ESTRO 36 Abstract Book

S776 ESTRO 36 2017 _______________________________________________________________________________________________

Garonne, Toulouse, France 2 SIMAD, Haute Garonne, Toulouse, France

Millennium 120. For this last machine, a RapidArc plan was also calculated. A HT treatment for Tomotherapy Hi-Art was also planned for every patient. Results ID and NTID are 27% and 33%, respectively, larger for HT compared to 6MV-IMRT Eclipse treatments. Statistically no difference has been found for ID and NTID values between RapidArc and IMRT treatments. For IMRT treatments, no influence has been observed on the size of MLC, the delivery technique (step-and-shoot or sliding-window) and the number of fields. However, an ID and NTID increments of 8% and 10%, respectively, are reported when moving a plan from Eclipse to XiO. (Table 1). The mean DVHs in Fig 1 show some differences depending on the isodose evaluated. Higher values calculated below 20 Gy are compensated by the region from 20 Gy to 30 Gy, where this technique minimizes the volume encompassed by these isodose curves. For HT, there is no compensation, as the volumes below 20 Gy are much higher than for the other techniques. From 20 Gy to 30 Gy, the values are comparable to IMRT, showing no advantage in terms of ID. NORMAL TISSUE INTEGRAL DOSE (NTID) (·10 7 cGy·g)

Purpose or Objective The INTRABEAM® system is a miniature accelerator producing low energy photons (50 keV maximum). The published dosimetric characterization of the INTRABEAM system for flat and surface applicators was based on detectors (radiochromic films or ionization chambers) not allowing measuring the absorbed dose in the first millimeters of the irradiated medium, where the dose is actually prescribed. This study aims at determining the sensitivity of a paramagnetic gel in order to measure the dose deposited with INTRABEAM surface applicators in the first millimeters of irradiated medium. Material and Methods The determination of paramagnetic gel sensitivity was performed with irradiations at different dose levels with the INTRABEAM® Carl Zeiss Surgical system (Oberkochen, Germany). The ferrous gel used in this study is a new « sensitis» material which is described by C. Stien et al and V. Dedieu et al. Gel irradiation in tin and capsule containers was carried out for twelve dose levels between 2 Gy and 50 Gy at the gel surface with a 4 cm surface applicators. The applicator was in contact of the gel during irradiation. For the calibration curve, one batch gel was measured without being irradiated. T 2 weighted multi echo MRI acquisitions were performed on a 1,5 T Magnetom Aera MR scanner of Siemens with surface flex head coil technology. Results The T 2 signal versus echo times can be fitted with a mono- exponential function with 95% of confidence. The first echo time was not considered for the fit. The calibration curve determined from experiments with tins is a linear function (R 2 =0.967) with a sensitivity of 1.04*10 -4 s -1 .Gy -1 . Gels Sensitivity with capsules are of 3.67*10 -4 s -1 .Gy -1 (R 2 =0.979) and 2.54*10 -4 s -1 .Gy -1 (R 2 =0.944). The calibration curve was applied to the irradiation of a surface applicator to obtain the 3D dose distribution in the gel. Conclusion The dose distribution obtained after irradiation at low energies with an INTRABEAM® miniature accelerator can be measured for the first millimeters thanks to ferrous gels. The determination of gel sensitivity was possible with MRI measurements. Results are relevant but must be confirmed with more irradiations with different dose levels at the surface and different surface and flat applicator diameters. EP-1471 Comparison of the integral dose of IMRT, RapidArc and helical tomotherapy prostate treatments J. Martinez Ortega 1 , P. Castro Tejero 2 , M. Pinto Monedero 1 , M. Roch Gonzalez 2 , L. Perez Gonzalez 2 1 Hospital Universitario Puerta de Hierro, Servicio de Radiofísica y PR, Majadahonda - Madrid, Spain 2 Hospital Universitario de la Princesa, Servicio de Radioterapia, Madrid, Spain Purpose or Objective Comparison of integral dose (ID) and normal tissue integral dose (NTID) for Helical Tomotherapy (HT), RapidArc and static fields IMRT. Material and Methods A cohort of ten prostate patients were selected whose prescription was 78 Gy mean dose to the Planning Target Volume (PTV). Seven different plans for every patient were computed. One sliding-window IMRT with XiO planning system and Varian Clinac 21EX, equipped with MLC Millennium 80. Four Intensity-Modulated Radiation Therapy (IMRT) plans were calculated with Varian Eclipse planning system, two step-and-shoot and sliding-window IMRT for a Varian Clinac 2100 C/D with Millennium 80 and two analogous plans for a Varian Clinac 2300iX with a

PTV Volu me (cm3 ) 181.2 0 140.9 4 228.9 6 180.4 2 234.2 2 204.1 4 175.3 8 276.0 4 209.7 8 256.6 0 208. 77

IMR T XIO SW 80

IMRT ECLI PSE SW12 0

IMRT ECLI PSE SS12 0

IMRT ECLI PSE SW80

IMRT ECLI PSE SS80

Patie nt

RAPIDA RC

HT

1.2 9 1.26 1.23 1.25 1.23 1.23 0.9 8 0.92 0.90 0.91 0.90 0.91 1.4 4 1.39 1.36 1.39 1.35 1.35 1.3 1 1.28 1.26 1.28 1.25 1.24 1.3 6 1.33 1.29 1.33 1.28 1.29

1.6 4 1.2 7 1.7 3 1.6 3 1.7 2 1.8 5 1.7 2 2.1 1 1.5 4 2.2 7

1

2

3

4

5

1.6 3 1.5 2 1.8 3 1.4 0

6

1.43 1.39 1.43 1.39 1.40

7

1.31 1.27 1.31 1.27 1.27

8

1.63 1.60 1.62 1.59 1.65

9

1.23 1.22 1.23 1.21 1.21

1.9 4 1.76 1.73 1.75 1.72 1.75

10

Aver age

1.4 7

1. 75

1.35 1.33 1.35 1.32 1.33

41.06 0.2 8

0.2 8

SD

0.23 0.23 0.23 0.22 0.24

Typi cal error (k=2)

0.1 8

0. 18

0.14 0.14 0.14 0.14 0.15

Table 1. NTID calculated from the dose volume histograms, for every treatment plan, IMRT, RAPIDARC or HT. For IMRT treatments, both delivering technique (SW for sliding-window and SS for step-and-shoot) and MLC