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. 2010 Dec 7;55(23):7097-106.
doi: 10.1088/0031-9155/55/23/S10. Epub 2010 Nov 12.

Adjustment of the lateral and longitudinal size of scanned proton beam spots using a pre-absorber to optimize penumbrae and delivery efficiency

Uwe Titt  1 , Dragan Mirkovic, Gabriel O Sawakuchi, Luis A Perles, Wayne D Newhauser, Phillip J Taddei, Radhe Mohan
Affiliations

Adjustment of the lateral and longitudinal size of scanned proton beam spots using a pre-absorber to optimize penumbrae and delivery efficiency

Uwe Titt et al. Phys Med Biol. .

Abstract

In scanned-beam proton therapy, the beam spot properties, such as the lateral and longitudinal size and the minimum achievable range, are influenced by beam optics, scattering media and drift spaces in the treatment unit. Currently available spot scanning systems offer few options for adjusting these properties. We investigated a method for adjusting the lateral and longitudinal spot size that utilizes downstream plastic pre-absorbers located near a water phantom. The spot size adjustment was characterized using Monte Carlo simulations of a modified commercial scanned-beam treatment head. Our results revealed that the pre-absorbers can be used to reduce the lateral full width at half maximum (FWHM) of dose spots in water by up to 14 mm, and to increase the longitudinal extent from about 1 mm to 5 mm at residual ranges of 4 cm and less. A large factor in manipulating the lateral spot sizes is the drift space between the pre-absorber and the water phantom. Increasing the drift space from 0 cm to 15 cm leads to an increase in the lateral FWHM from 2.15 cm to 2.87 cm, at a water-equivalent depth of 1 cm. These findings suggest that this spot adjustment method may improve the quality of spot-scanned proton treatments.

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Figures

Figure 1
Figure 1
Schematic diagram of the Monte Carlo model of the scanning beam treatment head and water phantom. Particles emerging from the source (SRC) located at the entrance of the treatment head travel along the z-axis through a beam profile monitor (BPM), followed by a helium chamber (He) which leads through bending magnets (x-Mag and y-Mag). The proton beam traverses a set of two dose monitor ionization chambers (M1 and M2), a spot position monitor chamber (SPM), a pre-absorber (PA), and finally enters the water phantom (H2O) where the beam stops.
Figure 2
Figure 2
Lateral spot sizes at dmax versus residual range in water without pre-absorbers (black) and with a fixed energy of 178.6 MeV at the treatment head entrance (corresponding to a range of 21 cm in water) and penetrating pre-absorbers of various thicknesses (red).
Figure 3
Figure 3
Longitudinal spot sizes between the proximal 80% and distal 80% dose location (black) as well as the proximal 90% and the distal 90% dose location (red) without a pre-absorber and as a function of the residual range in water. Longitudinal spot sizes with a pre-absorber in the beam line are also shown (blue: 80%–80% and green: 90%–90%). The data with the pre-absorber were simulated with a fixed proton beam energy corresponding to a range of 21 cm in water and with variable pre-absorber thicknesses.
Figure 4
Figure 4
Lateral spot sizes as a function of the residual range in water, without a pre-absorber (black) and with a 5 cm (magenta), a 10 cm (red), a 15 cm (green) and a 20 cm (cyan) thick pre-absorber, with a 5 cm air gap between the pre-absorber and the water phantom.
Figure 5
Figure 5
Longitudinal spot sizes without (black line) a pre-absorber and those with a pre-absorber of fixed thicknesses 5, 10, 15 and 20 cm as a function of the residual range. Only the 80%–80% longitudinal spot sizes are plotted. The 90%–90% longitudinal spot sizes (not shown) exhibited similar behavior but with smaller slopes.
Figure 6
Figure 6
Lateral spot size as a function of the residual range in water for three different air gap sizes, 5 cm (white squares), 10 cm (red squares) and 15 cm (green squares), between the downstream surface of the pre-absorber and the upstream surface of the water phantom. The pre-absorber had a water-equivalent thickness of 15 cm.
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