Abstract book - ESTRO meets Asia

S49 ESTRO meets Asia 2018

PV-124 Uncertainty analyses for Monte Carlo dosimetric characterization of Co0.A86 HDR brachytherapy source S. Abdollahi 1 , M. Dayyani 2 , L. Rafat Motavalli 3 , H. Miri Hakimabad 3 , E. Hoseinian Azghadi 3 1 Reza Radiotherapy & Oncology Center RROC, Medical Physics, Mashhad, Iran Islamic Republic of 2 Reza Radiotherapy & Oncology Center, Radiation Oncology, Mashhad, Iran Islamic Republic of 3 Ferdowsi University of Mashhad, Physics, Mashhad, Iran Islamic Republic of Purpose or Objective Overall uncertainty assessment and minimizing the dose delivery variation is a key task in brachytherapy to improve the local control and reduce the healthy organ toxicity. According to the Analysis guidelines of GEC- ESTRO and the AAPM, each type of uncertainty should be quantified and Monte Carlo dose estimation uncertainty analysis is required as a component affecting the overall uncertainty. Although there are three published studies in peer-reviewed journals as well as consensus datasets for Monte Carlo dosimetric characterization of Co0.A86, the detailed uncertainty analysis recommended by the TG-138 has not been published yet. This study attempted to report the detailed uncertainty analysis and providing the new dosimetric datasets for this source. Material and Methods Based on the following methods, an uncertainty analysis was done to evaluate the Type A and Type B uncertainties with a coverage factor of k = 1. For source and capsule dimensions and compositions, tolerances of 1.5% and 1% were respectively considered. To our knowledge, there is no report about the dynamic elements in the BEBIG source; but due to the existence of air, the moving of the active core seems probable. Therefore, calculations with an assumed possibility of 0.1 mm maximum displacement were performed. To investigate the source spectra effect, two main spectra were taken from NuDat and LUND/LBNL nuclear libraries based on which various spectra were additionally constructed, considering the reported energies and intensities uncertainties. To examine the dosimetric influence of phantom densities, water and air were modeled with 0.1% variations in densities. The dosimetric variations due to the phantom photoatomic cross section µ en /ρ were adopted from Andro et al. The µ/ρ uncertainty is assumed to be slightly lower than the uncertainty and its dosimetric influence was analytically estimated. To evaluate the effect of tally volume averaging, dose rates were calculated in voxels with 0.01 and 0.05 thickness in spherical, cylindrical and cartesian coordinates. Type A uncertainties were directly estimated based on Monte Carlo statistical uncertainties. Results Using a number of 6.4 × 10 10 for each photon and electron histories, the statistical uncertainties for water dose rates in 1 and 5 cm and air kerma strength were estimated as 0.064%, 0.065%, and 0.005%, respectively. Overall, the uncertainties for these quantities were respectively evaluated as ~ 0.088%, 0.112%, and 0.007%. Conclusion There is not any reported uncertainty which explicitly specified for a 60Co HDR brachytherapy source except for the recently published paper for Flexisource Co-60. Their result is also in line with the current study which the Type A and B uncertainties are comparable for this kind of the source. The other data that can be referred to is 1.6% for Monte Carlo dose calculation uncertainty expressed in TG- 138 for all high energy sources in water dose rate at 1 cm which is significantly greater than our estimation of <0.1%. PV-125 Evaluation of a high-resolution silicon detector for Quality Assurance of VMAT. F. Matar 1 , J. Davis 1 , D. Wilkinson 2 , T. Causer 1 , I. Fuduli 1 ,

7 University of Perugia, Radiation Oncology, Perugia, Italy 8 University of Perugia- Perugia, Department of Surgical and Biomedical Science, Perugia, Italy 9 Polo Scienze Oncologiche ed Ematologiche- Istituto di R adiologia- Università Cattolica del Sacro Cuore- Fondazio ne Policlinico Universitario Agostino Gemelli, Radiation Oncology, Rome, Italy 10 University of Perugia and Perugia general hospital, Radiation Oncology Unit- Department of Surgical and Bio medical Science, Perugia, Italy Purpose or Objective Several disadvantages are associated with the use of commercial or hand-made customized devices for the individual delivery of high-dose rate interventional radiotherapy (HDR-IRT). To demonstrate the potential of individual 3D printed devices we present an intraoral mould for delivering HDR-IRT treatment of hard palate cancer, an individual vaginal mould in a case of a vaginal recurrence in a vaginal stricture case, and the use of an individual device to perform contact HDR-IRT for penis cancer. Material and Methods Intraoral mould: Using an intra-oral scanner the patient's upper teeth and palate was scanned, and the structures transformed into a mesh. The mesh was aligned with DICOM CT images using free Blue-Sky Plan 4 planning software. The files and measurements were imported into CAD Design Software. Catheter ducts were created, and the mesh was optimized for 3D printing using the ready mould, a CT scan checked its placement and was used for treatment planning with a commercial Treatment Planning Software (TPS). Vaginal mould : Using a commercial vaginal cylinder CT images were prepared and transferred into the TPS, optimal source track channels defined, and the individual applicator printed. Commercial plastic tubes were placed into the pre-defined source track channels and used for radiation delivery. Penis mould : After the individually printed mould was adjusted to the patient’s penis, a CT scan (2.5mm slices) checked its placement and the data set was used for treatment planning. Treatment intention was to deliver ≥ 95% of the prescribed dose to the PTV, accepting 90% as satisfactory. The dose was prescribed at 5mm from the mucosal surface for penis and palate cancer, while for vaginal recurrence the treatment was performed PET/TC guided. Treatment was delivered by means of microSelectron digital afterloading. Results We used 3D printed devices in 3 cases (1x intraoral, 1x vaginal, 1x penis). Intraoral mould: the dental surgeon took 3 minutes to perform the intra-oral scan and the radiation oncologist 3 minutes to establish catheter number and placements. The 3D printer technician dedicated 30 minutes to programme the printer and printing took 15 minutes. The mould was ready for use within a total of 5 hours. Vaginal and penis mould: Preparation times are similar to the previous procedure. The 3D printed mould provided effective dose coverage in the PTV in all cases with V95%= 98%, 99,01% and 100% in vaginal, intraoral and penis respectively. The treatment was well-tolerated with no patient discomfort. Conclusion 3D printing is a promising technique for HDR-IRT. It was associated with more accurate dose distribution optimization with consequent better target coverage. Moreover it allows a less patient discomfort in all cases as it eliminated major steps in the mould-making procedure. Future studies are still needed to validate dosimetry advantages and economic aspects, as well as to characterize printing materials before 3D printing can be fully implemented in clinical practice.