ESTRO 2020 Abstract book

S745 ESTRO 2020

servo submersion in the scanning water phantom and its influence on dosimetry uncertainties Material and Methods An optical reticle with a 0.1 mm scale was attached to the external wall of the water phantom. A mobile phone camera held on a selfie stick tripod with remote control was used for reading and recording. A small 1D scanning water phantom (SNC 1D Scanner) and a 3D scanning water phantom (IBA Blue Phantom v2) were investigated. A Farmer type chamber was mounted on the chamber holder with its central axis aligned to the water surface as the origin. In-house and published dosimetry data were used for assessment of the influence of the depth variation on the dose calibration. Results Water level changes up to 0.8 mm were observed for the small 1D phantom when the servo was positioned about 4 cm or more below the origin with the moving part fully in water, with less water level change in the region between the origin and 4 cm. This variation made the actual depth more than the physical chamber movement, e.g. to 100.8 mm depth instead of 100 mm. Less variation (up to about 0.3 mm) was observed for the large 3-D phantom. The extra depth produced lower dose readings due to the small PDD decrease. Using in-house and published data, the relative variation to the reference dose for this amount of deviation from the designated depth is about 0.4%, 0.5%, 0.4% for 6, 6FFF & 10 MV photon beams, 0.2% & 0.3% for 16 & 20 MeV electron beams, and 1-2% for kV beams (HVL 3 & 4 mm Al). This depth variation results in a systematic increase in the dose at the true calibration depth for this kind of 1D moving servo phantoms. The scale is comparable to other correction factors like k pol , k s , k n (non-uniformity). This variation would also distort the PDD curves. The depth variation is proportional to the ratio of variable volume and the water surface area in tank. The actual amount of variation varies with different designs and needs to be assessed by physicists. Corrections can be made by using the corrected chamber position or through a correction factor k w (ratio of PDD’s of the reference and actual depth). Conclusion The variation of the water level with the moving parts and its influence on the dose accuracy needs to be assessed for reference dosimetry and other measurements with such techniques. The magnitude of this uncertainty is small and of similar influence to other correction factors, however large enough to warrant inclusion in the reference dosimetry. It is recommended that the phantom manufacturers look into this issue and reduce the volume variation during servo movement to reduce uncertainty. PO-1322 Advanced Marcus chamber in high dose-per- pulse electron beams.kpol and ksat inter-chamber dependence J. Chimeno 1 , J. Gimeno-Olmos 1 , J.C. Ruíz-Rodriguez 1 , V. Carmona 1 , F. Lliso-Valverde 1 , J. Pérez-Calatayud 1 1 Hospital Universitari i Politècnic La Fe, Unidad Radiofísica. Oncología Radioterápica, Valencia, Spain Purpose or Objective The Advanced Marcus parallel plate chamber (PTW Germany) is one of the preferred chambers for measurements in electron high dose-per-pulse irradiations since its low inter-electrode distance (1 mm) produces a low saturation correction factor (k sat ). The objective of this study was to evaluate the inter-chamber dependence of the polarization correction factor (k pol ) and the saturation correction factor (k sat ) for the Advanced Marcus when irradiated with highly dose-per-pulse electron beams. Material and Methods Six Advanced Marcus chambers embedded in a phantom attached to the mobile linac LIAC HWL (SIT, Sordina IORT Technologies, Vicenza, Italy) were irradiated with different energies (6, 8, 10 and 12 MeV) and, as a This abstract has been withdrawn from presentation

Material and Methods A rigid phantom was developed consisting of 6 coplanar, 5 mm diameter tungsten carbide spherical targets of which 4 are colinear. The maximum center-to-center distance between targets is 100 mm. The Winston-Lutz (WL) analysis framework was extended to encompass off- isocenter targets in order to calculate their 3D locations in the IEC 61217 fixed coordinate system. Applying a best-fit line and best-fit plane to the calculated 3D locations of the 6 targets enables the estimation the pitch, roll, and yaw of the phantom relative to the radiation isocenter. An analysis tool was developed and applied on data acquired on a Varian TrueBeam® equipped with Millennium multi- leaf collimators and on a Varian Edge® equipped with high- definition multi-leaf collimators. Results Optimized delivery plans were developed, which allow data acquisition to be completed within 10 minutes on either the Edge or TrueBeam. By introducing positioning errors of known magnitude, we demonstrated the ability of the tool to identify translational positioning errors to ± 0.1 mm and rotational positioning errors (pitch, roll, and yaw) ± 0.2 degrees. Correcting the positioning error allowed to quantify the targeting errors with the accuracy of ± 0.1 mm. We will present data demonstrating this tool’s ability identify targeting error due to couch and collimator. On a well-calibrated treatment delivery system, the targeting error was demonstrated to be less than 1 mm for off-center targets 7 cm off-isocenter.

Conclusion The MultiMet-WL QA phantom and the MultiMet-WL Analysis tool are a readily useable off-the-shelf solution and a clinically useful tool for daily or pre-treatment machine QA. Integration of the MultiMet-WL QA phantom with the StereoPHAN end-to-end phantom makes it an effective tool for end-to-end testing. PO-1321 Assessment of depth uncertainty and its influence on dose measurement in water phantoms G. Song 1 , D. Rajaratnam 1 , S. Saju 1 , N. Caswell 1 , A. Buddhavarapu 1 1 Ballarat Austin Radiation Oncology Centre, Medical Physics, Ballarat, Australia Purpose or Objective To introduce a simple method to investigate the variation of the water level and thus the true chamber depth due to