ESTRO 36 Abstract Book

S190

ESTRO 36 2017

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The ACE doses for the single seed in water agree with MC

simulations on average within 4.4 ± 2.1% in a 60x60x60

cube centered on the seed, with the largest

differences near the end-welds of the seed. Percent

differences between ACE and MC doses along the plaque

central axes (CAX) for all eye plaque plans are shown in

Figure 1. The agreement improves beyond ~3 mm from the

outer scleral surface, and is generally better for the fully

loaded plaques than the single seed plaques, due to more

overlapping dose from each seed washing out ray effects

caused by the ACE calculation. Compared to using the

previous minimum calculation grid size of 1 mm

, the

smaller 0.5 mm

grid size results in less voxel averaging,

and therefore more accurate doses immediately adjacent

to the plaques, though both agree well with MC in the eye

region (Figure 2).

Conclusion

Overall, good agreement is found between ACE and MC

dose calculations in front of the eye plaques in water. The

consistent difference of ~3-4% observed for all

comparisons with MC simulations is potentially due to

differences in the MC simulation codes used to generate

the data, and scaling of the ACE dose distribution in water

to match TG-43 data in OcB. Updated seed models will be

used to investigate this discrepancy. The good level of

agreement indicates that further investigation of ACE in

applications involving a virtual, voxelized eye phantom,

and patient CT datasets, is warranted.

OC-0359 Microdosimetric evaluation of intermediate-

energy brachytherapy sources using Geant4-DNA

G. Famulari

, P. Pater

, S.A. Enger

1,2

McGill University, Medical Physics Unit, Montreal,

Canada

McGill University Health Centre, Department of

Radiation Oncology, Montreal, Canada

Purpose or Objective

Recent interest in alternative radionuclides for use in high

dose rate brachytherapy (Se-75, Yb-169, Gd-153) with

average energies lower than Ir-192 has triggered the

investigation of the microdosimetric properties of these

radionuclides. A combination of Monte Carlo Track

Structure (MCTS) simulations and track sampling

algorithms was used to predict the clinical relative

biological effectiveness (RBE) for fractionated

radiotherapy at relevant doses and dose rates. Previous

studies have concluded that the dose mean lineal energy

in nanometre-sized volumes is approximately proportional

to the α-ratio derived from the linear-quadratic (LQ)

relation in fractionated radiotherapy in both low-LET and

high-LET radiation.

Material and Methods

Photon sources were modelled as point sources located in

the centre of a spherical water phantom with a radius of

40 cm using the Geant4 toolkit. The kinetic energy of all

primary, scattered and fluorescence photons interacting

in a scoring volume were tallied at various depths from the

point source. Electron tracks were generated by sampling

the photon interaction spectrum, and tracking all the

interactions following the initial Compton or photoelectric

interaction using the event-by-event capabilities of

Geant4-DNA. The lineal energy spectra were obtained

through random sampling of interaction points and

overlaying scoring volumes within the associated volume

of the tracks.

Results

For low-LET radiation, the dose mean lineal energy ratio

was approximately equal to the α-ratio in the LQ relation

for a volume of about 30 nm (Fig 1). The weighting factors

(often denoted clinical RBE) predicted were 1.05, 1.10,

1.14, 1.19 and 1.18 for Ir-192, Se-75, Yb-169, Gd-153, and

I-125, respectively (Fig 2). The radionuclides Se-75, Yb-

169, and Gd-153 are 5-14 % more biologically effective

than current Ir-192 sources. There is little variation in the

radiation quality with depth from the source.