Comment on “Dosimetric evaluations of the interplay effect in respiratory‐gated intensity‐modulated radiation therapy” [Med. Phys. 36, 893–903 (2009)]

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134968/1/mp2483.pd

Laser acceleration of protons from near critical density targets for application to radiation therapy

Laser accelerated protons can be a complimentary source for treatment of oncological diseases to the existing hadron therapy facilities. We demonstrate how the protons, accelerated from near-critical density plasmas by laser pulses having relatively small power, reach energies which may be of interest for medical applications. When an intense laser pulse interacts with near-critical density plasma it makes a channel both in the electron and then in the ion density. The propagation of a laser pulse through such a self-generated channel is connected with the acceleration of electrons in the wake of a laser pulse and generation of strong moving electric and magnetic fields in the propagation channel. Upon exiting the plasma the magnetic field generates a quasi-static electric field that accelerates and collimates ions from a thin filament formed in the propagation channel. Two-dimensional Particle-in-Cell simulations show that a 100 TW laser pulse tightly focused on a near-critical density target is able to accelerate protons up to energy of 250 MeV. Scaling laws and optimal conditions for proton acceleration are established considering the energy depletion of the laser pulse.Comment: 25 pages, 8 figure

Magnetic confinement of electron and photon radiotherapy dose: A Monte Carlo simulation with a nonuniform longitudinal magnetic field

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135106/1/mp1091.pd

On‐line monitoring of radiotherapy beams: Experimental results with proton beams

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135090/1/mp8491.pd

Magnetic confinement of radiotherapy beam-dose profiles

We have used electron and photon beams from the 50 MV electron microtron at UM Hospital together with a large-bore 3.5T superconducting solenoid to demonstrate the magnetic confinement of HE electron and photon beam-dose profiles for typical radiotherapy beams. The HE electron beams in particular exhibit a large reduction in penumbra when entering a tissue-equivalent phantom and, in addition, confinement of the secondary electrons produced by the primary beam. Likewise photon beams show a similar confinement of the dose from secondary electrons. While the results resemble features predicted from Monte Carlo calculations, there are a number of anomalous details in the actual experimental data which serve to illustrate the problems associated with practical clinical implementations. However the data suggest that in certain cases HE electrons may provide a cost-effective alternate to proton or HI radiotherapy beams and, also, improve the dose profile for HE photon beams. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87615/2/44_1.pd

NOTE: An apparatus for applying strong longitudinal magnetic fields to clinical photon and electron beams

Monte Carlo studies have recently renewed interest in the use of the effect of strong transverse and longitudinal magnetic fields to manipulate the dose characteristics of clinical photon and electron beams. A 3.5 T superconducting solenoidal magnet was used to evaluate the effect of a longitudinal field on both photon and electron beams. This note describes the apparatus and demonstrates some of the effects on the beam trajectory and dose distributions for measurements in a homogeneous phantom. The effects were studied using film in air and in phantoms which fit in the magnet bore. The magnetic field focused and collimated the electron beams. The converging, non-uniform field confined the beam and caused it to converge with increasing depth in the phantom. Due to the field's collecting and focusing effect, the beam flux density increased, leading to increased dose deposition near the magnetic axis, especially near the surface of the phantom. This study illustrates some benefits and challenges associated with the use of non-uniform longitudinal magnetic fields in conjunction with clinical electron and photon beams.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48970/2/m105n1.pd

Electron contamination modeling and reduction in a 1 T open bore inline MRI-linac system

Purpose: A potential side effect of inline MRI-linac systems is electron contamination focusing causing a high skin dose. In this work, the authors reexamine this prediction for an open bore 1 T MRI system being constructed for the Australian MRI-Linac Program. The efficiency of an electron contamination deflector (ECD) in purging electron contamination from the linac head is modeled, as well as the impact of a helium gas region between the deflector and phantom surface for lowering the amount of air-generated contamination. Methods: Magnetic modeling of the 1 T MRI was used to generate 3D magnetic field maps both with and without the presence of an ECD located immediately below the MLC's. Forty-seven different ECD designs were modeled and for each the magnetic field map was imported into Geant4 Monte Carlo simulations including the linac head, ECD, and a 30 × 30 × 30 cm water phantom located at isocenter. For the first generation system, the x-ray source to isocenter distance (SID) will be 160 cm, resulting in an 81.2 cm long air gap from the base of the ECD to the phantom surface. The first 71.2 cm was modeled as air or helium gas, with the latter encased between two windows of 50 μm thick high density polyethlyene. 2D skin doses (at 70 μm depth) were calculated across the phantom surface at 1 × 1 mm resolution for 6 MV beams of field size of 5 × 5, 10 × 10, and 20 × 20 cm. Results: The skin dose was predicted to be of similar magnitude as the generic systems modeled in previous work, 230% to 1400% of\documentclass[12pt]{minimal}\ begin{document}\rm D-{\rm max}\end{document}D max for 5 × 5 to 20 × 20 cm, respectively. Inclusion of the ECD introduced a nonuniformity to the MRI imaging field that ranged from ∼20 to ∼140 ppm while the net force acting on the ECD ranged from ∼151 N to ∼1773 N. Various ECD designs were 100% efficient at purging the electron contamination into the ECD magnet banks; however, a small percentage were scattered back into the beam and continued to the phantom surface. Replacing a large portion of the extended air-column between the ECD and phantom surface with helium gas is a key element as it significantly minimized the air-generated contamination. When using an optimal ECD and helium gas region, the 70 μm skin dose is predicted to increase moderately inside a small hot spot over that of the case with no magnetic field present for the jaw defined square beams examined here. These increases include from 12% to 40% of \documentclass[12pt]{minimal}\ begin{document}\rm D-{\rm max}\end{document}D max for 5 × 5 cm , 18% to 55% of \documentclass[12pt]{minimal}\begin{document}\rm D-{\rm max}\end{document}D max for 10 × 10 cm, and from 23% to 65% of \documentclass[12pt]{minimal}\begin{document}\rm D-{\rm max}\end{document}D max for 20 × 20 cm. Conclusions: Coupling an efficient ECD and helium gas region below the MLCs in the 160 cm isocenter MRI-linac system is predicted to ameliorate the impact electron contamination focusing has on skin dose increases. An ECD is practical as its impact on the MRI imaging distortion is correctable, and the mechanical forces acting on it manageable from an engineering point of view

Electron contamination modeling and reduction in a 1 T open bore inline MRI-linac system

A potential side effect of inline MRI-linac systems is electron contamination focusing causing a high skin dose. In this work, the authors reexamine this prediction for an open bore 1 T MRI system being constructed for the Australian MRI-Linac Program. The efficiency of an electron contamination deflector (ECD) in purging electron contamination from the linac head is modeled, as well as the impact of a helium gas region between the deflector and phantom surface for lowering the amount of air-generated contamination

On-line monitoring of radiotherapy beams: Experimental results with proton beams

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135090/1/mp8491.pd