A dynamic match-line wedge

Purpose: To implement match-line wedges at the abutting edges of x-ray fields using dynamic collimation, Methods and Materials: Experiments were made using a computer-controlled linear accelerator equipped with developmental software that allows for collimator jaw motion while the beam is on. The jaws defining the abutting field edges were programmed to move from 1.5 cm inside to 1.5 cm outside the prescribed field during irradiation, Films were taken in plastic phantoms to assess the resulting edge gradient and to evaluate the sensitivity of this technique to setup errors, Results: The measured edge gradient for a single field was 30% per cm. Parallel-opposed lateral fields produced a gradient of 28% per cm along their midline, A simulated central nervous system irradiation with cranial and spinal fields kept dose variations in the field-match region to less than 10% with setup errors of 3 mm. Conclusion: The use of collimator motion during irradiation is an effective and simple means of reducing the dose variation in a field-match region due to setup errors and system tolerances. Treatment time is not increased and labor savings can be achieved when compared to feathering techniques commonly used.