Minimal skin dose increase in longitudinal rotating biplanar linac-MR systems: examination of radiation energy and flattening filter design

Abstract

The magnetic fields of linac-MR systems modify the path of contaminant electrons in photon beams, which alters patient entrance skin dose. Also, the increased SSD of linac-MR systems reduces the maximum achievable dose rate. To accurately quantify the changes in entrance skin dose, the authors use EGSnrc Monte Carlo calculations that incorporate 3D magnetic field of the Alberta 0.5 T longitudinal linac-MR system. The Varian 600C linac head geometry assembled on the MRI components is used in the BEAMnrc simulations for 6 MV and 10 MV beam models and skin doses are calculated at an average depth of 70 μm using DOSXYZnrc. 3D modeling shows that magnetic fringe fields decay rapidly and are small at the linac head. SSDs between 100 and 120 cm result in skin-dose increases of between ~6%-19% and ~1%-9% for the 6 and 10 MV beams, respectively. For 6 MV, skin dose increases from ~10.5% to ~1.5% for field-size increases of 5 × 5 cm(2) to 20 × 20 cm(2). For 10 MV, skin dose increases by ~6% for a 5 × 5 cm(2) field, and decreases by ~1.5% for a 20 × 20 cm(2) field. Furthermore, the proposed reshaped flattening filter increases the dose rate from the current 355 MU min(-1) to 529 MU min(-1) (6 MV) or 604 MU min(-1) (10 MV), while the skin-dose increases by only an additional ~2.6% (all percent increases in skin dose are relative to D max). This study suggests that there is minimal increase in the entrance skin dose and minimal/no decrease in the dose rate of the Alberta longitudinal linac-MR system. The even lower skin dose increase at 10 MV offers further advantages in future designs of linac-MR prototypes.

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom

“…However, to date, EGSnrc depth dose calculations in phantom or patient in the presence of a parallel magnetic field have not been verified by experiment. The purpose of the current study is to experimentally explore the accuracy of EGSnrc calculated depth doses in a homogeneous tissue‐like phantom, with a slight modification to the EGS code required to read in the 3D magnetic field map and use it in the standard electromagnetic field macros as previously described . This is achieved by verifying the agreement between the EGS calculated percent depth doses (PDDs) and the measurements performed using a parallel plate ion chamber in a polystyrene phantom placed inside the bore of a solenoidal electromagnet and irradiated using a clinical linac.…”

“…As the main magnetic field is parallel to the beam's CAX, this is called a parallel configuration Linac‐MR. Both these configurations have been previously described in detail …”

Future of medical physics: Real‐time MRI‐guided proton therapy

“…These may be above the patient, or scattered from the patient on the exit side. 41,42,43,44,45,46,47,48,49,50 With MRPT, the therapeutic proton beam (being charged particles), is subject to the Lorentz deflection force when transporting towards the patient at the center of the MRI, as well as within the patient while slowing down. The planning process is best broken down into two components as described in Fig.…”

Technical Note: Experimental verification ofEGSnrc calculated depth dose within a parallel magnetic field in a lung phantom

“…Systems with the radiation beam central axis parallel to the main magnetic field have a minimal dose increase at tissue — air surfaces . However, the parallel magnetic field is preventing the contaminant electrons accompanying the photon beam from scattering laterally, causing an entrance surface dose increase that can be substantial, measurable, or minimal depending on the MRI's fringe magnetic field and design.…”

“…All these dose modifications have previously been investigated using several Monte Carlo (MC) packages: Geant4, EGSnrc, and PENELOPE . While Geant4 dose calculations in the presence of a magnetic field have been previously verified experimentally for multiple materials, to date, EGSnrc MC has only been recently experimentally verified in polystyrene .…”