Determination of beam profile characteristics in radiation therapy using different dosimetric set ups.

PURPOSE

The purpose of this study was to analyze and to compare results regarding the penumbra size, flatness and symmetry obtained using six different measuring systems.

METHODS

Beam profile measurements were performed in standard water phantom set-up for two photon beams for various square field sizes and for five electron beams for several applicator sizes at several depths. Six measuring systems were used: three ionization chambers; a Semiflex (31002, PTW), a Markus (23343, PTW) and a Roos (34001, PTW); Two semiconductor detectors; a p-type diode (60008, PTW) and an e-type diode (60017, PTW) and a one dimensional Linear Array (LA48, PTW).

RESULTS

Our results indicate that penumbra size determination is strongly dependent on the measuring system. For the photon measurements the diodes showed the narrowest penumbra followed by the LA48, while the largest penumbra was presented by the Semiflex. The unshielded diode overestimates the penumbra in large field sizes and big depths. The parallel plate ionization chambers overestimate the penumbra width of electron beam profiles. The LA48 presents the most symmetric beam profiles.

CONCLUSIONS

Regarding penumbra size determination, the LA48 can be considered acceptable in terms of accuracy, and is the most time-effective system. It is also adequate for symmetry and flatness measurements. For greatest possible accuracy silicon diode is recommended. Parallel plate ionization chambers are not appropriate for penumbra measurements.