Surface and Buildup Dose Effects in a Linac-MR

• Author / Creator

  Ghila, Andrei D

• Magnetic resonance imaging (MRI) can visualize solid tumours in real time and its integration with a linear accelerator offers the possibility to track tumours during irradiation. However, irradiating a patient inside a linac-MR poses several challenges. MRI uses radiofrequency (RF) coils to acquire images, and with existing RF coil designs, the patient will have to be irradiated through the RF coil. Also a linac-MR in which the main magnetic field is parallel to the radiation beam central axis is expected to cause surface and buildup dose modifications. The purpose of the current work is to experimentally investigate and quantify the surface and buildup dose modifications caused by irradiating through typical RF coil materials and by irradiating in the presence of a strong parallel magnetic field. The surface and buildup dose measurements in a parallel magnetic field are used to verify the ability of the EGSnrc Monte Carlo system to accurately calculate magnetic field dose effects.
  An imitation RF coil (layers of polycarbonate, copper tape, and Teflon) was placed at various distances from the surface of a polystyrene phantom and irradiated using a 6 MV photon beam. The depth dose in polystyrene was measured in three cases: no magnetic field, a 0.22 T magnetic field perpendicular to the radiation beam central axis, and a 0.21 T magnetic field parallel to the radiation beam. With the imitation RF coil in direct contact with the surface of the polystyrene phantom the surface and buildup dose increased considerably irrespective of a magnetic field’s presence or orientation. With no magnetic field, moving the coil away from the surface of the phantom gradually decreased the surface dose. When the measurements were repeated in a transverse magnetic field, the surface dose decreased even faster with increasing coil to phantom separation. In a parallel magnetic field, increasing the separation between the coil and the phantom had a minimal effect on decreasing the surface and buildup dose. The influence of other RF coil materials (RF coil casing, RF coil plastic sheet, RF coil foam padding, thin copper sheet, thin copper pipe) on the surface dose was also investigated, and displayed the same trends. Thus, in order to use surface coils in a linac-MR, the current coil design has to be modified.
  An electromagnet (two coils) provided a magnetic field parallel to the radiation beam central axis of a clinical linac, and offered a central bore for phantom placement. A polystyrene, and subsequently a Gammex lung phantom, were placed inside the bore at two locations: top of the phantom coinciding with the top of the bore and top of the phantom coinciding with the centre of the bore. COMSOL Multi-physics created a 3D magnetic field map that was validated against magnetic field measurements taken along three orthogonal axes. A benchmarked (against commissioning measurements) BEAMnrc model of the clinical linac was used to simulate the phase space of particle fluence. A parallel plate ion chamber measured the doses in the phantoms. The dimensions of the ion chambers' air cavity and entrance window had to be included in the simulations. The 3D magnetic field map was implemented in DOSXYZnrc and the charge particle deflection per step, due to the magnetic field, was restricted compared to default values. With these modifications, EGSnrc was able to accurately calculate the surface and buildup dose increases caused by the parallel magnetic field at both locations within the bore, for polystyrene and lung.

• Subjects / Keywords

  • Surface Dose
  • Linac-MR
  • Magnetic field
  • Buildup Dose
  • MR-Linac
  • dose effects

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3HX16668

• License

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