ESTRO 36 Abstract Book

S964 ESTRO 36 _______________________________________________________________________________________________

in CT and MRI as contrast agents, but also can be feasible radiosensitizers in radiotherapy. Hence they are attractive candidates for multimodal dose enhancement studies. In this study, the ability of dose enhancement of these nanoparticles using MAGIC-f polymer gel under the internal Iridium-192 and the external Cobalt-60 radiotherapy practices were investigated. Material and Methods The Bi2O3-NPs less than 40 nm in diameter and 0.1 mM concentration were synthesized. To increase the precision of the gel dosimetry a Plexiglas phantom was designed and made, all of the gel filled vials (with and without the nanoparticles) were irradiated to an Ir-192 radioactive source simultaneously. Also, Irradiation was carried out with a Co-60 teletherapy unit. Results The maximum dose enhancement factors under the internal Iridium-192 radiotherapy were 31% and 22% in the presence of Bi2O3-NPs and Gd2O3-NPs, respectively, whereas these amounts were reduced to 1% in external radiotherapy by Co-60 photons. Conclusion The results of our research approves the use of Bismuth and Gadolinium based nanoparticles in brachytherapy. Additionally, the polymer gel dosimetry can be a feasible material for verification and estimation of dose enhancements in the presence of nanoparticles. EP-1751 A comparison of tools for Delivery Quality Assurance in TomoTherapy T. Santos 1 , T. Ventura 2 , J. Mateus 2 , M. Capela 2 , M.D.C. Lopes 2 1 Faculty of Sciences and Technology, Physics Department, Coimbra, Portugal 2 IPOCFG- E.P.E., Medical Physics Department, Coimbra, Portugal Purpose or Objective A TomoTherapy HD unit has recently been installed in our hospital. The purpose of the present work is to establish an accurate and efficient method of patient specific delivery quality assurance (DQA). Four available tools (EBT3 Grafchromic film, Dosimetry Check –DC –, ArcCHECK TM and RadCalc®) have been tested and compared. Material and Methods Standard patient plan verification in TomoTherapy is done through film dosimetry in the Cheese Virtual Water TM phantom. Also point dose measurements can be performed inserting ionization chambers in this phantom. A well- established film absolute dosimetry methodology using EBT3 Gafchromic film and applying a multichannel correction method was developed in-house, adapting the standard approach in the DQA Tomo station. The treatment plans of the first 21 patients were retrospectively verified using also Dosimetry Check software (Math Resolutions, LLC) and ArcCHECK TM (Sun Nuclear). A beta version of RadCalc®6.3 (LifeLine Software Inc.) for TomoTherapy has been used for independent treatment time calculation. DC uses the MVCT detector sinogram to reconstruct the 3D dose distribution. In this work it was used in pre-treatment mode with the couch out of the bore. The transit dose mode where the patient delivered dose reconstruction is obtained was not assessed in this work. ArcCHECK TM records the signal of 1386 diodes embedded as a helical grid on a cylindrical phantom, enabling 4D volumetric measurements. The Gamma passing rate acceptance limit was 95% using a 3%/3 mm criterion in all cases. Results All the used QA tools showed a good agreement between measured and planned doses. Film and DC achieved similar

results with mean Gamma passing rates of 98.8±1.6% (1SD) for EBT3 film and 97.9±1.6% (1SD) for DC. Moreover, a correlation was found between those results: when passing rates using film were poorer (<97%), the same happened with DC, while passing rates over 97% for DC corresponded to the same range using film. This correspondence was not verified with ArcCHECK TM where Gamma passing rates were always close to 100% (99.6±0.7% (1SD)). Concerning the independent treatment time verification with Radcalc®, the percentage difference to the Tomo TPS calculation was 0.2±2.5% (1SD), on average. Conclusion DC and ArcCHECK TM allow volumetric dose comparisons between calculated and measured doses. Moreover DC displays DVHs and isodose lines for the considered structures in the plan while 3D-DVH in ArcCHECK TM is not available for TomoTherapy. DC seems to be a valuable tool for performing patient- specific DQA however, considering the present Pencil Beam algorithm and its known limitations, a verification using film dosimetry and ionization chamber measurements should be done in case of any significant discrepancy. Concerning the beta version for TomoTherapy in RadCalc® software, it seems to be a valid tool for independent treatment time verification, easily incorporable in routine treatment workflow. EP-1752 A simple technique for an accurate shielding of the lungs during total body irradiation H. Mekdash 1 , B. Shahine 1 , W. Jalbout 1 , B. Youssef 1 1 American University of Beirut Medical Center, Radiation Oncology, Beirut, Lebanon Purpose or Objective During total body irradiation (TBI), customized shielding blocks are fabricated and positioned in front of the lungs at a close distance from the patient’s surface to protect the lungs from excessive radiation dose. The difficulty in such treatments is to accurately position the blocks to cover the entire lungs. Any error in the positioning of lung blocks can give higher doses in the lungs than intended and can lead to underdosage in the body/target volume. The conventional technique for the positioning of lung blocks is based on a time-consuming trial and error procedure verified at each trial with radiographic films. A new technique based on CT simulation was developed to determine the exact position of lung blocks prior to treatment for each specific patient. This technique of accurate shield positioning serves the purpose of reducing lung toxicities and most importantly reduces patient’s pain and discomfort by minimizing the length of the overall treatment session. Material and Methods Patients were CT simulated in their lateral recumbent treatment position and lungs were contoured with the aid of a treatment planning system. Horizontal AP/PA fields were designed with MLC aperture conforming to lung contours. The fields were used to project a light field on the patient’s skin representing the extent of the lungs, which was subsequently marked on the patient’s anterior and posterior skin as seen in Figure 1. Prior to each fraction, the lung blocks were positioned with their shadow matching the lungs’ marks. The position of the shielding blocks was radiographically verified prior to the delivery of each beam as per the usual procedure (Figure 2). Three patients underwent TBI with this new technique. Each patient received in total six fractions of AP/PA beams including two fractions with shielded lungs. The lungs received in total 8 Gy and the rest of the body was irradiated with the prescribed dose of 12 Gy. To evaluate the efficiency of this technique, the number of repeated