The effect of independent collimator misalignment on the dosimetry of abutted half-beam blocked fields for the treatment of head and neck cancer

Background and purpose: Independent collimation conveniently allows for the junctioning of abutting fields with non-diverging beam edges. When this te chnique is used at the junction of multiple fields, e.g. lateral and low an terior fields in three-field head and neck set-ups, there should be a dosim etric match with no overdose or underdose at the matchline. We set out to e valuate the actual dosimetry at the central match plane. Materials and methods: Independent jaws were used to mimic two half-beam bl ocked fields abutting at the central axis. X-Ray verification film was expo sed in a water-equivalent phantom and the dose at the matchline was evaluat ed with laser densitometry. Collimators were then programmed to force a gap or overlap of the radiation fields to evaluate the effect of jaw misalignm ent within the tolerance of the manufacturer's specification. Diode measure ments of the field edges were also performed. Four beam energies from four different linear accelerators were evaluated. Results: Small systematic inhomogeneities were found along the matchline in all linear accelerators tested. The maximum dose on the central axis varie d linearly with small programmed jaw misalignments. For a gap or overlap of 2 mm between the jaws, the matchline dose increased or decreased by 30-40% . The region of overdose or underdose around the matchline is 311 mm wide. The discrepancy between the width of jaw separation and the width of the re gion of altered dose is explained by a penumbra effect. Conclusion: We recommend that independent jaw alignment be evaluated routin ely and provide a simple method to estimate dose inhomogeneity at the match plane. If there is a field gap or overlap resulting in a clinically signif icant change in dosimetry, jaw misalignment should be corrected. If it cann ot be corrected, part of the benefit of asymmetric collimation is lost and other methods of field junctioning may have to be considered. We routinely use a small block over the spinal cord at the mono-isocenter set-up plane f or three-field head and neck treatments to prevent an overdose. (C) 1999 El sevier Science Ireland Ltd. All rights reserved.