Radiation leakage dose from Elekta electron collimation system

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

Evaluation of prototype of improved electron collimation system for Elekta linear accelerators

“…For the 20 MeV profiles, the MC calculations in general slightly underpredicted the measured dose within approximately 7 cm of central axis, but slightly overpredicted the measured dose outside of 7 cm. These differences are similar to those found in the MC‐calculated and measured dose profiles of the current clinical system as shown by Harris and Pitcher et al …”

“…Each of these five applicator designs was modeled in BEAMnrc by inserting them into a previously verified model of our Elekta Infinity accelerator (MLCi2 treatment head) . Simulations were performed with these prototype collimation systems with the 7 MeV (E p,0 = 7.14 MeV, R 90 = 2.0 cm), 13 MeV (E p,0 = 13.12 MeV, R 90 = 4.0 cm), and 20 MeV (E p,0 = 20.47 MeV, R 90 = 6.0 cm) beams to score phase space files 1 cm upstream of isocenter.…”

“…In‐field flatness was evaluated according to the criteria described by Hogstrom, which states that off‐axis dose vary from the central‐axis dose by no more than ±3% along the major axes (in‐plane and cross‐plane) and ±4% along the diagonal axes. These specifications are assessed at 1 cm depth in water for E p,0 ≤ 9 MeV and 2 cm for E p,0 > 9 MeV within a region 2 cm inside the field edge for the major axes and 2 cm for the diagonal axes …”

“…Consistent with IEC specifications, mean and maximum leakage doses were calculated from measured dose (relative dose divided by central‐axis percent of D max , i.e., 95.2%, 94.5%, and 97.8%, respectively) points sampled at 2 cm spacing along the in‐plane, cross‐plane, and diagonal axes at 1 cm depth in water at 100 cm SSD. These sampled points spanned from an inner boundary located 2 and 4 cm outside the field edge for maximum and mean dose determination, respectively, out to the geometric projection of the primary collimator at isocenter, located 24.8 cm off‐axis for the Elekta infinity accelerator …”

“…Our cancer clinic has seven Elekta Infinity radiotherapy accelerators (MLCi2 treatment head) with matched, custom electron beams spanning 7–20 MeV (R 90 values of 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, and 6.0 ± 0.1 cm) and slightly modified scattering foils . Although they have exceptional in‐field flatness laterally (±3% along major axes; ±4% along diagonals) and out‐of‐field leakage doses well below IEC standards for all applicators (6 × 6–25 × 25 cm 2 ), there is opportunity to improve the delivery technology by reducing the weight of the electron applicators, particularly those for larger fields .…”