ESTRO 2021 Abstract Book

S1323

ESTRO 2021

Results In Fig. 2 the results are represented for two different phantom thicknesses. It was verified that the dispersion between the areas of the PSF calculated for the same phantom as a function of depth, were below of 5%. The PSF of the conjugate is found to be independent of depth for each material thickness.

Conclusion The quasi invariance with depth is confirmed. This makes it possible to use a single system response function for certain measurement conditions. The PSF* obtained allow to find the original dose distribution by deconvolution. PO-1601 Energy correction for TLD100 measurements up to 10 cm from the field edge for 3 different linacs B. Sanchez nieto 1 , L. Irazola Rosales 2 , F. Sánchez-Doblado 3 , J.L. Rodriguez-Mongua 4 , E. Doerner 5 , I. Espinoza 5 1 Pontificia Universidad Católica de Chile, Instituto de física, Santiago, Chile; 2 Clínica Universidad de Navarra, Servicio de Radiofisica y protección radiológica, Pamplona, Spain; 3 Universidad de Sevilla, Departamento de Fisiología Médica y Biofísica, Sevilla, Spain; 4 Pontificia Universidad Católica de Chile, Instituto de Física, santiago, Chile; 5 Pontificia Universidad Católica de Chile, Instituto de Física, Santiago, Chile Purpose or Objective It is well-known that if the TLD100 dosimeters are calibrated in a primary MV beam, the TLD100 signal per delivered dose outside of the treatment field will be greater than expected because of the dosimeter’s overresponse to the softer spectrum outside the field (TG-158). Therefore, out-of-field photon dose measurements with TLD-100 require the use of a correction to the calibration factor that depends on the energy spectra at each measurement position. The majority of second cancers appear within the first few centimeters from the field-edge, and there is also interest in the dose at points close to the field (e.g., pace markers during thoracic irradiation). Thus, this work concentrates on the first 10 cm from the field edge. We calculated energy correction factors using a previously published model [1] for the Elekta Axesse (Agility MLC), Varian 21EX (Millenium 120 MLC), and Siemens Primus (with Toshiba MLC), based on the mean energy, calculated by MC simulations, as a function of isocenter distance. Materials and Methods The BEAMnrc code, built on the EGSnrc system, was used to create all three linacs models. Energy spectra were computed at different positions outside 10x10 cm 2 square fields using 6 MV photon beams from which mean energies were calculated. The spectra were calculated at 5 cm depth in water for Elekta (SSD = 95 cm) and Varian (SSD = 100 cm) [2] and at 10 cm depth for Siemens (SSD = 100 cm) [3]. It is expected that depth and SSDs differences at which simulations were performed do not impact the mean energy dramatically [4]. Multiplicative energy correction to the calibration factor for TLD100 measured under standard conditions were calculated as 1/R, where R is given by the expression presented in [1] for the following model parameters: α 1 =10,92 keV -1 , E 1 =-4,32 keV, α 2 =3,5x10 6 keV 3 and E2=-66,7 keV Results Simulated mean energies at different distances from the field-edge are shown in figure 1A for the three cases (error bars are within the symbol size). The corresponding energy correction factors (R -1 ) are shown in figure 1B.