ESTRO 38 Abstract book

S943 ESTRO 38

the developed backscatter correction matrix (Fig.1 (a)). This provide an easy-to-apply method to correct for backscatter from the linac arm that may be applied to all clinical images before dose reconstruction (Fig. 1 (b)). Dose profiles of some static fields were then being assessed for the effectiveness of AdABS application against the Matlab-based scripted system externally to DC. TPS profiles were compared with (i) DC profiles without backscatter correction, (ii) DC profiles corrected with the Matlab script and (iii) DC profiles corrected with AdABS.

C. Valdés 1,2 , F. Ballester 1 , J. Vijande 1 , J. Perez- Calatayud 1,3 1 Universitat de Valencia, Departamento de Física Atómica- Molecular y Nuclear, Valencia, Spain ; 2 Centro Oncológico de Antofagasta, Radioterapia, Antofagasta, Chile ; 3 La Fe Hospital, Radiofisica, Valencia, Spain Purpose or Objective There is a consensus on the use of the HVL (mm of Al) as an indicator of the beam quality for electronic brachytherapy equipments (eBT). Usually, the manufacturer supplies dosimetric data as percent depth dose (PDD) and HVL among others, being the responsibility of the final user to verify this information. HVL is obtained through a cumbersome procedure, which requires absorbent layers of high purity with very accurate thickness. The aim of this work is to investigate the relation between PDD and HVL for two eBT systems widely used in surface skin treatments, to evaluate the plausibility of verifying the PDD and the HVL supplied with a single measurement in water (or water-equivalent material). Material and Methods The simulations of the Intrabeam® (Carl Zeiss Meditec AG, Oberkochen, Germany) and the Esteya® (Elekta Brachytherapy, Veenendaal, The Netherlands) systems; which emit flattened bremsstrahlung beams of 50 kVp and 69.5 kVp, respectively; were performed with penEasy for PENELOPE2014. In both cases, surface applicators of 10 mm in diameter were used. To obtain beams with different HVLs, the original beams were filtered (filtered beams) with different amounts of Al, calculating the new HVL and the ratio of the absorbed doses at 5 cm and 2 cm depth (D 5,2 ), for both the original and the filtered beams.

Fig. 1 (a) and (b) Results

Profiles corrected for backscatter with the scripted method and with AdABS showed excellent agreement, with percentage differences of less than 1.5% relative to each other suggesting that AdABS is a feasible correction method that may be applied in the clinic for more accurate treatment verification (Fig. 2).

Conclusion This technique provided a feasible way of applying the correction within DC without the need to generate corrected images using external software (Matlab) scripts for every plan. Ultimately, this will benefit clinical advanced radiotherapy treatment plan verification by providing a ready solution that will help in implementing a quick and efficient workflow in Dosimetry Check. References: [1] Antonuk, L. E. 2002. Electronic portal imaging devices: a review and historical perspective of contemporary technologies and research. Phys Med Biol, 47 , R31-R65. [2] Mijnheer, B., et al. 2013b. 3D EPID-based in vivo dosimetry for IMRT and VMAT. Journal of Physics: Conference Series, 444 , 1-7. [3] Narayanasamy, G., Zalman, T., Ha, C. S., Papanikolaou, N., & Stathakis, S. 2015. Evaluation of Dosimetry Check software for IMRT patient-specific quality assurance. J Appl Clin Med Phys, 16 , 329-338. [4] Md Radzi, Y., Windle, R. S., Lewis, D. G., & Spezi, E. 2017. EP-1473: Improving the accuracy of dosimetry verification by non-uniform backscatter correction in the EPID. Radiotherapy and Oncology, 123 , S787-S787. [5] Md Radzi, Yasmin 2018. Development of techniques for verification of advanced radiotherapy by portal dosimetry. PhD Thesis, Cardiff University. EP-1749 Relation between depth dose and HVL for electronic brachytherapy systems: a Monte Carlo study

Results HVLs of the original beams were 0.304 ± 0.004 mm and 1.72 ± 0.04 mm for the Intrabeam and the Esteya systems, respectively. The D 5,2 of the original beams were 0.108 ± 0.002 and 0.205 ± 0.005 for the Intrabeam and the Esteya systems, respectively. We found that a change of 5% in D 5,2 deviates the HVL by approximately 40% and 20% in the Intrabeam and Esteya systems, respectively.

Conclusion The present work has proven that small deviations detected in a PDD measurement can predict noticeable changes in the HVL for the eBT investigated. The finding