Jy. Ting et al., SCATTERED RADIATION FROM LINEAR-ACCELERATOR AND CO-60 COLLIMATOR JAWS, International journal of radiation oncology, biology, physics, 30(4), 1994, pp. 985-992
Citations number
19
Categorie Soggetti
Oncology,"Radiology,Nuclear Medicine & Medical Imaging
Purpose: Solid state diodes and/or thermoluminescent dosimeters (TLDs)
are often used to measure scattered radiation doses to critical organ
s immediately adjacent to radiation field sites. The energy-dependent
response of these commonly used in vivo dosimeters sometimes makes the
interpretation of measured values uncertain. This study investigates
scattered radiation arising from the collimator jaws of linear acceler
ators and the treatment head of a cobalt-60 teletherapy unit. Methods
and Materials: A thin window Markus-type parallel-plate ionization cha
mber placed in a polystyrene phantom was employed to document the magn
itude, energy composition, and sources of scattered radiation at surfa
ces near radiation fields. Measurements were taken both with and witho
ut additional phantom material covering the ionization chamber, as wel
l as with various distances between the ionization chamber and edges o
f the radiation fields tested. Results: Data was collected, analyzed a
nd compared for treatment units produced by different manufacturers. I
t was found that the magnitude of scattered radiation to surfaces imme
diately adjacent to radiation fields ranged from 1% to 15% of the maxi
mum dose along the beam central axis. These values showed a strong dep
endence upon distance from the edge of the radiation field, beam energ
y, collimator setting (field size), and the presence of externally mou
nted accessories. Teletherapy unit differences due to manufacturing fi
rm origins were found to only slightly affect scattered radiation magn
itude, while the orientation of upper and lower collimator jaws had ab
solutely no effect. Conclusions: Percent depth dose curves of scattere
d radiation were obtained and analyzed. The shapes of these depth dose
curves suggest the presence of complex energy spectra from secondary
electrons and scattered x-rays. Because of the presence of these compl
ex energy spectra in areas immediately adjacent to radiation fields, c
aution should be observed when interpreting patient doses near radiati
on fields, if dose values have been measured in vivo using thermolumin
escent dosimeters (TLDs) or solid state diodes. Many of these on-patie
nt dosimetry devices are strongly energy dependent and may demonstrate
large over- or under-responses in areas dominated by scattered radiat
ion. The results of this study, thus, suggest that ionization chambers
are preferred for determination of scattered radiation doses in such
regions.