ESTRO 36 Abstract Book

S791 ESTRO 36 2017 _______________________________________________________________________________________________

Figure 1 : Spot size over initial proton energy with (full symbols) and without (empty symbols) passive elements, for the non-optimized (Basic) and the most compacted nozzle. Conclusion The optimum in terms of spot size can be reached if all nozzle elements are as close as possible to the nozzle exit as a reduction in distance to the isocenter proved very effective. Therefore, MedAustron will focus on a compact nozzle design with retractable snout. EP-1495 Should we use correction factors for skin dose measurements with radiochromic films? P. Carrasco de Fez 1 , M.A. Duch 2 , L. Muñoz 2 , N. Jornet 1 , M. Lizondo 1 , C. Cases 1 , A. Latorre-Musoll 1 , T. Eudaldo 1 , A. Ruiz 1 , M. Ribas 1 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain 2 Universitat Politècnica de Catalunya, Institut de Tècniques Energètiques, Barcelona, Spain Purpose or Objective The election of detector for skin dose measurements is critical (see fig. 1a). This work is aimed to study the surface dose in high-energy x-ray beams and to derive potential correction factors (CFs) to be applied for in-vivo skin dose measurements when using EBT3. Material and Methods • 6 and 15 MV x-ray beams from a Clinac 2100 CD (Varian) • EBT3 radiochromic films + Film QA Pro 2014 software (Ashland) + EPSON EXPRESSION 10000XL scanner Methods TLDs and EBT3 films were attached to the centre of the PW phantom surface side facing the radiation beam. The main parameters affecting surface dose as reported in literature were studied (field size and angle of incidence) with both EBT3 and TLDs. The field size was changed between 3.5 and 25 cm, the angle of incidence between 0 and 90º, and the SSD between 75 and 100cm. The effect of a plastic sleeve to be used for in vivo measurements was assessed. Incidence angle and field size CFs for EBT3 films could be derived from comparison against measurements made with TLDs because TLDs are known as not having any dependence on the incidence angle or the field size. The equivalent depth correction factors (EDCF) for EBT3 films have been determined using measurements made with a PTW 23392 Extrapolation ion chamber in a previous work [1] and measurements made with EBT3 films in this work. EDCF allows determining dose @ the ICRU skin depth (70 µm) from EBT3 measurements (active depth@120µm). The effect of SSD was studied with EBT3. For film dosimetry, EBT3 films were cut into 3 cm 2 square pieces marked to keep track of their orientation for scanning. Readout of each film corresponded to the mean value within a 1×1cm 2 ROI centred in the film piece. Several pieces for each measurement were read 3 times with random position in the central part of the scanner to account for scanner and film non-uniformity in the uncertainty. Results Fig 1b shows surface dose increases linearly as a function of the field size measured with every detector. Fig 1c shows that surface dose increases slowly up to an angle of incidence of 30º and very fast from angles between 60 and 80º. There is no significant difference between measurements • 30X30X30 cm 3 Plastic Water (PW) phantom (CIRS) • • Low-density polyethylene plastic sleeve TLD-2000F (Conqueror)

made with EBT3 and with TLD. The effect of the plastic sleeve is negligible considering the uncertainty. SDD: Fig 1d shows deviation to the inverse square law of 0.004% for 6 MV and 0.13% for 15 MV, much lower than EBT3 overall uncertainty (≈3%).

EDCF were 0.709±0.044 for 6MV and 0.872±0.127 for 15 MV. Fig 2 shows the consistency of applying EDCF on data of Fig 1b: EBT3 agree with TLD within the uncertainty. All error bars are k=2.

Conclusion

No other CFs than the EDCF have to be applied for skin dose measurements with EBT3 films. This work has been partially financed by the grant Singulars Projects 2015 of the Spanish Association Against Cancer (AECC). [1]Detector comparison for dose measurements in the build-up zone. M.A Duch et al. 3rd ESTRO FORUM. 2015. EP-1496 A portal dosimetry dose prediction method based on CT images of Electronical Portal Imaging Device J. Martinez Ortega 1 , N. Gomez Gonzalez 1 , P. Castro Tejero 2 , M. Pinto Monedero 1 , N. Tolani 3 , L. Nuñez Martin 1 , R. Sanchez Montero 4 1 Hospital Universitario Puerta de Hierro, Radiofisica y PR, Majadahonda - Madrid, Spain 2 Hospital Universitario La Princesa, Radioterapia, Madrid, Spain 3 ME De Bakey VA Medical Center, Radiotherapy Department, Houston, USA 4 Universidad de Alcala, Signal Theory and