ESTRO 38 Abstract book

S476 ESTRO 38

preliminary dosimetric characterization was performed, the studied parameters were repeatability, response linearity versus absorbed dose and dose rate dependence. In-plane and cross-plane beam profiles were acquired with the setup configuration (SSD=90 cm and depth=10 cm). Beam profiles were acquired scanning the field with 0.1mm step. A comparison with single crystal diamond detector specifically developed for small field (PTW60019) was also performed. Signal ratios were obtained by considering the 3DDOSE charge value for different nominal field size. For each acquisition 100 cGy were delivered at a Dose rate of 430 cGy/min. The 3.2× 3.2 cm 2 FS was used as reference. Values were compared with PTW60019 and a plastic scintillator dosimeter. Results The 3DDOSE detector was tested for time stability and repeatability showing excellent performance with less than 0.6% signal variation. It also showed a linear response for low dose rates with a deviation from linearity of 2%. As an example in fig.1 the 0.8×0.8 cm 2 in-plane profiles for 3DDOSE, and the single crystal diamond PTW60019 were shown. In table1 the signal ratios as function of the nominal field size are reported and compared with PTW60019 and the plastic scintillator detector EXRADIN W1 used as reference in the hospital. Even if this study is very preliminary and more works must be done, both on the detector contacts and in testing it for small fields measuremnts , the obtained results show a good agreement both with the single crystal diamond and the plastic scintillators and are good starting point for developing a new device for QA.

Poster: Physics track: Dose measurement and dose calculation

PO-0899 Validation of dose calculation accuracy on daily cone-beam CT scans in the thoracic region L. Hoffmann 1 , M. Alber 2 , U.V. Elstrøm 1 , L.P. Kaplan 1 , D.S. Møller 1 1 Aarhus University Hospital, Department of Oncology- Medical Physics, Aarhus, Denmark ; 2 Heidelberg University Hospital, Department of Radiation Oncology-, Heidelbarg, Germany Purpose or Objective Large anatomical changes may occur during RT of lung cancer. Daily image guidance combined with adaptive RT can correct for these changes. However, the quality of cone-beam CT (CBCT) is often regarded as problematic for dose computation. This study investigates the accuracy of daily dose calculation on CBCT using three different algorithms with the purpose of daily assessment of delivered dose. Material and Methods Stoichiometric calibration was performed for the CT and CBCT images. Twelve patients were set up using soft tissue matching based on daily CBCTs. All patients had a surveillance CT scan (sCT) at fraction 10. Target and OAR structures were delineated on the sCT and rigidly transferred to CBCT at fraction 10. The treatment plan was recalculated on the sCT and CBCT using the AAA, Acuros (Varian Medical Systems) and Monte Carlo SciMoCa (ScientificRT) algorithms. On CBCT, large regions with HU<950 were seen. Density overrides with HU=900 were performed for these regions followed by recalculation of dose (CBCTlift). Dose to 98% (D 98 ), 2% (D 2 ) and mean dose to CTV was analyzed in addition to dose to the hottest 1 cm 3 of the spinal cord and oesophagus (D 1cc ). Dose parameters were compared for the algorithms between sCT and CBCT or CBCTlift.

Results The CTV dose distribution calculated by Acuros is shown for one patient in Fig. 1. A very similar dose distribution is seen, except for underdosage of the CTV on the CBCT in the lung region with low HU density. Density override removes the discrepancy. In Fig. 2, a box plot for CTV dose difference between sCT and CBCT is shown for all 12 patients and all three algorithms. The median difference of the mean dose is <0.5% for all algorithms with a maximum deviation of 1.3%. The dose difference range between the 1 st and 3 rd inter quartile ranges (IQR) for D 98 is 6.4%, 5.0% and 2.7% for Acuros, SciMoCa and AAA, respectively. The deviations between sCT and CBCT mainly stem from lung regions with very low HU only in the CBCTs affecting primarily Acuros and SciMoCa. Density override in these regions reduces the IQR to 1.0%, 0.8%

Conclusion 3DDOSE demonstrated its potentiality to be used as dosimeter for beam quality assurance, moreover a 3DDose array or a 2D matrix could be realized to speed up QA tests on clinical beams.