Measurement and Analysis of PDDs Profile and Output Factors for Small Field Sizes by cc13 and Micro-Chamber cc01

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 mea-surement 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.

The effect of SSD, Field size, Energy and Detector type for Relative Output Factor measurement in small photon beams as compared with Monte Carlo simulation

“…Nasir et al compared the use of CC13 and CC01 ionization chamber for small field ROFs (1 × 1 -5 × 5 cm 2 at 100 cm SSD for 6 and 15 MV). The study shows that the measurements performed with the CC13 chamber underestimates the ROF and are insensitive to a relative error of 22.89% smallest field considered as compared to CC01 [9]. Accordingly, the detector material will decrease/increase interactions according to its physical density [10].…”

Total scatter factor for small fields in radiotherapy: a dosimetric comparison

“…CC04 did not measure scattering for the field sizes <1•0 cm 2 due to perturbation and volume averaging effects. 12 TSF increased with the increasing field sizes as measured by both detectors. 13 For 18 MV beam, both detectors measured TSF for the same set of field sizes as for 6 MV beam.…”

“…CC01 detector measured TSF for all the given field sizes. CC04 did not measure scattering for the field sizes <1·0 cm 2 due to perturbation and volume averaging effects 12 . TSF increased with the increasing field sizes as measured by both detectors 13…”

“…This type of discrepancy was also observed by Muhammad Kamran et al They analysed the TSFs for 6 and 15 MV photon beams by using CC01 and CC13 detectors. They found underestimation in the measurement of TSF by CC13 due to larger volume averaging and disequilibrium of the charged particles, as compared with CC01 12 …”

“…They found underestimation in the measurement of TSF by CC13 due to larger volume averaging and disequilibrium of the charged particles, as compared with CC01. 12 Reduan et al, discussed about the minimum scattering of photons through small field sizes due to reduced lateral scattering of the photons from the primary beam and it becomes greater for the larger field sizes. 22 This study also revealed that TSF went on increasing with the field sizes whether they are formed by the collimator jaws or by the MLCs because the increased collimator openings cause more scattering of the photon beam.…”