ESTRO 36 Abstract Book

S801 ESTRO 36 _______________________________________________________________________________________________

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 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%).

use of similar components as in the other beam lines as well as a static beam monitoring system. Simulations were performed for 5 representative energies (62, 96.5, 157.4, 205, and 252.3 MeV) also considering the use of range shifter and ripple filter. Nozzle designs were based on the MC model of the MedAustron fixed beam line nozzle, which was verified with measurements at isocenter for 20 representative energies. Results The influence of the gantry nozzle filling with vacuum or helium (as used in some commercial systems) was found to have only a minor impact on spot size (< 2%). Compacting all nozzle elements towards the nozzle exit and reducing the nozzle dimension in beam direction by 25% lead to a reduction of the spot size of up to 20%, depending on the initial energy as depicted in Fig. 1. Using higher energies in combination with range shifter also decreased the delivered spot size for shallow seated tumors, as already demonstrated in other studies (Parodi, et al. PMB 57(12), 2012).

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

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.

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30X30X30 cm 3 Plastic Water (PW) phantom (CIRS)

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Low-density polyethylene plastic sleeve

TLD-2000F (Conqueror)