Photon and electron beam penumbra determination using different measuring systems

Introduction The penumbra region is an intrinsic characteristic of any beam, weather photon or electron. Therefore, its correct determination constitutes a vital part in the dosimetric chain of a modern radiation therapy department. Purpose The purpose of this study is to compare and analyze results concerning the penumbra size determination obtained using six different measuring systems. Materials and methods Penumbra measurements were performed for two photon beams (6, 15 MV) for 4×4, 10×10 and 25×25 cm 2 square field sizes and for five electron beams (6, 9, 12, 16, 20 MeV) using a 10×10 cm 2 applicator at several depths. Six different measuring systems were used: (a) a Semiflex, (b) a Markus, (c) a Roos ionisation chamber, (d) a one dimensional Linnear Array (LA48, PTW), (e) a p-type diode (PTW 60008) and (f) an e-type diode (PTW 60017). All measurements were taken with standard water phantom geometry (MP3, PTW) using water as medium. Results Our results indicate that penumbra size determination is strongly dependant on the measuring system. More specifically, the diodes showed the narrowest penumbras followed by the LA48, while the largest penumbras were presented by the ionisation chambers. In terms of effectiveness, the LA48 method showed significant lower beam-on time. Conclusion Regarding penumbra size determination, the LA48 method is acceptable in terms of accuracy, and it is associated with considerable time-saving compared to other measuring systems. When the greatest possible accuracy is required, the use of silicon diode dosimeters is recommended.