Radiation therapy often requires the treatment of large volumes requiring multiple fields of radiation having different isocenters. Due to machine limitations, the maximum field size for Varian high energy linear accelerators is 40 × 40 cm at isocenter with a source-to-axis distance (SAD) of 100 cm. To deal with larger field sizes, radiation therapy requires the junction of multiple fields and care must be taken when aligning adjacent fields to avoid the formation of hot and cold spots. To attempt to about one field up against another field is not feasible due to different directions of beam divergences. The alternative is to overlap field edges in such a way as to be able to control the combined dose in the junction region. A number of approaches have been developed to deal with the dose junction issue with varying levels of success. When aligning fields, one must also consider the possibility of set-up error; even a small setup error can lead to an unexpected increase or decrease in field overlap or mismatching resulting in unintentional high or low doses. We have developed a novel IMRT technique called Jagged- Junction-IMRT (JJ-IMRT) that addresses the dose junction issues associated with multiple radiation fields. The method was originally developed for cranio-spinal irradiation (CSI). JJ-IMRT for CSI uses a three-isocenter IMRT plan that handles the junction issues by intentionally overlapping adjacent fields, with a different field edge location for each field, and letting the optimization process smoothly blend the dose from these fields. The intentional ‘jagging’ of the field edges results in the junction being insensitive to set-up error. Wang et al.  have also developed an IMRT method for CSI which they claim, when compared to the JJ-IMRT technique, further simplifies the planning process for CSI. Their IMRT technique, referred to here as Non-Jagged-Junction- IMRT (NJJ-IMRT) simplifies the steps required to manually set field widths and boundaries by allowing the optimization to freely open the field size as needed. As a result, all the fields open to their maximum size and all field edges from the same iso-center end on the same line. Because of this, we believe the NJJ-IMRT method becomes more sensitive to setup error.

With Regards,
Sara Giselle
Associate Managing Editor
Journal of Medical Physics and Applied Sciences