A three-isocenter jagged-junction IMRT approach for craniospinal irradiation without beam edge matching for field junctions

Abstract

Purpose: Traditionally craniospinal irradiation treats the central nervous system using two or three adjacent field sets. We propose a technique using a three-isocenter intensity-modulated radiotherapy (IMRT) plan (jagged-junction IMRT) which overcomes problems associated with field junctions and beam edge matching and improves planning and treatment setup efficiencies with homogenous target dose distribution.

Methods and materials: Treatments for 3 patients with a prescription of 36 Gy in 20 fractions were retrospectively planned with jagged-junction IMRT and compared to conventional treatment plans. Planning target volume (PTV) included the whole brain and spinal canal to the S3 vertebral level. The plan used three field sets, each with a unique isocenter. One field set with seven fields treated the cranium. Two field sets treated the spine, each set using three fields. Fields from adjacent sets were overlapped, and the optimization process smoothly integrated the dose inside the overlapped junction.

Results: For jagged-junction IMRT plans vs. conventional technique, the average homogeneity index equaled 0.08 ± 0.01 vs. 0.12 ± 0.02, respectively, and conformity number equaled 0.79 ± 0.01 vs. 0.47 ± 0.12, respectively. The 95% isodose surface covered (99.5 ± 0.3)% of the PTV vs. (98.1 ± 2.0)%, respectively. Both jagged-junction IMRT plans and the conventional plans had good sparing of organs at risk.

Conclusions: Jagged-junction IMRT planning provided good dose homogeneity and conformity to the target while maintaining a low dose to organs at risk. Results from jagged-junction IMRT plans were better than or equivalent to those from the conventional technique. Jagged-junction IMRT optimization smoothly distributed dose in the junction between field sets. Because there was no beam matching, this treatment technique is less likely to produce hot or cold spots at the junction, in contrast to conventional techniques. The planning process is also simplified as only one IMRT plan is required for the entire target volume.