ESTRO 36 Abstract Book

S786 ESTRO 36 _______________________________________________________________________________________________

IMRT for a Varian Clinac 2100 C/D with Millennium 80 and two analogous plans for a Varian Clinac 2300iX with a 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)

Fig 1. Dose volume histogram for the whole body averaged over the 10 patients of this study, comparing every treatment technique. Conclusion The source for higher values of ID and NTID for HT is the larger volume receiving dose below 20 Gy. No differences were found in the election of IMRT delivery. For RapidArc plans, ID and NTID values are similar to IMRT. EP-1472 Dosimetric E2E verification using 3D printing and 3D dosimeter for brain stereotactic radiotherapy M.S. Kim 1 , K.H. Chang 1 , J. Kwak 1 , G.M. Back 1 , T.Y. Kang 1 , S.W. Kim 1 , Y. Ji 1 1 Asan Medical Center- Univ of Ulsan, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To evaluate the dosimetric accuracy of brain stereotactic radiotherapy (SRT) with a 3D dosimetry system and MRI, we investigated dosimetric end-to-end verification using 3D printing technology and 3D dosimeter. Material and Methods We implemented an anthropomorphic head and neck phantom with a 3D printed insert made using a 3D printer designed by the Autodesk software and two gel-filled spherical glass flasks as a patient having multiple target brain cancer. For the feasibility study of the gel dosimeter, the dose linearity, dose rate dependence, and reproducibility for the gel dosimeter were verified. Gel- filled vials were irradiated with 6 MV beams to acquire a calibration curve of dose relation to R2 (1/T2) values in 9.4T MR images. Graded doses from 0 to 8 Gy with an interval of 2 Gy were delivered. Two PTVs (PTV1,2) were contoured on the MR images of phantom have dosimetric gel tumor. To evaluate geometric and dosimetric accuracy, a treatment plan was created such that D95s for PTV1 and intentional PTV2 were more than the prescribed dose. The intentional PTV2 was produced by intentionally shifting by 5mm from the true target position. 2 arc VMAT plan was created to deliver 35 Gy in 5 fractions. After irradiation, calibration vials and phantom were scanned by 9.4T MRI and then acquired images were analyzed using an ImageJ and DCMTK software libraries. Scanned MRI images of phantom were imported to a treatment planning system and registered to CT images to compare dose distributions. We also compared the agreement result between the planned and the measured data in 1D (ion chamber), 2D (gafchromic film), and 3D (Gel dosimeter). Results The best dose linearity was 0.99 (R 2 ) at 180 TE (ms). Reproducibility and dose rate dependency were less than 2.2% and 3.5%, respectively for 180 TE. Point dose differences in plan vs. ion chamber were 1.08%, 0.47%, and -2.82%, respectively, for PTV 1, 2, and intentional PTV. And its differences between plan and gel were 0.98%, 1.66% and 3.76%, respectively, for PTV 1, 2, and shifted PTV. Gamma passing rates with 3%/3mm criteria were greater than 99% for all plans. Isodose distributions and

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 characteristics (80 or 120 leaves) are indicated.