ESTRO 36 Abstract Book

S802 ESTRO 36 _______________________________________________________________________________________________

Matlab code was used to calculate EPID arm backscattering and output factor corrections. Gamma index comparison (3 %, 3 mm) was made for the EPID and MatriXX dose planes versus the calculated dose planes with OmniPro ImRT (IBA Dosimetry).

Figure 1. Acquired CT images of the Varian aS1000 EPID. Results For plans verified with EPID, Gamma index pass rate were 98.6% and 96.5% for prostate (Table 1) and head and neck case, respectively. Dose differences (EPID vs planned) were -0.7% and -0.4%. For MatriXX measurements, the results are very similar: gamma pass rate of 97.2% for prostate and 97.9% for head and neck, and dose differences (MatriXX vs planned) of - 1.4% and -0.8%, respectively.

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 Communications Department, Alcala de Henares-Madrid, Spain Purpose or Objective In this study, we present a new method for portal dosimetry. CT images of the Electronical Portal Imaging Device (EPID) were used as phantom images for dose calculation. The clinical beam model and beam energy, in the treatment planning system, were used to calculate dose over the EPID. Material and Methods The method was developed for a Varian Clinac 21-EX (Varian Medical Systems, USA), with a nominal photon energy of 6 MV, equipped with a Varian aS1000 EPID. Pinnacle 8.0m (Philips Medical Systems, NL) was used for treatment planning calculations. Matlab® v2012a (Mathworks, USA) was employed to develop code for calculations involving backscatter and output correction factors. The EPID was calibrated, following the manufacturer procedure, and then unmounted from the linear accelerator and scanned to acquire CT images of the EPID (Fig. 1) on an Aquilion LB (Toshiba Medical Systems, Japan). These CT images were imported into the Pinnacle planning system. The imported images were used as a quality assurance phantom to calculate dose on the image plane, which was considered as the predicted portal dose. Two sliding-window IMRT treatment plans, a prostate and a head and neck case, were delivered, measured and analyzed with both with the EPID and with MatriXX (IBA Dosimetry, Germany), as an independent measurement method.

Gamma (3 %, 3 mm)EPID

Dose diff EPID (%)

Gamma (3 %, 3 mm)MatriXX

Dose diffMatriXX (%)

Field

1 2 3 4 5 6 7

98.5% 98.6% 98.9% 98.6% 99.0% 98.7% 98.0%

-2.0 -1.0 -0.5 -0.6 -0.3 -0.1 -0.6 -0.7

96.5% 98.1% 96.0% 96.5% 99.0% 97.3% 97.2%

-1.6 -1.1 -1.5 -1.1 -1.5 -1.2

Average 98.6% -1.4 Table 1. Gamma index and dose difference results for prostate treatment. Conclusion The obtained results show the validity of the method presented here. This method can be easily implemented into clinic, as no additional modeling of the clinical beam is necessary. The main advantage of this method is that portal dose prediction is calculated with the same algorithm and energy beam model used for patient treatment planning dose distribution calculations. EP-1497 Dosimetric effect of the Elekta Fraxion cranial immobilization system and dose calculation accuracy Purpose or Objective Devices external to the patient may cause an increase in the skin dose, as well as modify the dose distribution and hence the tumor dose. This study describes the effect on this parameters caused by the Elekta Fraxion cranial immobilization system. The effect of the inclusion of Fraxion in ElektaMonaco treatment planning system (v. 5.00.00) was also checked. Material and Methods To study the dose attenuation a cylindrical phantom was placed over the Elekta Fraxion with a CC13 Scanditronix- Wellhofer ionization chamber located in the central insert at the linac isocenter. Dose measurements were performed for two open fields, 10x10 cm and other smaller 5x5 cm, as Fraxion is used mainly for radiosurgery treatments. The gantry angles were the ones which cross Fraxion (135º - 225º, 5º-10º increment, IEC gantry angles). C. Ferrer 1 , C. Huertas 1 , R. Plaza 1 , A. Serrada 1 1 Hospital universitaria La Paz, Radiofísica y Radioprotección, Madrid, Spain