ESTRO 37 Abstract book

S988

ESTRO 37

The differences between the mean Dmax values for AAA and AXB were significant for both AXB reporting modes. Although inter-patient variability exists between AAA and AXB Dw, CI show that these differences are not clinically relevant in any case. For AXB Dm, we must take into account that the Dmax we would have obtained with AAA would be approximately 1 Gy higher. No significant differences were observed between the variances of AAA and AXB Dm, indicating that for this particular case, changing from AAA to AXB can be interpreted as a renormalization. This difference of 2% is of the order of the inaccuracy of the calculation algorithms, so its clinical impact is probably very limited. No complications were observed in this study, with a minimum follow-up of 1 year.

Data obtained from such simulations are compared to experimental ones. For the latters, shielding disks between two radiochromic films are inserted at build-up depth in a water-equivalent solid phantom irradiated with a Mobetron accelerator. 6 MeV nominal beam energy and the reference field are used for both MC and experimental disks study. Results The MC code is validated for the nominal energy of 6 MeV and 9 MeV. In Figure 1 simulated PDD and BF are shown for 6 MeV nominal energy.

Conclusion When switching from AAA to AXB Dw or AXB Dm in H&N VMAT treatments, combination of mean differences and their inter-patient variability in the SC Dmax suggest a very limited clinical impact, so we can conclude that if the current restrictions are maintained an increased probability of complications is not likely to occur. EP-1831 IORT: Monte Carlo simulation of Mobetron accelerator and experimental comparison of shielding disk L. Toscano 1 , M. Severgnini 2 , F. Longo 1,3 , D.D. Mario 2 , A. Beorchia 2 1 University of Trieste, Physics, Trieste, Italy 2 Azienda Sanitaria Universitaria Integrata, medical Physics, Trieste, Italy 3 INFN, Physics, Trieste, Italy Purpose or Objective In Intra Operative Radiation Therapy (IORT) high doses are involved without a pre-treatment planning. This leads to the necessity of a deep knowledge of beam characteristic to carry out dosimetric evaluations. Such experimental measurements can be difficult so huge uncertainties are associated to them. The aim of this work is to use Monte Carlo (MC) code in order to simulate the Mobetron accelerator and so obtain dosimetric evaluations to be compared with experimental data. Material and Methods MC simulations are computed using Geant4. Measures and materials of simulated accelerator are in according to the real one thanks to a collaboration with the manufacturer. In order to validate MC code, simulated Percentage Depth Dose (PDD) and Beam Profile at the build up (BF) have to be in agreement with experimental ones with a gamma analysis 2%/2mm. In order to achieve this, primary electrons source and secondary scattering foil are tuned. Once validated, the MC code is used to study shielding disk solutions for breast treatment. Four solutions were considered involving different setup and materials (A: 5mm PMMA+3mm Cu+2mm PMMA, B: 2mm PMMA+3mm Cu+5mm PMMA, C: 4mm Tecapeek+ 2mm Cu+4mm Tecapeek, D: 4mm Tecapeek+1mm Pb+4mm Tecapeek).

In Table 1 are resumed results regarding the four different shielding disks, named A, B, C, D as above. Percentages are referred to the dose at build-up without the shielding disk. Dis k typ e % dose beforeMC % dose beforeexperim ental % dose afterMC % dose afterexperime ntal

0,454±0, 002 0,519±0, 002 0,723±0, 003 1,710±0, 004

A 101,34±0 ,05

101,79±4,16

4,00±2,23

B 111,19±0 ,05

116,18±4,57

5,36±3,04

C 103,14±0 ,06

102,29±4,78

2,48±1,78

D 109,68±0 ,06 3,24±1,80 Considering the dose before the disk, both MC and experimental data show a dose increment due to the backscattering electrons. A and B solutions consider a correct (A) and a wrong (B) positioning of an asymmetric disc: in the B solution less PMMA thickness before the metal leads to a minor backscattering electrons absorption and so to a greater percentage dose (see Figure 2). 116,01±4,53