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

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Document pages: 22 pages

Abstract: Small radiation fields are abundantly used in modern radiotherapytechniques like in IMRT and SRS. In order to commission these techniques,dosimetric data for small fields is required. The purpose of this study is tocompare dosimetric measurements with two different ion chambers cc13, and cc01for smaller fields. Dosimetric measurements are beam profile, output factor,pdds, and collimator factor. Dosimetric data is acquired in water phantom fortwo different photon beam energies 6 MV and 15 MV with zero gantry angle. Inbeam profiles cc13 chamber, measure wider penumbra as compare to cc01. And thiswider measurement of penumbra occurs for smaller as well as for larger fieldsizes. Accumulated relative error in the measurement of penumbra for number offield 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 fornumber of field sizes and 15 MV at dmax, and at 10 cm depth are 28.49 and23.92 . In case of output factor for smaller fields cc13 underestimates theoutput factor relative to cc01, with non-linear increase for smaller fields.But for larger fields, this increase in output factor is almost lineardifference of two chambers is decreased. For very smaller fields × 2 cm, relative error inoutput factor of cc13 and cc01 is greater than 5 and rapidly increases withdecreasing field size. But for lager fields, this relative error is negligible.In measurement of pdds after the buildup region difference occurs in theresponse of two chambers cc13 and cc01 for smaller fields. For field sizes ≤2 cm × 2 cm averagecc13-cc01 at various depths 30 cm, 40 cm, 50 cm, 60 cm, 70 cm, and 80 cm isalmost greater than 0.5 cm. And similarly as output factor, this difference (cc13-cc01)increases with field size decreasing.

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