ESTRO 37 Abstract book

S959

ESTRO 37

statistically significant (p = 0.002), highlighting that phant-MV is more accurate than phant-CT mod, likely due to the more realistic modeling of phantom inhomogeneities. Higher differences, although not significant, are found for abdominal cases (maximum 9.4%), probably because of a larger weight of anterior and posterior dose contributions that emphasized phant-CT mod inaccuracy. Instead, lower differences (mean 1%) are found for breast plans where the higher contribution to the dose distribution is due to the oblique entries, confirming the previous hypothesis. Considering g values-4%/3mm criteria, the differences among phant-CT mod and phant- MV largely reduce, ranging from 0% to 4.2% (mean 1%), while remaining statistically significant (p = 0.03).

using an in-house developed software by shifting the target point within the planning CT based on the motion information and beam status recorded with the Calypso system. To assess the accuracy of the dose calculation algorithms of both TPS and PG, a PinPoint® chamber measurement was used for normalization. Results Both PG measurement and 4D dose calculation algorithm were able to reproduce the dose fall-off in the direction of dominant motion. A representative dose profile is shown in Fig.1 (a). However, the calculated dose showed a dose overestimation of up to 0.4Gy. Analysis of the motion trajectory, however, showed small deviations between the motion trajectory derived from the Calypso TM -system and that acquired directly from the motion robot. Deviations were especially large for regions of maximum elongation, where the container position was underestimated with a mean deviation of −1.07 ± 0.38mm. To analyze this effect in more detail, the 4D dose calculation was repeated using the robot-based rather than the Calypso TM -system-based container trajectory. As a result, the shape of the profiles in the dose gradient showed a better agreement with the measured data (see Figure 1 (b)) with a high 3D-gamma passing rate of 91.8%.

Conclusion Regardless of the method used, the high g values obtained (> 95%) confirm the reliability of Acuros algorithm. The inhomogeneities of the phantom used for pre-clinical verification and the consequent artifacts in the CT images, slightly decrease the agreement between measured and calculated planar dose distributions. Phant-MV showed better results, providing a robust method for pre-clinical verifications. EP-1786 4D dose calculations validated by combing 3D polymer gel dosimetry and the Calypso tracking system P. Mann 1,2 , N. Saito 1,2 , C. Lang 1,2 , A. Runz 1,2 , W. Johnen 1,2 , M. Witte 1,2 , C.P. Karger 1,2 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany Purpose or Objective Modern adaptive radiotherapy techniques have the potential of significant normal tissue sparing. Clinical implementation of these techniques, however, requires validation of the intended workflow. The aim of this study was to validate an in-house developed 4D dose accumulation algorithm, which uses motion tracking data from the Calypso TM -system in combination with a polymer gel (PG)–based 3D dose measurement. Material and Methods In this study, a cylindrical phantom was designed which allows to insert a PG container made of BAREX. In addition to this, three Calypso beacons can be attached allowing for online target tracking. This phantom was then placed on top of a computer-controlled motion robot to which arbitrary motion patterns can be applied. For this study, a 1D cos 4 motion trajectory in z-direction with a 2.5cm peak-to-peak amplitude and a period of 7.5s was used. The 3D dose measurement was performed with the in-house produced PAGAT PG. It consists of 88% H 2 O and two type of monomers embedded within a gelatine matrix that start to polymerize upon irradiation. This process causes the relaxation time T 2 to locally vary which can be measured in 3D by MRI. A multi-spin echo sequence with 32 equidistant echoes (TE=22.5ms – 720ms) was used for the quantitative T 2 measurement. The 3D dose distribution was retrospectively calculated

Conclusion The 4D dose calculation algorithm in combination with the PG is very accurate to calculate dynamic target irradiation without the application of motion- compensation techniques. It should be emphasized that the PG was able to detect small deviations being caused by the Calypso TM . Therefore, the PG can be considered as a valuable and robust tool for the purpose of workflow verification in adaptive radiation therapy. EP-1787 Use of two in vivo monitoring devices in the breast irradiation: an anthropomorphic phantom study C. Arilli 1 , Y. Wandael 2 , M. Casati 1 , L. Marrazzo 1 , C. Galeotti 2 , I. Meattini 2 , P. Bonomo 2 , G. Simontacchi 2 , S. Pallotta 3 , C. Talamonti 3 1 Azienda Ospedaliera Universitaria Careggi, Medical Physics Unit, Florence, Italy 2 Azienda Ospedalieria Universitaria Careggi, Radiotherapy Unit, Florence, Italy 3 University of Florence, Biomedical Experimental and Clinical Science "Mario Serio", Florence, Italy Purpose or Objective Two systems for in vivo dosimetry, the IQM detector (Integral Quality Monitoring, iRT Systems GmbH, Koblenz, Germany) and the SoftDiso software (Best Medical Italy Srl), were tested to evaluate their sensitivity in detecting some delivery errors which can occur in standard 3DCRT external breast irradiation. Material and Methods The irradiations were performed with a Precise Elekta linac (Crawley, UK) equipped with silicon amorphous portal imaging (EPID). IQM detector is designed to be mounted below the MLC during treatment irradiation. The IQM signal (dependent of the radiation fluence) is compared with a reference signal. SoftDiso is a software which reconstructs the dose distribution at the isocenter