ESTRO 37 Abstract book

S1242

ESTRO 37

Results The dose measured by MOSFET/PRODOSE showed a minimum, average and maximum deviation of, respectively, 1.45/0.49, 16.81/18.43 and 46.84/55.27%, regarding the planned dose. Comparing the same measurement point, PRODOSE presented smaller deviations than MOSFET in 5 out of 9 patients. Partial and total dwell times measured by PRODOSE in each catheter showed a maximum deviation of less than 8.5% (with an average deviation of 3.0 and 0.86%, respectively), whereas the maximum deviation regarding the total treatment times for each patient decreased to values less than 1%. Conclusion Our initial measurements demonstrated the interest in implementing an in vivo procedure in our department. The deviations from the prescribed dose obtained with both dosimetric systems was due to a possible variation of the dosimeter position in relation to the planned dose point (even a 1mm offset can lead to a large data deviation, since the measurements were made in high dose gradient regions), and due to the fact that our TPS does not include heterogeneities in the dose calculation (particularly important in tissue-air barrier). Moreover, MOSFET also have a high angular dependence, and the presented MOSFET data was an average of the acquired data of 3 treatment fractions. PRODOSE dosimeter presented a smaller deviation than MOSFET from the dose prescribed in 5 out of 9 patients, and allowed the reading of the source dwell times, information not provided by MOSFET. The higher deviations from the partial and total dwell times in each individual catheter were expected, considering that the dosimeter temporal resolution is 50ms. However, this deviations dramatically decreased for values less than 1% for the total treatment time of each patient (well above the dosimeter temporal resolution). These systems validation were done for BT APBI treatments, but in the future they will be used in other BT treatments with different pathologies, and at measurement points inside the patient. EP-2248 Absorbed dose distribution on an anterior eye tumor using measured heterogeneous Ru-106 eye plaques. F.J. Zaragoza Serrano 1 , W. Sauerwein 1 , L. Brualla 1 1 Universitätsklinikum Essen, NCTeam- Strahlenklinik, Essen, Germany Purpose or Objective The emitter substance in Ru-106 eye plaque is assumed heterogeneously distributed. Monte Carlo simulations are used to determine the effect of these heterogeneities over modeled tumors using an adapted version of the PENELOPE code for radiation transport. Material and Methods The actual heterogeneous distributions of the specific CCA1364 and CCB1256 plaques are considered. For comparison purposes the homogeneous distribution of the CCA plaque is considered too. A theoretical tumor of 10.0mm of basal diameter and 3.0mm of apical height is modeled. The geometry of the plaques and tumors were embedded in a computerized tomography scan of a real patient where three materials are used, namely, air water and bone. The heterogeneous CCA1364 and the homogenous CCA eye plaques were simulated in an anterior position placed centrically with respect to the tumor, that means the symmetry axis of the tumor and the eye plaque are coincident. The CCA1364 eye plaque was also simulated eccentrically with respect to the tumor, that is the edge of the plaque was coincident with the basal line of the tumor. In this eccentric positioning, the commonly used safety margin of 2.0mm used in treatment planning was not respected. The CCB1256 eye plaque was simulated only in an equatorial placement, that is eccentrically with respect to the anterior position

graphical user interface (GUI) that enables the user to explore improving any structure at the expense of one or more other structures. The GUI displays pair-wise dose volume histogram (DVH) tradeoffs among all structures and has a selection tool for controlling the magnitude of improvement for the selected structure. The GUI also provides pairwise metric tradeoff ratios (e.g. slopes) local to the current plan. Results Proof-of-principle was demonstrated using a clinical case for the prostate treatment site and five DVH metrics, PTVD95, PTVD10, UrethraD10, BladderD1cc, and RectumD1cc. 625 candidate plans that were distributed over the trade-off surface were generated. The candidate plans were generated by varying the optimization constraints for four of the metrics (all of the metrics except for PTVD95, which was the optimization objective) by ± 10% variation in 5% increments. Plans chosen using the GUI were deliverable, i.e. it was composed of feasible dwell times that produce the displayed plan doses. The loss of quality from interpolation was less than 1%. The user may elect to perform a final optimization after navigation if coarser plan spacing in the library generation or less quality loss from interpolation is desired. The method is generalizable to any number of criteria and any treatment site. Conclusion Proof-of-principle was demonstrated for multi-criteria optimization in HDR brachytherapy. This HDR brachytherapy treatment planning approach efficiently generates a trade-off surface consisting of high-quality plans that span a wide range of DVH values for each structure of interest. Represented as an intuitive GUI, this tool could improve both treatment planning time and quality for brachytherapy. EP-2247 In vivo dosimetry in APBI brachytherapy S. Pinto 1,2 , A. Pereira 1,2 , J.A.M. Santos 1,2 , L.M. Moutinho 3,4 , I.F. Castro 3,4 , H. Freitas 4 , J. Melo 4 , M. Torres 3 , M. Costa 5 , J.F.C.A. Veloso 3,4 1 Portuguese Oncology Institute of Porto IPO Porto, Medical Physics, Porto, Portugal 2 Medical Physics- Radiobiology and Radiation Protection Group, IPO Porto Research Center CI-IPOP, Porto, Portugal 3 I3N, Physics Department- University of Aveiro, Aveiro, Portugal 4 NU-RISE, IEUA, Aveiro, Portugal 5 APNOR - Associação de Politénicos do Norte, Escola Superior de Saúde, Porto, Portugal Purpose or Objective In vivo dosimetry should be a reality in BT treatments. In order to make it a standard procedure in our department, we performed in vivo dosimetry in APBI patients, using In vivo dosimetry was performed in 9 patients (4Gy/8 fractions). The dosimetric systems used were based on a well known MOSFET dosimeter, and a fiber optic dosimeter (PRODOSE, still under evaluation), both placed on the breast surface. The mosfets were placed in 3 different positions, while the fiber optic was placed in 1 position (common to the mosfet). The mosfets data presented are an average of the data acquired in 3 treatment fractions, whereas the fiber optic data was acquired in a single treatment fraction. The two dosimetric systems recorded the skin dose, and the PRODOSE system also recorded the total treatment time, and the total and partial dwell times of the source in each catheter. The results obtained were compared with TPS parameters (Oncentra ® Brachy - Elekta). two dosimetry systems. Material and Methods