Abstract Book

S958

ESTRO 37

Our preliminary results show a good agreement with reference data in terms of relative depth dose profiles (PDD). Figure 1 shows the result of such a PDD measurement, performed on the Mobetron linear accelerator.

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).

Conclusion Rare-earth based optical fiber dosimetry, thanks to their high light yield and favorable spectral properties, offer a true alternative to perform real-time optical fiber dosimetry, with effective suppression of the stem effect. These devices are characterized by minimal invasiveness, capability to work under high instantaneous dose rates and real-time performances. We are currently completing the development of a device that will allow real-time dose monitoring at multiple positions. We believe that our device will eventually increase the quality and safety of IORT treatments. EP-1785 Impact of CT artifacts of planar phantoms on VMAT pre-clinical dosimetry using the Acuros algorithm P. Colombo 1 , G. Pallazzi 1 , L. Perna 1 , S. Broggi 1 , G. Cattaneo 1 , C. Fiorino 1 , P. Mangili 1 , R. Parisi 1 , R. Calandrino 1 1 IRCCS San Raffaele Scientific Institute, Medical Physics, Milano, Italy Purpose or Objective The recent upgrade of the Treatment Planning System (TPS) in our Institute led to investigate the accuracy of the implemented calculation algorithm (Acuros 13.5). As a part of the TPS commissioning program, we performed pre-clinical verifications of VMAT plans comparing two different methods; the purpose was to evaluate the dependency of the calculation accuracy on the CT imaging artifacts of the phantom used for measurements. Material and Methods Based on the delivery reliability from the previous experience, we compared measured planar dose against calculated dose distributions using the g function without dose threshold and 3%-3mm pass/fail criteria (4%-3mm for complex case). The measured and calculated dose maps were obtained respectively with the Mapcheck2+Mapphan and the CT-scan of this system. However, the presence of different materials on the detector plane creates artifacts in the CT images, reducing the reliability of calculation. This effect was analysed in two ways. First, we manually assigned the Hunsfield Unit (HU) of the water to artifacts and air (phant-CT mod ). Second, we acquired the MVCT of the phantom (on an Helical Tomotherapy unit available in the Department), as MV-Xray beams reduce artifacts (phant- MV); dose distributions were calculated considering the previously calibrated HU curve of the MV scanner. The dosimetry impact of phant-MV against phant-CT mod was investigated by comparing the g values of 30 pre-clinical verifications of VMAT plans delivered with 6MV XRays on a Varian Clinac-2300ix: 6 breast, 12 pelvis (prostate, rectum), 5 thorax-abdominal (lung, pancreas, stomach) 7 head and neck. Statistical differences were analyzed using a t-test. Results The results in terms of g values and p-values are summarized in table 1. The differences of g-3%/3mm between the two methods (mean 2.2% range 0-9.4%) are

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