ABSTRACT
Small radiation fields are abundantly used in modern radiotherapy
techniques like in IMRT and SRS. In order to commission these techniques,
dosimetric data for small fields is required. The purpose of this study is to
compare dosimetric measurements with two different ion chambers cc13, and cc01
for smaller fields. Dosimetric measurements are beam profile, output factor,
pdds, and collimator factor. Dosimetric data is acquired in water phantom for
two different photon beam energies 6 MV and 15 MV with zero gantry angle. In
beam profiles cc13 chamber, measure wider penumbra as compare to cc01. And this
wider measurement of penumbra occurs for smaller as well as for larger field
sizes. Accumulated relative error in the measurement of penumbra for number of
field sizes and 6 MV at dmax, and at 10 cm depth are 34.32% and 27.72%
respectively. Accumulated relative error in the measurement of penumbra for
number of field sizes and 15 MV at dmax, and at 10 cm depth are 28.49% and
23.92%. In case of output factor for smaller fields cc13 underestimates the
output factor relative to cc01, with non-linear increase for smaller fields.
But for larger fields, this increase in output factor is almost linear
difference of two chambers is decreased. For very smaller fields <2 cm × 2 cm, relative error in
output factor of cc13 and cc01 is greater than 5% and rapidly increases with
decreasing field size. But for lager fields, this relative error is negligible.
In measurement of pdds after the buildup region difference occurs in the
response of two chambers cc13 and cc01 for smaller fields. For field sizes ≤2 cm × 2 cm average
cc13-cc01 at various depths 30 cm, 40 cm, 50 cm, 60 cm, 70 cm, and 80 cm is
almost greater than 0.5 cm. And similarly as output factor, this difference (cc13-cc01)
increases with field size decreasing.