Purpose: To develop a modified planning and delivery technique that reduces dose nonuniformity for tomographic delivery of intensity-modulated radiation therapy (IMRT).
Methods and materials: The NOMOS-CORVUS system delivers IMRT in a tomographic paradigm. This type of delivery is prone to create multiple dose nonuniformity regions at the arc abutment regions. The modified technique was based on the cyclical behavior of arc positions as a function of a target length. With the modified technique, two plans are developed for the same patient, one with the original target and the second with a slightly increased target length and the abutment regions shifted by approximately 5 mm compared to the first plan. Each plan is designed to deliver half of the target prescription dose delivered on alternate days, resulting in periodic shifts of abutment regions. This method was experimentally tested in phantoms with and without intentionally introduced errors in couch indexing.
Results: With the modified technique, the degree of dose nonuniformity was reduced. For example, with 1 mm error in couch indexing, the degree of dose nonuniformity changed from approximately 25% to approximately 12%.
Conclusion: Use of the modified technique reduces dose nonuniformity due to periodic shifts of abutment regions during treatment delivery.