Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25

Abstract

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Comparison of measured electron energy spectra for six matched, radiotherapy accelerators

“…Measured ionization values at the four depths were normalized to the maximum ionization. Percent ionization vs effective depth (depth minus 0.5 r cav ) points were converted into percent dose (% D ) vs depth ( d ) points following AAPM TG‐25 protocol3 and its TG‐70 supplement11 with TG‐51 values for relative stopping powers,12 as implemented in the IBA data acquisition system (IBA, Louvain‐la‐Neuve, Belgium). Lastly, R 80 , R 50 , and R 20 were determined from a nonlinear, least‐squares fit to the four (% D , d ) points in the falloff region using$\%Dfalse(dfalse)=false(100\%-D_{x}false)\frac{\text{erfc}(a_{1}d+a_2)}{2}+D_x,$where d is the effective depth, D x is the energy‐dependent bremsstrahlung dose percent at R p   + 2 cm, erfc is the complimentary error function, and a 1 and a 2 are parameters determined by the fit using the nonlinear, Levenberg‐Marquardt algorithm option in MATLAB (MathWorks, Natick, MA).…”

Design and production of 3D printed bolus for electron radiation therapy

“…The density is obtained from the HU to density lookup table in the planning system which, in turn, was obtained during eMC commissioning from a HU calibration phantom (Catphan, the Phantom Laboratory, Salem, NY). The accuracy of the CET method has been discussed in AAPM Report of Task Group 70; (22) the approach is useful for simple geometries, but the accuracy is compromised in the presence of complex inhomogeneities. Each iteration of the algorithm includes calculation by the eMC algorithm such that subsequent modifications are based on an accurate dose distribution.…”

Experimental assessment of out‐of‐field dose components in high energy electron beams used in external beam radiotherapy

“…To date, most studies on out‐of‐field dose estimation focus on photon beams 3 , 4 . Nevertheless, electron beams are still an important component of RT, for treating superficial tumors (at depths $<5\hspace{0.17em}\text{cm}$) (5) . In high‐energy electron beam RT ($4–25\hspace{0.17em}\text{MeV}$); applicators with cutouts are used to confine the beams to the target area.…”