A method for delivering accurate and uniform radiation dosages to the head and neck with asymmetric collimators and a single isocenter.

PURPOSE To investigate the use of asymmetric collimators and a single isocenter for delivering a uniform, accurate dose of radiation to the head, neck, and supraclavicular lymph nodes. METHODS AND MATERIALS A linear accelerator with a pair of asymmetric collimators is required for this technique. An isocenter was placed at the junction of the lateral head and neck fields and the anterior supraclavicular field. The asymmetric collimators were set longitudinally, by collimator rotation if necessary. The collimators split the radiation beam to all portals. Dose uniformity was measured at the junction with films in solid-water phantoms. RESULTS Film dosimetry showed a uniform dose at the junction without hot or cold regions. A digital display tolerance of +/- 1.0 mm for a field size maintained an acceptable uniform dose (+/- 5% dose variation) at the junction. The single isocenter and asymmetric collimators reduced field setup time by half. No table rotation was required to match fields. CONCLUSION The asymmetric collimators lead to easy and accurate patient setup. The absence of the trapezoid effect resulted in the complete coverage of the submandibular and cervical nodes without any hot spots.