ESTRO 2021 Abstract Book

S1558

ESTRO 2021

schemes.

PO-1827 Validation of a direct method to convert Acuros XB dose-to-medium to dose-to-water in lung SBRT C. Muñoz-Montplet 1 , R. Fuentes-Raspall 2 , D. Jurado-Bruggeman 3 , S. Agramunt-Chaler 3 , A. Onsès 3 , J.C. Vilanova 4 1 Institut Català d'Oncologia, Medical Physics and Radiation Protection, Girona, Spain; 2 Institut Català d’Oncologia, Radiation Oncology, Girona, Spain; 3 Institut Català d’Oncologia, Medical Physics and Radiation Protection, Girona, Spain; 4 Clínica Girona, Radiology, Girona, Spain Purpose or Objective Acuros XB (AXB) enables computation of dose-to-medium (Dm) and dose-to-water (Dw). It is unclear which reporting mode is optimal for clinical practice. The aim of this study is to validate a direct method to convert Dm DVH parameters to Dw in lung SBRT. Materials and Methods 54 patients treated with SBRT in our hospital were enrolled. A total dose of 60 Gy was delivered in 5 or 8 fractions, depending on tumor location. Plans were created for a 6 MV photon beam using 7 to 13 non-coplanar fields in the Eclipse TPS v13. Dose calculations were performed with AXB Dm and Dw for same number of monitor units and identical beam and MLC setup. We obtained the following DVH parameters: D mean , D 98 , D 95 , D 50 , and D 2 to the PTV; D mean to the lungs; D 2 and D mean to the heart; D 2 to the spinal cord, esophagus, trachea and proximal bronchi, ribs, and great vessels. We adopted the proposal of Jurado-Bruggeman et al 1 , which uses solely dose distributions calculated within the algorithm for the considered beam to derive a set of correction factors ( CF ) to switch from Dm to Dw using the composition of the involved structures:

Where f i is the fraction corresponding to material i in the composition of volume vol , and mAXB are the materials included in the AXB CT-to-material conversion table. We used Python to determine the PTV and OAR compositions, taking patients’ CT images and RT structure DICOM files exported from the TPS as inputs and mimicking AXB media characterization for each CT voxel. Dw DVH parameters were predicted from Dm using this method and compared to Eclipse calculations. The method assumes a uniform dose in the volumes involved in DVH parameters and that these volumes are representative of the whole organ in composition. Results

DVH parameter (Gy)

Eclipse Dm value Eclipse Dw value Predicted Dw value

Difference (%)

PTV D mean

66.4

66.4

66.5

0.1

D 98

58.3

58.4

58.4

0.0

D 95

60.2

60.2

60.2

0.1

D 50

66.9

66.9

66.9

0.1

D 2

71.8

71.9

71.8

-0.1

Ipsilateral lung D mean

9.4

9.4

9.4

0.0

Contralateral lung D mean

1.7

1.7

1.7

0.0

Heart D 2

7.4

7.4

7.4

0.3

D mean

1.9

1.9

1.9

0.2

Spinal cord D 2 Esophagus D 2

11.2

11.3

11.3

-0.1

13.1

13.2

13.2

-0.1

Trachea and proximal bronchi D 2

13.7

13.6

13.5

-1.2

Ribs D 2

52.3

54.0

54.0

-0.1

Great vessels D 2

19.2

19.5

19.5

0.1