Room: ePoster Forums
Purpose: Establish new remote dosimetry audit criteria for verification of percentage depth dose for electron beams.
Methods: Acrylic phantoms with a dosimeter insert that hold nanodots are irradiated with a vertical electron beam with energies ranging from 6 to 22 MeV, under reference conditions. Two OSLD are located a depth close to dmax and two OSLD are located at depth between 80% and 50% of the maximum. Depth dose is calculated as ratio of doses from OSLD at depth to OSLD at dmax. The difference between the depth were OSLD are located and the depth derived from the institution depth dose data at the value defined from the OSLD ratio was compared. The average difference and standard deviation (sigma) were calculated. A normal distribution was assumed. Acceptance criteria was defined to be 3 sigma of experimental data.
Results: A total of 14,230 irradiations from 12,620 electron beams from 1,753 institutions were evaluated. The values included in this analysis had dose agreements at dmax within 5% and PDD agreements within 10mm. The data were analyzed by nominal electron beam energy. The sigma value for energies between 6 and 16 MeV was close to 1.0mm; 1.3mm for energies between 18 and 20 MeV and 2.0mm for energies greater that 20 MeV.
Conclusion: The origin of the original 5mm criterion for electron beam percentage depth dose audits is not known. A more scientific assessment of the data have indicated that the criteria should change. The new acceptance criteria, supported by audit data from the past 9 years, will be 3mm for 6-16 MEV electron beams and 4mm for electron energies greater than 16 MeV.