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Details Count! Micrometric Airgaps Between Phantom Slabs Influence Film Dosimetry When Micro-Fractionated X-Ray Beams Are Used

P Pellicioli1*, M Donzelli1, R Hugtenburg2, F Esteve3, E Brauer-Krisch1, M Krisch1, (1) European Synchrotron Rad. Fac.,Grenoble, FR (2) Swansea University, Swansea, UK (3) UA7 Strobe, Grenoble, FR

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
The spatial fractionation of synchrotron X-rays into arrays of micro-beams shows an extraordinary normal tissue sparing, thus opening new opportunities in radiation therapy.
Film dosimetry at the micrometric scale still needs further investigation because calculated and measured doses are not in agreement.
In this work, the impact of micrometric air gaps is studied as possible source of error when film dosimetry is performed in a plastic slab phantom.

Methods:
Monte Carlo radiation transport simulations were implemented to evaluate the dose distribution when an air gap is created with variable thickness (from 0 to 30 µm) next to the film position inside the phantom.
The orthovoltage X-ray radiation field consists of an array of 50 µm wide micro-beams spaced by 400 µm pitch.
The surface roughness of the utilized HD-V2 Gafchromic® films and phantom plastic surfaces was measured as indication for simulation comparison.

Results:
The presence of 30 µm large air gaps decreases the dose delivered in the beam peak areas by up to 3% and increases the valley dose (between beams) by up to 15-20%.
The measured maximum roughness of the film active layer was 4 µm while for the slab surface it was 10 µm.
With such an air gap, simulations estimate 2.3% peak dose reduction and 7.0% valley dose increase at 2 cm depth in the phantom.
Machining the phantom’s slab, a smoother surface improved film dosimetry accuracy.
We conclude that secondary electrons, produced inside the phantom in proximity of the air gap, travel along the gap and reach the center of valley regions before being absorbed.

Conclusion:
When dosimetry on the micrometric scale is performed with plastic slab phantoms and radiochromic films, it is mandatory to pay attention to the roughness level of the surfaces and to any setup imperfections that create air gaps.

Download ePoster [PDF]

Keywords

Radiochromic Film, Dosimetry Protocols, Low Energy X-rays

Taxonomy

TH- External Beam- Photons: Small field experimental dosimetry

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