Abstract
This study provided baseline data required for a greater project, whose objective was to design a new Elekta electron collimation system having significantly lighter electron applicators with equally low out‐of field leakage dose. Specifically, off‐axis dose profiles for the electron collimation system of our uniquely configured Elekta Infinity accelerator with the MLCi2 treatment head were measured and calculated for two primary purposes: 1) to evaluate and document the out‐of‐field leakage dose in the patient plane and 2) to validate the dose distributions calculated using a BEAMnrc Monte Carlo (MC) model for out‐of‐field dose profiles. Off‐axis dose profiles were measured in a water phantom at 100 cm SSD for 1 and 2 cm depths along the in‐plane, cross‐plane, and both diagonal axes using a cylindrical ionization chamber with the 10×10 and 20×20 cm2 applicators and 7, 13, and 20 MeV beams. Dose distributions were calculated using a previously developed BEAMnrc MC model of the Elekta Infinity accelerator for the same beam energies and applicator sizes and compared with measurements. Measured results showed that the in‐field beam flatness met our acceptance criteria (±3% on major and ±4% on diagonal axes) and that out‐of‐field mean and maximum percent leakage doses in the patient plane met acceptance criteria as specified by the International Electrotechnical Commission (IEC). Cross‐plane out‐of‐field dose profiles showed greater leakage dose than in‐plane profiles, attributed to the curved edges of the upper X‐ray jaws and multileaf collimator. Mean leakage doses increased with beam energy, being 0.93% and 0.85% of maximum central axis dose for the 10×10 and 20×20 cm2 applicators, respectively, at 20 MeV. MC calculations predicted the measured dose to within 0.1% in most profiles outside the radiation field; however, excluding modeling of nontrimmer applicator components led to calculations exceeding measured data by as much as 0.2% for some regions along the in‐plane axis. Using EGSnrc LATCH bit filtering to separately calculate out‐of‐field leakage dose components (photon dose, primary electron dose, and electron dose arising from interactions in various collimating components), MC calculations revealed that the primary electron dose in the out‐of‐field leakage region was small and decreased as beam energy increased. Also, both the photon dose component and electron dose component resulting from collimator scatter dominated the leakage dose, increasing with increasing beam energy. We concluded that our custom Elekta Infinity with the MLCi2 treatment head met IEC leakage dose criteria in the patient plane. Also, accuracy of our MC model should be sufficient for our use in the design of a new, improved electron collimation system.PACS number(s): 87.56.nk, 87.10.Rt, 87.56.J
Highlights
Four functions of the treatment head of radiotherapy accelerators used for electron beam therapy are: 1) to redirect the beam toward the patient, typically using an achromatic bending magnet; 2) to broaden and flatten the beam, typically using a dual scattering foil system; 3) to collimate the beam, typically using the X-ray jaws and an electron applicator; and 4) to monitor beam output, flatness, and symmetry, typically using a dual, segmented, transmission ionization chamber.[1]
In electron mode the multileaf collimator (MLC) are parked in their farthest position off-axis (± 20 cm off central axis projected to isocenter in the cross-plane direction with the treatment head at 0°) for all energies and applicators, and the underlying X-ray jaws are used for initial collimation
Our out-of-field leakage results do not apply to the newer Agility treatment head, which utilizes curved MLC ends and curved X-ray jaw ends to collimate the beam in the cross-plane and in-plane directions, respectively, both different from the MLCi2 X-ray jaw configuration
Summary
Four functions of the treatment head of radiotherapy accelerators used for electron beam therapy are: 1) to redirect the beam toward the patient, typically using an achromatic bending magnet; 2) to broaden and flatten the beam, typically using a dual scattering foil system; 3) to collimate the beam, typically using the X-ray jaws and an electron applicator; and 4) to monitor beam output, flatness, and symmetry, typically using a dual, segmented, transmission ionization chamber.[1]. Beam flatness is specified for off-axis relative dose (normalized to 1.00 on central axis) 2 cm inside the field edges for all energy-applicator combinations at a depth of 1.0 cm in water phantom for beams with most probable electron energies, Ep,0 ≤ 9 MeV and 2.0 cm for most probable electron energies, Ep,0 > 9 MeV. We find Elekta electron applicators substantially heavier than comparable Varian applicators, as compared, making their handling by our radiation therapy staff more difficult. We find their latching mechanism for attaching the applicators to the treatment head awkward, and projection of the optical distance indicator (ODI) on the patient surface is obscured by the applicators, making setting the treatment SSD difficult.
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