ESTRO 36 Abstract Book

S941 ESTRO 36 2017 _______________________________________________________________________________________________

An optimization bolus was added from the body and 12 mm expansion, defined as mass density 1.0 g/cm 3 . The prescribed dose was 12 Gy delivered in 6 fractions. The dose planning was aimed to keep D 95% > 95% to PTV and minimizing dose to organs at risk, which was defined as the rest of the body. An optimization structure was used to create a tangential irradiation of the skin, minimizing the dose to normal tissue. We tested the bolus effect of Neoprene with Gafchromic EBT3 film by irradiating slabs of 7mm, dry and soaked in water. To verify skin doses, the phantom with wet suit was irradiated with several 2x2 cm 2 slabs of film taped to the body. The film were evaluated at least 24 hours after irradiation. Corresponding detector array measurements (Delta4, Scandidos) were done and evaluated with gamma analysis. Further, a robustness test was done by moving the phantom 10 mm in the x, y and z directions, to evaluate the effect of mispositioning. Results Results of planning and robustness tests are presented in table 1. Measured data fit to depth dose data yields a dose maximum at 28 mm for Neoprene. Hence, 7 mm is equivalent to 3 mm thick water bolus and lightly soaked Neoprene adds another 1.2 mm thickness of water. Delta4 gamma analysis with 2 mm and 3%, global dose, is clinical acceptable with regards to deliverability (M = 93%, SD = 3%). The verification of 27 film slabs for skin dose gave an average difference from TPS dose of 4% (SD = 3%), figure 1.

toxicity.

EP-1736 Radiation and lasers isocenters coincidence with ArcCheck phantom F. Tato de las Cuevas 1 , J. Yuste Lopez 1 1 Hosp. Univ. de Canarias, Medical Physics Dept., Santa Cruz de Tenerife, Spain Purpose or Objective One tool of Machine QA module of ArcCheck phantom ( AC ) software checks Radiation and Lasers Isocenters Coincidence ( RLIC ). The purpose of this work is to evaluate the precision and accuracy of this software tool, comparing it to the same test made with EPID (Electronic Portal Imaging Device). Material and Methods The LINAC is an Elekta Synergy with Agility MLC and 6 MV energy. The RLIC with ArcCheck phantom ( AC ) are obtained following the instructions of the software manual. The measurements are done in continuous gantry movement and for discrete gantry angles. Measurements are made at 9 º collimator angle for a 1x25 cm field. A series of measurements were made also in 99º to see the MLC effect, as Agility head has not backup jaws. The AC displacements from laser isocenter in two directions are made in order to check software sensitivity. RLIC are made with EPID, positioning a Bearing Ball ( BB ) in the lasers isocenter of a 5x5 cm field and acquiring Images from 0º to 360º gantry angles in 45 º steps. The radiation center of the squared field and the center of the BB are calculated with a MATLAB in-house software. BB center is calculated with sub-pixel accuracy in each direction, 3 profiles are obtained and fitted to Gaussian curves, and the mean maximum of the 3 curves is calculated. Radiation field center is obtained calculating the 50% pixel value of a vertical and horizontal profile. The difference between BB center and radiation field center are computed for each gantry angle for in-plane and cross-plane directions. The RLIC for EPID measurements are computed using these values. Results The RLIC results obtained with AC for each gantry are compared with EPID in the first figure. The mean distance over all gantry angles, for AC (for 9 and 99 º collimator degrees) and EPID are: 0.3, 0.6 and, 0.7 mm, respectively. The AC results are just distance (because this phantom is not capable of give deviation in in-plane direction for each gantry angle). The results for AC for 9º are higher than for 99º because of the irregular MLC radiation field limit exposed for 9º to the AC diodes. The RLIC for EPID are given in in-plane and cross-plane directions, the distance for each gantry angle is calculated from both directions and show a bigger mean value than for AC, because of being calculated in just one direction in this phantom.

Conclusion The difference of measured dose compared to TPS, for both film and Delta4 dosimetry is larger than most types of targets treated with HT which is to be expected considering the technique and size of target. The deliverability is within limit of our clinic action levels (gamma pass rate < 90%) and neoprene is feasible as bolus. The benefits, in comparison with reported electron treatments, are target homogeneity and target coverage with good immobilization and complete irradiation with two positions. The higher dose to organs at risk than reported with electrons needs to be addressed if acceptable with regards to