A study on beam homogeneity for a Siemens Primus linac

Abstract

Asymmetric offset fields are an important tool for radiotherapy and their suitability for treatment should be assessed. Dose homogeneity for highly asymmetric fields has been studied for a Siemens PRIMUS clinical linear accelerator. Profiles and absolute dose have been measured in fields with two jaws at maximal position (20 cm) and the other two at maximal overtravel (10 cm), corresponding to 10 cm x 10 cm fields with extreme offset. Measured profiles have a marked decreasing gradient towards the beam edge, making these fields unsuitable for treatments. The flattening filter radius is smaller than the primary collimator aperture, and this creates beam inhomogeneities that affect large fields in areas far from the collimator axis, and asymmetric fields with large offset. The results presented assess the effect that the design of the primary collimator and flattening filter assembly has on beam homogeneity. This can have clinical consequences for treatments involving fields that include these inhomogeneous areas. Comparison with calculations from a treatment planning system, Philips Pinnacle v6.3, which computes under the hypotheses of a uniformly flattened beam, results in severe discrepancies.

IMRT Commissioning and Verification Measurements on Siemens (ARTISTE)Linear Accelerator

“…The resulting beam profile scans demonstrate the variation in flatness and symmetry of the field size as a function of the specified depths (1.5, 5, 10, 15, 20, 25 and 30 cm) 23 . The beam homogeneity increases with the field size and depth in which affecting in the beam uniformity 24 . This increase in the homogeneity value is illustrated in more detail in Figure (4), in this figure the beam profiles for field size 10 x 10 cm 2 at depths 1.5, 10, and 20 cm are normalized to the dose at the center of the field, thus obtaining an off-axis ratio (OAR) for each off axis distance.…”