The Dose Junction Issue Associated with Photon Beams for Large Volume Radiation Therapy and the Sensitivity to Set-up Error

Purpose: Radiation therapy involving large treatment volumes requires multiple radiation fields having different isocenters; dosing across field junctions and sensitivity to setup error must be addressed. Some published papers presume that using IMRT techniques alone will ensure treatment is not sensitive to setup error. We compared Jagged-Junction-IMRT (JJ-IMRT), Non-Jagged-Junction- IMRT (NJJ-IMRT) and VMAT to demonstrate how these different techniques address the dose junction issue and how this made each technique more or less sensitive to setup error. Method: A JJ-IMRT plan was developed and used to treat a patient using 6-MV photons and a 120-leaf multi-leaf collimator. NJJ-IMRT and VMAT plans were retrospectively developed with the same patient. Verification of the dose delivered to the cranio-spinal junction region for JJ-IMRT, NJJ-IMRT and VMAT were done through ion chamber and film measurements on a human-shaped wax phantom. For film measurements, each verification plan was delivered twice, with and without setup error in the longitudinal direction. Results: The difference between measured dose with ion chamber and planned dose from the verification plans was 1.59% for JJ-IMRT, 1.35% for NJJ-IMRT and 1.68% for VMAT. The maximum difference in dose profiles between film measurements with and without setup error was less than 4% for JJ-IMRT, less than 6% for VMAT and about 10% for NJJ-IMRT. Conclusion: IMRT and VMAT techniques alone will not make treatment plans insensitive to setup error. Ensuring that beam field edges are not all placed on the same line is the reason the plans are not sensitive to setup error.