ESTRO 2023 - Abstract Book

S1487

Digital Posters

ESTRO 2023

Materials and Methods X-ray dense markers (fiducials) were added to the myQA SRS array to be able to track its position with the Cyberknife (CK; Accuray Inc.) system. Different fiducial configurations were tested to achieve the lowest positioning uncertainty and highest flexibility for treatment plan verification. The array characteristics such as a possible angular dependence and a dependence on the source-axis distance (SAD) were investigated and the possibility to correct for these effects was added to the evaluation software myQA (IBA). Measurements of 12 typical patient treatment plans were carried out, a gamma index analysis with 1mm/3% criterion comparing the measured data with the data provided by the treatment planning system (TPS) were performed. The results were compared to the results from an ionization chamber (IC) array which has been used for treatment plan quality assurance (QA) in our clinic so far. Results Best positioning results were achieved with an additional plate added on top of the array containing eight fiducials and one fiducial below the array’s active area. The myQA SRS array showed angular dependence resulting in deviations in the measured values of up to 20%; the SAD dependence was significantly lower (1-2%). Without any correction of these dependencies, the measured dose distributions deviated from the TPS data with results of 22-78% gamma pass rate. Applying the corrections in the myQA software, the results were significantly improved with more than 95% pass rate for nearly all treatment plans used. In contrast, pass rates of only 50-60% were achieved in some cases when using the IC array. These results are provided in detail in Table 1.

Table 1: Parameters of the patient treatment plans applied and gamma pass rates (1mm/3% criterion) of the comparison of the IC array used in routine today (grey) / the IBA myQA SRS array with angular and SAD correction applied (green) with the TPS data. Conclusion With angular and SAD corrections applied, the myQA SRS array provided very good results with significantly higher agreement with the TPS data than the IC array. With the fiducials added, it is therefore well suited for the verification of CK SRS plans.

PO-1767 Hounsfield units to electron density conversion: inhomogenous phantoms, user experience and RTP dose

W. Baus 1 , G. Altenstein 2 , H. Hermani 1 , C. Moustakis 3

1 Robert Janker Klinik, Radiotherapy and Radiooncology, Bonn, Germany; 2 University Hospital of Cologne, Radiooncology, Cologne, Germany; 3 University Hospital Muenster, Radiotherapy and Radiooncology, Münster, Germany Purpose or Objective Conversion of computed tomography (CT) Hounsfield Units (HU), to electron density (ED) or material via lookup-table (LUT) is the basis of heterogeneity corrected dose calculation for radiotherapy treatment planning (RTP). Though the necessary precision of the conversion might be debatable [1], using an ED-phantom at commissioning of the RTP-System (RTPS) is standard [2]. However, CT measures mainly absorption by photo effect (depending on effective atomic number, Zeff), whereas dose in radiotherapy is caused rather by Compton scatter (depending on ED). Neglecting this fact by using unsuitable phantom material can lead to an inappropriate LUT (iLUT) as was observed for one of our RTPS. Therefore we investigated phantoms and materials. In addition, the LUTs used by the participants of an RTP study [3] are compared and the clinical impact is investigated. Materials and Methods We measured 3 commercial (CIRS 062, RMI 467, Catphan 504) and self-built phantoms (SBP) with CT-scanners from 3 vendors using various scan parameters. The SBP contained, CaCO3, hydroxylapatite, ethanol, n-propane, aqueous solutions (aqus.) of CaCl2, and water. For theoretical understanding, Zeff of various compounds was calculated after modifying a python script gratefully provided by Liu ([4]). To estimate the influence of the iLUT, patient plans covering bony anatomy were calculated and compared. In addition, plans using different LUTs where compared investigating the influence on plan metrics as part of a multicentre study on pancreas planning (PACA). Results Fig. 1 shows a graph of ED over HU for different materials, including iLUT and standard curves ([1], [5]). Nearly all data points lie in the vicinity of the standard curves – except the ones of the Catphan-phantom. These lie on a straight line