Assessment of an unshielded electron field diode dosimeter for beam scanning in small- to medium-sized 6 MV photon fields

Introduction: Radiotherapy planning systems require many percentage depth dose (PDD) and profile measurements and there are various dosimeters that can be used to obtain these scans. As dose perturbation is particularly troublesome in smaller photon fields, using a low-perturbation, unshielded electron field diode (EFD) in these fields is of interest. The aim of this work was to investigate the suitability of an unshielded diode for beam scanning in 3×3 cm2, 5×5 cm2, and 10×10 cm2, 6 MV fields. Materials and Methods: An EFD was used for all the scans. For comparison, in profile measurements, a tungsten-shielded photon field diode (PFD) was also used. PDDs were measured using the PFD and an RK ionization chamber. Results: Very good agreement (0.4%) was found between the PDDs measured with EFD and PFD for the two larger fields. However, the difference between them exceeded 1.0% slightly for the smallest field, which may be attributed to the effect of the larger PFD perturbation. The RK chamber PDDs around 10 cm depth were 1- 2% lower than those measured with the diodes. There was good agreement ( 1 mm) between EFD- and PFD-measured penumbra widths. Conclusion: The EFD generally agrees well with the PFD and may even perform better in smaller fields.