion detector, the response in the build-up region for small inserts is significantly lower (up to 5%). RS is able to predict the dose within the NCS-15 tolerances for the dose region at dmax or deeper but for some energies an overdosage (5%) is observed in the build-up zone. The R85 value is smaller ( ∑ =0.82 mm) and all individual data remain within 3mm. At SSD=115 cm the measured data are in good agreement for the entire profile (3%;2mm). RS data match well around the R50 values but an overdosage is observed around dmax. Absolute dose agreement is for most fields within 3% of dose max, with maximum of 5%, but the local relative difference ranges from 1.5% to 16%. Conclusion This study confirms that reproducible data can be obtained but a validation using different detectors is mandatory before RS validation is started. Using the same tolerance (5%) for small photon beams, the RS MC code is validated for fields with no significant air gaps between the applicator and patient. An underestimation of the LSE in the MC model could explain the observed differences around dmax and R85 and overestimation of the VSD results in the errors observed at extended SSD. Detector based measurements are used to validate the E-MC code and help in adjusting parameters to optimize the beam model. Clinical use of small inserts is feasible with MC based treatment planning.

PO-1789 Ionisation chamber constancy check : a 3D printed device associated with an Ir192 afterloader

L. Parent 1 , P. Zidane 1

1 Institut Universitaire du Cancer Toulouse Oncopole, Medical physics department, Toulouse, France

Purpose or Objective The ionisation chamber constancy check is part of good practice in medical physics. Strontium 90 check sources have long been used for this test. However, they are no longer marketed. In this study, a 3D printed device was developed and tested to perform ionisation chamber constancy check with an Iridium 192 high-dose rate afterloader. Materials and Methods The device was printed on the Raise3D Pro 2 printer with a polylactic acid (PLA) filament. It consisted of a 10x10x10 3 cube in which a catheter connected to the Iridium 192 high-dose rate afterloader can be inserted in its centre. The cube is attached with bars to PTW RW3 water equivalent slabs where the ionisation chamber can be inserted. Bars were designed to position the cube on the slab centre with a minimum error at each measurement. The tested chamber models were PTW TM31016 (0.016 3), TM31010 (0.125 3), TM31013 (0.3 3), TM30013 (0.6 3) and TM34001 (ROOS ). The following tests were performed: repeatability, reproducibility, influence of the measurement environment, effect of the source position and the source replacement. Results Repeatability and reproducibility results were lower than 2%. The positioning of the device on 10 cm RW3 plates makes the measurements insensitive to the material on which the measuring device is positioned (metal plate or additional RW3