• K Abdel-Hakim, T Nishimura, M Takai, S Suzuki, and H Sakahara.
• Departments of Radiology, Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu, 431-3192 Japan.
• Br J Radiol. 2002 May 1; 75 (893): 428-34.

AbstractCreating non-divergent field edges using asymmetric collimators and a single isocentre can improve matchline dosimetry owing to decreased reliance on operator skills and avoidance of couch movement. However, asymmetic jaws have an associated tolerance that can cause abutment to be misaligned. The matching area dose for monoisocentric three-beam split fields commonly used in head and neck cancer treatments using mismatched and matched collimators is the subject of this work. X-ray verification film was exposed in a solid-water phantom, and the dose at the matching area was evaluated using mismatched and matched collimators. In the case of mismatched (consistently overlapped) collimators, digital displays of an asymmetric collimator position within the tolerance indicated in the manufacturer's specifications were investigated for the three-beam split field technique. The effect of this technique on the junctional dose was also determined using matched collimators. Although the collimators showed a consistent overlap, a perfect dose distribution could be obtained at the matching area. The three-beam split field technique yielded an 8% overdose at the matchline using matched collimators. In conclusion, an awareness of the effects of the abutting technique and digital display tolerance is necessary to achieve good junction uniformity using asymmetric collimators.

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