Limitations of silicon diodes for clinical electron dosimetry.
This work investigates the relevance of several factors affecting the response of silicon diode dosemeters in depth-dose scans of electron beams. These factors are electron energy, instantaneous dose rate, dose per pulse, photon/electron dose ratio and electron scattering angle (directional response). Data from the literature and our own experiments indicate that the impact of these factors may be up to +/-15%. Thus, the different factors would have to cancel out perfectly at all depths in order to produce true depth-dose curves. There are reports of good agreement between depth-doses measured with diodes and ionisation chambers. However, our measurements with a Scantronix electron field detector (EFD) diode and with a plane-parallel ionisation chamber show discrepancies both in the build-up and in the low-dose regions, with a ratio up to 1.4. Moreover, the absolute sensitivity of two diodes of the same EFD model was found to differ by a factor of 3, and this ratio was not constant but changed with depth between 5 and 15% in the low-dose regions of some clinical electron beams. Owing to these inhomogeneities among diodes even of the same model, corrections for each factor would have to be diode-specific and beam-specific. All these corrections would have to be determined using parallel plane chambers, as recommended by AAPM TG-25, which would be unrealistic in clinical practice. Our conclusion is that in general diodes are not reliable in the measurement of depth-dose curves of clinical electron beams.
Song
Haijun
H
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT, USA. haijun.song@duke.edu
Ahmad
Munir
M
Deng
Jun
J
Chen
Zhe
Z
Yue
Ning J
NJ
Nath
Ravinder
R
D004307:Dose-Response Relationship, Radiation; D004583:Electrons / Q000627:therapeutic use*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D011879:Radiotherapy Dosage; D015203:Reproducibility of Results; D012666:Semiconductors; D012680:Sensitivity and Specificity; D012825:Silicon / Q000737:chemistry* / Q000528:radiation effects*; D013819:Thermoluminescent Dosimetry / Q000295:instrumentation* / Q000379:methods
D012825:Silicon