estimation of dosimetric parameters based on k_nr and k_ncsf correction factors for small field radiation therapy at 6 and 18 mv linac energies using monte carlo simulation methods

Background: estimating dosimetric parameters for small fields under nonreference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in nonreference conditions at different depths and field sizes.objective: determining correction factors (k_nr and k_ncsf) recommended recently for small field dosimetry formalism by american association of physicists in medicine (aapm) for different detectors at 6 and 18 mv photon beams.methods: egsnrc monte carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended k_nr and k_ncsf correction factors for various circular small field sizes ranging from 530 mm diameters. k_nr and k_ncsf correction factors were determined for different active detectors (a pinpoint chamber, edp20 and edp10 diodes) in a homogeneous phantom irradiated to 6 and 18 mv photon beams of a varian linac (2100c/d). results: knr correction factor estimated for the highest small circular field size of 30 mm diameter for the pinpoint chamber, edp20 and edp10 diodes were 0.993, 1.020 and 1.054; and 0.992, 1.054 and 1.005 for the 6 and 18 mv beams, respectively. the kncsf correction factor estimated for the lowest circular field size of 5 mm for the pinpoint chamber, edp20 and edp10 diodes were 0.994, 1.023, and 1.040; and 1.000, 1.014, and 1.022 for the 6 and 18 mv photon beams, respectively.conclusion: comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (edp20 and edp10) can confidently be recommended for small field dosimetry as their correction factors (k_nr and k_ncsf) was close to 1.0 for all small field sizes investigated and are mainly independent from the electron beam spot size.