The dosimetric effect of asymmetric collimator misalignment on matched half beam blocked fields.
OBJECTIVE
The purpose of this study was to assess the dosimetric effect of the asymmetric jaw misalignment on the junction region for 6 MV photon beams.


METHODS
Dose uniformity was measured at the junction with film dosimetry. The Kodak XOmat V film was exposed in a solid water phantom at 0 degrees and 180 degrees collimator position for exact matching. This procedure was repeated for 1 mm, 2 mm, and 4 mm overlaps and gaps. Furthermore, the dose distributions were obtained by mathematical summation using the dose profile data for appropriate overlaps and gaps.


RESULTS
Film dosimetry showed that the collimators underlapped and the fields overlapped for exact matching for this machine. When the two asymmetric fields were matched without gap, both the calculated values and film dosimetry results showed that there was approximately 9% inhomogeneity above the prescribed dose. A 2 mm overlap and gap produced inhomogeneities nearly of 35% and 30% above or below the prescribed dose, respectively. The 4 mm overlap and gap created an inhomogeneity of +65% and -50%. The dose inhomogeneity produced for 1 mm overlap and gap was 22% above and 6.8% below the prescribed dose, respectively.


CONCLUSIONS
Asymmetric collimators should be evaluated routinely related to inhomogeneity at the junction, especially for the mono-isocentric set-up technique. Small misalignments of asymmetric collimators cause serious inhomogeneity at the junction. If the homogeneity can not be improved, other methods of field matching have to be developed.
Kemikler
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Istanbul University, Oncology Institute, Department of Medical Radiophysics, Istanbul, Turkey.