ESTRO 2023 - Abstract Book

S2029

Digital Posters

ESTRO 2023

• That the Water Equivalent Thickness (WET) of decorated Q-Fix™ BoS immobilisation shells is modelled sufficiently by PBT planning systems Materials and Methods The first evaluation determined if the paints had any the effect on MRI image quality. A homogenous phantom was scanned on the Phillips Ingenia 1.5T MRI with an immobilisation mask decorated with standard liquid acrylic paints fixed around it, followed by one decorated with POSCA™ pens. A T2 sequence from the departmental Head and Neck protocol was used and images then evaluated by radiographer and physics staff. The second evaluation aimed to ensure that the WET modelled by Eclipse matched measured values on the treatment unit using an MLIC (Multi-layer Ionisation Chamber). The decorated shell was scanned using the Head and Neck protocol on the PBT CT scanner and imported to Eclipse. The WET was measured with the tools available in Eclipse in a variety of places where physical measurements could be taken on set using the MLIC. A 160MeV beam was shot into the MLIC as a reference. The distal 80% fall off of the Bragg peak was measured with and without theses mask sections in the beam path. The difference in the measured D80 then represented the measured WET of that mask region. Results The MRI image on the left of Figure 1 shows significant artefact given by the standard liquid acrylic paints, deeming them inappropriate for use in PBT due to their lack of compatibility with MRI. Opposingly, the right of Figure 1 shows the phantom with a mask painted with POSCA™ paints around it and shows no artefact from this. If these paint pens would have no impact on dosimetry, these looked to be the better solution.

In the second evaluation, both sections of the painted mask had D80 values of 17.47cm which means a measured WET of 0.14cm, lying within the values measured in Eclipse.

Conclusion The POSCA™ pens used to decorate immobilisation shells do not cause issues with the quality of MRI scans and can be appropriately modelled by the planning system for accurate dosimetry. The service is recommended to only use POSCA™ pens to decorate immobilisation.

PO-2257 An evaluation of IGRT strategies in N1 prostate cancer patients receiving nodal irradiation

D. Fairweather 1 , W. Kinnaird 1 , H. Grimes 2 , R. Davda 3 , L. Taylor 1 , A. Webster 1

1 University College London Hospital, Radiotherapy and Proton Beam Therapy, London, United Kingdom; 2 University College London Hospital, Medical Physics, London, United Kingdom; 3 University College London Hospital, Department of Uro- oncology, London, United Kingdom Purpose or Objective The benefits of pelvic ± para-aortic nodal (PAN) irradiation in N1 prostate cancer patients are currently being debated. However, data regarding the optimal IGRT strategy for these patients is limited. This evaluation aims to quantify the inter-