A linear diode array (JFD-5) for match line in vivo dosimetry in photon and electron beams; evaluation for a chest wall irradiation technique.
OBJECTIVE
In vivo dosimetry using thermoluminiscence detectors (TLD) is routinely performed in our institution to determine dose inhomogeneities in the match line region during chest wall irradiation. However, TLDs have some drawbacks: online in vivo dosimetry cannot be performed; generally, doses delivered by the contributing fields are not measured separately; measurement analysis is time consuming. To overcome these problems, the Joined Field Detector (JFD-5), a detector for match line in vivo dosimetry based on diodes, has been developed. This detector and its characteristics are presented.


METHODS
The JFD-5 is a linear array of 5 p-type diodes. The middle three diodes, used to measure the dose in the match line region, are positioned at 5-mm intervals. The outer two diodes, positioned at 3-cm distance from the central diode, are used to measure the dose in the two contributing fields. For three JFD-5 detectors, calibration factors for different energies, and sensitivity correction factors for non-standard field sizes, patient skin temperature, and oblique incidence have been determined. The accuracy of penumbra and match line dose measurements has been determined in phantom studies and in vivo.


RESULTS
Calibration factors differ significantly between diodes and between photon and electron beams. However, conversion factors between energies can be applied. The correction factor for temperature is 0.35%/ degrees C, and for oblique incidence 2% at maximum. The penumbra measured with the JFD-5 agrees well with film and linear diode array measurements. JFD-5 in vivo match line dosimetry reproducibility was 2.0% (1 SD) while the agreement with TLD was 0.999+/-0.023 (1 SD).


CONCLUSIONS
The JFD-5 can be used for accurate, reproducible, and fast on-line match line in vivo dosimetry.
Essers
M
M
Division of Clinical Physics, University Hospital Rotterdam, Daniel den Hoed Cancer Center, Groene Hilledijk 301, 3075 EA, Rotterdam, The Netherlands.
van Battum
L
L
Heijmen
B J
BJ
D004583:Electrons; D006801:Humans; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D011834:Radiation Monitoring / Q000295:instrumentation; D011882:Radiotherapy, High-Energy; D013909:Thorax