Monte Carlo calculated correction factors for diodes and ion chambers in small photon fields

The application of small photon fields in modern radiotherapy requires the determination of total scatter factors S-cp or field factors Omega(fclin,fmsr)(Qclin,Qmsr) with high precision. Both quantities require the knowledge of the field-size-dependent and detector-dependent correction factor k(Qclin,Qmsr)(fclin,fmsr). The aim of this study is the determination of the correction factor k(Qclin,Qmsr)(fclin,fmsr) for different types of detectors in a clinical 6 MV photon beam of a Siemens KD linear accelerator. The EGSnrc Monte Carlo code was used to calculate the dose to water and the dose to different detectors to determine the field factor as well as the mentioned correction factor for different small square field sizes. Besides this, the mean water to air stopping power ratio as well as the ratio of the mean energy absorption coefficients for the relevant materials was calculated for different small field sizes. As the beam source, a Monte Carlo based model of a Siemens KD linear accelerator was used. The results show that in the case of ionization chambers the detector volume has the largest impact on the correction factor k(Qclin,Qmsr)(fclin,fmsr); this perturbation may contribute up to 50% to the correction factor. Field-dependent changes in stopping-power ratios are negligible. The magnitude of k(Qclin,Qmsr)(fclin,fmsr) is of the order of 1.2 at a field size of 1 x 1 cm(2) for the large volume ion chamber PTW31010 and is still in the range of 1.05-1.07 for the PinPoint chambers PTW31014 and PTW31016. For the diode detectors included in this study (PTW60016, PTW 60017), the correction factor deviates no more than 2% from unity in field sizes between 10 x 10 and 1 x 1 cm(2), but below this field size there is a steep decrease of k(Qclin,Qmsr)(fclin,fmsr) below unity, i.e. a strong overestimation of dose. Besides the field size and detector dependence, the results reveal a clear dependence of the correction factor on the accelerator geometry for field sizes below 1 x 1 cm(2), i.e. on the beam spot size of the primary electrons hitting the target. This effect is especially pronounced for the ionization chambers. In conclusion, comparing all detectors, the unshielded diode PTW60017 is highly recommended for small field dosimetry, since its correction factor k(Qclin,Qmsr)(fclin,fmsr) is closest to unity in small fields and mainly independent of the electron beam spot size.