Entrance and exit dose measurements with semiconductors and thermoluminescent dosemeters: a comparison of methods and in vivo results.
OBJECTIVE
In order to compare diodes and TLD for in vivo dosimetry, systematic measurements of entrance and exit doses were performed with semiconductor detectors and thermoluminescent dosemeters for brain and head and neck patients treated isocentrically with external photon beam therapy.


METHODS
Scanditronix EDP-20 diodes and 7LiF thermoluminescent chips, irradiated in a 8 MV linac, were studied with similar build-up cap geometries and materials in order to assure an equivalent electronic equilibrium. Identical calibration methodology was applied to both detectors for the dose determination in clinical conditions.


RESULTS
For the entrance dose evaluation over 249 field measurements, the ratio of the measured dose to the expected dose, calculated from tabulated tissue maximum ratios, was equal to 1.010 +/- 0.028 (1 s.d.) from diodes and 1.013 +/- 0.041 from thermoluminescent crystals. For the exit dose measurements, these ratios were equal to 0.998 +/- 0.049 and 1.016 +/- 0.070 for diodes and TLDs, respectively, after application of a simple inhomogeneity correction to the calculation of the expected exit dose.


CONCLUSIONS
Thermoluminescence and semiconductors led to identical results for entrance and exit dose evaluation but TLDs were characterised by a lower reproducibility inherent to the TL process itself and to the acquisition and annihilation procedures.
Loncol
T
T
Cliniques Universitaires Saint Luc, Service de Radiothérapie Oncologique, Université Catholique de Louvain, Bruxelles, Belgique. loncol@rbnt.cl.ac.be
Greffe
J L
JL
Vynckier
S
S
Scalliet
P
P
D001932:Brain Neoplasms / Q000532:radiotherapy*; D002138:Calibration; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D011874:Radiometry / Q000295:instrumentation / Q000379:methods; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy; D012666:Semiconductors; D013819:Thermoluminescent Dosimetry / Q000295:instrumentation