SU‐E‐T‐126: Non‐Reference Condition Correction Factor KNR of Typical Radiation Detectors for the Dosimetry of High‐Energy Photons

Purpose: To correct for the deviations of the detector response when typical radiation detectors are used under non‐reference conditions, factor kNR was calculated from the known energy dependence of the detector response at photon energies from 10 keV upwards and from clinical photon spectra within a large water phantom beneath a Siemens Primus 6/15 MV linac. A Farmer type ion chamber (NE2571), two TLD detector types and two diodes were investigated. Methods: Factor kNR was obtained as the ratio of the weighted responses Yt of a given detector t under reference conditions xref (axial distance r = 0 cm, depth d = 10 cm, field size 10 × 10 cm2 and SSD = 100 cm) and that under non‐reference conditions × (off‐axis points and depths for various field sizes); kNR = Yt(xref)/Yt(x). For small field (SF) dosimetry, we evaluated correction factor kNRSF, which refers to small field reference conditions (4 × 4 cm2 field). Results: For all detectors investigated, the deviations of kNR from unity were highest outside the field, due to prevailing low‐energy scatter contributions. For the Farmer chamber and EDP‐10 diode, the kNR deviations did not exceed 2%, but were up to 60% for the EDD‐5 diode, while kNR values for LiF:Mg,Cu,P and LiF:Mg,Ti deviated at most 15% and 5% respectively. kNR values appear as unique functions of the mean photon energy at the point of interest. Conclusions: Air‐filled ion chambers show only small kNR variations, while for non‐water equivalent detectors, kNR variations depend on the detector response at low photon energy. kNR can be presented as a unique function of the mean photon energy at the point of interest. A 4 × 4 cm2 reference field is recommended for small fields, with correction factor kNRSF varying almost negligibly from kNR except for unshielded Si diodes.