Dosimetric evaluation of abutted fields using asymmetric collimators for treatment of head and neck

The objective of this study was to reevaluate the dose nonuniformity of abutted fields defined using asymmetric collimators and one isocenter for treatment of the head and neck region.

Bilateral parallel-opposed fields abutted to the anterior field at one isocenter were implemented in the treatment of head and neck. The effect of digital display tolerance can produce dose nonuniformity at the junction of the abutted fields. The amount of dose nonuniformity was quantified using both mathematical summation of dose profiles and by direct measurement of doses at the junction of the two abutted fields. The dose nonuniformity was obtained by irradiating the superior part of a film using bilateral parallel-opposed fields and the inferior part by an anterior field with a gap or an overlap. Dose profiles were taken at the depth of maximum dose for the anterior field across the abutted fields. The dose nonuniformity was determined for the case where the asymmetric jaw was set at -2 mm, -1 mm, 0, +1 mm, and +2 mm from the beam central axis.

The dose at the junction increases systematically as the abutment of the fields changes from a gap to an overlap. The dose nonuniformity with 1-mm gap and 1-mm overlap is about 15% underdose and overdose, respectively.

Imperfect abutment of split fields due to digital display tolerance (+/-1 mm) of asymmetric collimator can cause an underdose or overdose of 15% of the delivered dose.